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  • 1990-1994  (4,266)
  • Engineering General  (3,144)
  • Physical Chemistry  (1,121)
  • Nuclear reactions
  • 101
    Electronic Resource
    Electronic Resource
    Instability, chaos, and growth and decay of energy of time-stepping schemes for non-linear dynamic equations (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 393-401 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this paper we present numerical experiments made to investigate the behaviour of the Newmark time-stepping scheme applied to non-linear dynamic systems. Our attention is focused on the instability and chaos in the Newmark scheme when it is applied to the equation ü + P(u) = 0, representing a non-linear elastic spring. Some unusual modes of behaviour, which are of substantial interest, have been observed. In the first case, a stable but chaotic solution is found. In the second case, while a stable solution is obtained with a certain time step, an unstable solution is found by decreasing the time step. In the third case, instability is triggered by neglecting the initial acceleration. A simple modification of the Newmark scheme is proposed which keeps the energy constant for the equation ü + P(u) = 0 and thereby guarantees unconditional stability. Numerical examples in support of such an energy-conserving scheme are presented.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100505
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  • 102
    Electronic Resource
    Electronic Resource
    The effects of a discontinuity in the forcing function of a singular integral equation (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 421-423 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A note on the effects of a weak discontinuity in the forcing function g(x) of a singular, integral equation of the first kind and the resulting strong discontinuity that can appear in the solution f(x) is presented.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100508
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  • 103
    Electronic Resource
    Electronic Resource
    Unconditionally stable FEM for transient linear heat conduction analysis (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 427-435 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper presents a new method for the numerical solution of transient linear heat conduction problems. In the proposed method, the transient linear heat conduction equation is first integrated with respect to time over subsequent intervals. Then the resulting set of elliptic equations is discretized in space according to a finite-element procedure. The method is unconditionally stable. At the end of the paper, the effectiveness of the proposed method is assessed through some numerical examples.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100602
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  • 104
    Electronic Resource
    Electronic Resource
    Criteria for eliminating oscillation in analysis of heat-conduction equation by finite-element method (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 453-460 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In finite-element analysis of transient heat conduction, oscillatory results are quite common. Several criteria for eliminating oscillation which permit choice of time step length and/or element mesh and type are sensibly derived and proved with a different methodology from those of previous work. Numerical examples are presented to show their validity and usefulness.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100604
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  • 105
    Electronic Resource
    Electronic Resource
    Numerical procedure for calculation of temperature evolution in composite elements with internal cavities (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 437-451 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the paper a computation code based on the finite-element method for temperature evolutions in composite elements with internal cavities is presented. It is suitable for the analysis of various thermal problems encountered in mechanical and civil engineering such as fire risk assessment. It is based on a plane isoparametric triangular element with six nodes allowing the modelling of curved boundaries. In order to analyse composite steel-concrete elements and to take into account the non-linearities coming from the temperature-dependent material properties and from the boundary conditions, an implicit integration scheme is used. Another advantage of the program is the possibility of describing internal cavities with convection and radiation exchanges. In order to show the capacity of the program two examples are presented. Comparisons have been made with experimental and other numerical results, exhibiting very good agreement.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100603
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  • 106
    Electronic Resource
    Electronic Resource
    Numerical study of withdrawal and return of water following a volcanic eruption at sea (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 461-468 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A speculation is presented to explain in physical terms the biblical story in which the withdrawal and return of the sea saved the Hebrews but drowned the army of Pharaoh. The explosion of the volcano on the island of Thira (Santorini) is speculated to be connected with this biblical story.To support our proposition concerning depression and flooding waves, we have made a numerical calculation with a highly accurate computer code and provided an outline of the PLIM method (piecewise linear interpolation method) used. This calculation is numerically very challenging because the volcano is located far from the shore and hence detailed discretization is not practical. In addition, we show that if a less accurate numerical method is used, the whole phenomenon is lost because of numerical errors.Our simple approach provides a withdrawal and return of the water to the shore that is sufficiently large, but the duration of this occurence is only a few minutes. The opportunity for a much longer duration as a consequence of other phenomena has not been excluded, however.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100605
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  • 107
    Electronic Resource
    Electronic Resource
    Direct simulation of turbulent flow and heat transfer in a channel. Part II: A Green's function technique for wavy walls (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 489-496 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The possibility of improving heat transfer rates by the imposition of a wavy boundary led to this extension of direct simulation of heat transfer in turbulent flow in a channel. This part of the paper shows how a pattern of micro-riblets on the surface and parallel to the flow is incorporated into the solution of the Navier-Stokes and temperature equations. The channel is two-dimensional with walls at different but uniform temperatures, and the fluid properties are constant. The presence of the patterns presents a considerable computational problem that can be largely overcome by the use of a discrete Green function technique. The method is illustrated by making short-time calculations of the velocity and temperature fields for a Prandtl number of 0.72, a Reynolds number based on channel half-width and bulk velocity of 2800, and a sinusoidal pattern with a height of 5 wall units.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100608
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  • 108
    Electronic Resource
    Electronic Resource
    Masthead (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994) 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100701
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  • 109
    Electronic Resource
    Electronic Resource
    Non-linear analysis of plane elastic bodies with inclusions by a BEM-FEM approach (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 511-521 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the present study a hybrid boundary-element and finite-element method is developed for the non-linear analysis of two-dimensional elastic media with inclusions, and it is utilized in defining the micro-mechanical stress field in composite materials. The linear elastic, isotropic and homogeneous matrix is treated with the BEM, while the non-linear elastic and orthotropic inclusions (fibres) are treated with the FEM.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100703
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  • 110
    Electronic Resource
    Electronic Resource
    Finite-element method for EHL analysis of spherical bodies (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 501-510 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A finite-element solution for elastohydrodynamic lubrication (EHL) of spherical bodies undergoing elastic deformation is presented. The Galerkin finite-element approach is employed and the domain under investigation is discretized using isoparametric quadrilateral elements. Isothermal conditions are assumed and the generation of the pressure is assumed to be governed by the Reynolds equation. The viscosity and the density of the oil are assumed to be functions of the pressure. It is assumed that the underlying surface deforms elastically under the hydrodynamic pressure distribution. The accuracy of the technique is illustrated in particular examples.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100702
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  • 111
    Electronic Resource
    Electronic Resource
    Revised Greedy algorithm for formation of a minimal cycle basis of a graph (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 523-530 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An efficient algorithm is developed for the formation of a minimal cycle basis of a graph. This method reduces the number of cycles to be considered as candidates for being the elements of a minimal basis and makes practical use of the Greedy algorithm feasible. A comparison is made between the existing methods and the present algorithm. A counter-example is presented for Kaveh's algorithm from planar graphs.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100704
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  • 112
    Electronic Resource
    Electronic Resource
    Masthead (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994) 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100901
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  • 113
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    Electronic Resource
    Explicit expressions of shape functions for the modified 8-node serendipity element (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 711-716 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: We derive explicit expressions of shape functions for the 8-node serendipity element with bilinear element geometry, which is modified so that it can represent any quadratic fields in Cartesian co-ordinates. The modification is similar to the one by MacNeal and Harder, but is slightly different and based on our own idea. We also show that the usual 6-node triangular element can be obtained by applying the node degeneration technique to this modified element.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100905
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  • 114
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    Derivation of shape functions for an exact 4-D.O.F. Timoshenko beam element (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 673-681 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Timoshenko beam elements have been the subject of numerous publications. The difficulty was that of arriving at a superconvergent element with four degrees of freedom, as is the case for the Bernuli-Euler classical beam element. Two different approaches are presented here for the derivation of the shape functions. The first is based on the flexibility matrix, where utilizing the unit load method, including the term that accounts for the shear deformations in the virtual work expression, the stiffness matrix is derived. Then, a second method is presented to derive the exact shape functions, directly from the differential equations of the Timoshenko beam theory. The resulting shape functions are the same in both methods.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100902
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  • 115
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    Convergence of stress maxima in finite element computations (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 683-697 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The convergence of stress maxima, computed directly from finite element solutions, is investigated with respect to a family of exact solutions characterized by varying degrees of smoothness. The performances of h- and p-extensions and the product and trunk spaces are evaluated and documented with respect to a family of benchmark problems. In uniform p-extensions a characteristic pattern in the convergence of stress maxima was observed. There does not appear to be a clear-cut advantage of the product space over the trunk space in this respect. The much faster convergence of stress maxima in the case of p-extensions, as compared with h-extensions, is evident from the results.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100903
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  • 116
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    Enhanced 8-node three-dimensional solid and 4-node shell elements with incompatible generalized displacements (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 699-709 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Enhanced 8-node three-dimensional solid and 4-node shell elements are proposed in the paper. Improvement of the response of the 3-D solid is achieved through additional strains corresponding to generalized displacements incompatible between elements. In the case of the 4-node shell, the membrane behaviour is improved through addition of incompatible membrane strains.Some linear and geometrically non-linear examples demonstrate significant improvements of the elements. Results are compared with solutions obtained with the standard, displacement-based elements and with results available in the literature.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100904
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  • 117
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    A three-dimensional symmetric linear equation solver (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 717-730 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Factorization procedures for the efficient solution of large sparse linear finite difference systems have been introduced recently. In these procedures the large sparse symmetric coefficient matrix of a certain structure is factorized exactly, yielding a direct solution method. Furthermore, approximate factorization procedures yeild implicit preconditioning iterative methods for the finite difference solution. The numerical implementation of these algorithms is presented and Fortran subroutines for the efficient solution of the resulting sparse symmetric linear system of algebraic equations are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100906
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  • 118
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    Note on finite element predictions of natural frequencies (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 731-734 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: It is well known that the finite element discretization with a distributed mass matrix gives overstimates of the natural frequencies of a system. The note shows that these do not necessarily become progressively worse as the frequency increases.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100907
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  • 119
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    Semi-discretization method for the heat equation with non-local boundary conditions (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 751-758 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Semi-discretization methods with iterated corrections are considered for solving the heat equation with boundary conditions containing integrals over the interior of the interval. The given problem is transformed into an ordinary differential system of equations, when we substitute the spatial derivative by finite differences. Such a system is then solved with an implicit integrator together with a quadrature method for the boundary integrals and, for each time step, the numerical scheme - implicit integrator and quadrature method - is repeated iteratively in order to achieve a given accuracy. The extra work we need with the iterated corrections enables us to take care of the interrelation of the solution u(x, t) at the interior and at the boundary points and also to start the numerical process with rough approximations of u(0, t) and u(1, t).An improvement of the results is achieved when we estimate the local spatial truncation error and make the injection of that estimation into the ordinary differential system. Numerical experiments are presented.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100910
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  • 120
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    Applying dispersion correction to numerical approximations of the two-dimensional wave equation - eigenproblems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 735-742 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A technique is presented whereby numerical calculations of vibration modes can be improved. The paper looks at the classical two-dimensional wave equation using finite difference approximations. Analysis of the numerical dispersion of the approximations is used to develop a correction method. In general the numerical dispersion is dependent upon both the frequency and the direction of a wave, but if a 9-point formula is used the directional dependence is much reduced. This enables correction factors to be obtained using only the frequency of a vibration mode. The method was tested on the vibration of a square membrane and of an L-shaped region; in both cases a marked improvement in accuracy was obtained, at very little computational cost.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640100908
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  • 121
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    Reliable Delaunay-based mesh generation and mesh improvement (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 773-783 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper describes a method for generating triangulations of two-dimensional domains. Firstly, a description is given of a reliable algorithm that creates a constrained Delaunay triangulation for a multiply-connected planer domain, without performing any explicit visibility computations. Then, several techniques to improve an existing triangulation are discussed. It turned out that a new variant of mesh relaxation was most effective in improving an existing triangulation. Several examples are included to illustrate the method's overall behaviour.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101003
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  • 122
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    Discretization error dominance over subgrid terms in large eddy simulation of compressible shear layers in 2D (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 785-790 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Second- and fourth-order-accurate spatial discretization methods give rise to discretization errors which are large than the corresponding subgrid terms in large eddy simulation of compressible shear layers in 2D, if the ratio between the filter width and the grid spacing is close to one. Even if an exact representation for the subgrid-scale contributions is assumed, large eddy simulation is accurate only if this ratio is sufficiently larger than one. In that regime fourth-order methods are more accurate than second-order methods. An analysis of the data obtained from two-dimensional direct numerical simulations of compressible shear layers substantiates these assertions.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101004
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  • 123
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    Approximate solution of the Fokker-Planck-Kolmogorov equation by finite elements (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 763-771 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper considers the solution of the Fokker-Planck-Kolmogorov equation by the finite element method (FEM). The problem is set in a variational formulation suitable for the FEM. Some theoretical aspects related to applying the method are discussed. Discretization of the problem is carried out and questions of convergence and order of the method are also addressed. A numerical example is given to verify the validity of the approach.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101002
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  • 124
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    A fourth-order difference scheme for quasilinear poisson equation in polar co-ordinates (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 791-797 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: We present in this paper a nine-point, fourth-order difference scheme for the numerical solution of the quasilinear Poisson equation in polar co-ordinates, \documentclass{article}\pagestyle{empty}\begin{document}$$ Au_{rr} + \frac{1}{r}u_r + \frac{1}{{r^2 }}u_{\theta \theta } = f\left({r,\theta,u,u_{r,} u_\theta} \right) $$\end{document} with appropriate boundary conditions. A separate difference scheme of order four valid at r = 0 has also been obtained. The method is based on five evaluations of the function f. The numerical results of two problems obtained using this scheme are listed. The numerical results demonstrate the fourth-order accuracy of the scheme.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101005
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  • 125
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    Statistical analysis for static responses of structures with stochastic shape (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 809-814 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper presents a probabilistic boundary element method for analysis of structural responses to static loading, when the shape parameters of structures are considered as random variables. We can use this method to evaluate the mean values and standard deviations of the responses, and estimate the stochastic errors of structural character resulting from manufacturing errors.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101007
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  • 126
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    Two-dimensional adaptive quadrilateral mesh generation (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 815-825 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In the paper we address the problem of 2D adaptive quadrilateral mesh generation by using the variational principles. We first find the variational integral which generates the known grid system of curve-by-curve error equidistributions. We then use the same intergral to generate a new adaptive grid system which is superior to the known system in that the new system produces smoother meshes. Moreover, the new integral may be combined linearly with existing smoothness control integrals to yield more robust adaptive grid systems. Numerical results and comparisons are reported.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101008
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  • 127
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    Application of direct boundary integration method to wave diffraction by submerged bodies (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 799-808 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The diffraction of waves due to the presence of submerged objects in the ocean environment is solved by a relatively novel approach involving the direct boundary integration method (DBIM). Firstly, the method is tested for the case of a submerged circular cylinder and compared with the conventional boundary element method (BEM) results. The agreement between the results using this method and the BEM is good. Further, the DBIM has been applied to the case of a submerged horizontal plate for which the horizontal and vertical force coefficients are computed and compared with a semi-analytical solution using the matched eigenfunction expansion method (EFEM). It is found that the computed results using DBIM are in good agreement with those of EFEM and BEM. Further, the DBIM is found to be computationally more efficient than the BEM for the water wave diffraction problems presented in this paper.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101006
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    An adaptive time-stepping algorithm for quasistatic processes (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 837-844 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A scheme is presented that automatically determines the magnitude of the time step in quasistatic boundary value problems discretized via some customary method (finite elements, finite differences). The basic idea stems from the path-following techniques widely employed in purely static analyses in the sense that the incremental displacements, measured in some suitable norm, are constrained to a prefixed value. Examples of trusses composed of a viscoelastic material are included to demonstrate the effectiveness of the method.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101010
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    Integral transform solution of eigenvalue problems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 827-835 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The generalized integral transform technique is used to reduce eigenvalue problems described by partial differential equations to algebraic ones, that can be solved by existing codes for matrix eigensystem analysis. The method is illustrated for the operators that correspond to heat and mass diffusion, but can be employed in different fields. Three examples demonstrate the potential of the method.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/cnm.1640101009
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    Conference diary (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 857-858 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101012
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    Announcements (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 859-860 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101013
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994) 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101101
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  • 133
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    Finite element solution of steady natural convection equations by a false diffusion method (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 845-856 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Use of upwind finite element methods was so far confined to forced convection problems. The present work is devoted to the application of false diffusion techniques to phenomena involving natural convection. A range of Rayleigh numbers in which the discretization by the conventional Galerkin scheme fails is considered. A false diffusion method similar to the Streamline upwind technique has been employed to avoid cross-wind diffusion. The upwind parameter is changed continually until a true solution is obtained. The present scheme provides convenient means for obtaining gradually improved guesses from which a more accurate solution can be obtained using the conventional Galerkin method. The criterion for the determination of a true solution is established, and hot-wall Nusselt number values are documented in tabular form.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101011
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    A note on the treatment of frictionless contact between non-smooth surfaces in fully non-linear problems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 869-878 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An approach is presented for the formulation of the unilateral contact constraint in the presence of contact surface discontinuities. Such discontinuities may be due to physical corners on the surfaces of contacting bodies, or may be introduced by a discretization process (e.g. finite elements). It is asserted that a strong analogy exists between this problem and the one describing inelastic evolution in the presence of a discontinuous yield surface. This analogy is exploited to produce an effective treatment of the frictionless corner problem, complete with an effective augmented Lagrangian implementation for accurate constraint enforcement.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101103
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    Incompressibility and axisymmetry: A modified B method (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 861-867 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper deals with a B-type strain projection method with specific reference to axisymmetric incompressible solids. Essentially the \documentclass{article}\pagestyle{empty}\begin{document}$ \overline{\overline {\rm{B}}} $\end{document} (B double bar) method, which is reported in the present study, is based on Hughes' B-bar approach utilizing modified shape functions to represent the radial displacement and geometry. As a consequence of adopting these procedures the \documentclass{article}\pagestyle{empty}\begin{document}$ \overline{\overline {\rm{B}}} $\end{document} method results in an exact satisfaction of incompressibility constraints. The rationale of the \documentclass{article}\pagestyle{empty}\begin{document}$ \overline{\overline {\rm{B}}} $\end{document} method leads to the modified mean-dilatation approach for the case of an axisymmetric incompressible problem. It is interesting to note in the present work that the explicit influence of enhanced hoop strain is eliminated from the discrete divergence condition.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101102
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  • 136
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    Finite-deformation interface formulation for frictionless contact problems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 879-893 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: An interface finite element for non-linear analysis of frictionless contact problems is presented. A constitutive equation relates stresses in the interface layer to the deformation gradient with respect to an imaginary reference configuration where the layer thickness is constant and finite. The equation of equilibrium is written in the same configuration, while the boundary conditions involve stresses related to an actual reference configuration used in the formulation for the contacting bodies. Finite element discretization is introduced for the layer in order to calculate the unknown field of displacements. Computational examples demonstrate properties of the contact element and its range of applicability in the analysis of large deformation contact problems including relative sliding of curved surfaces.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101104
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    A BEM formulation of new boundary stress components (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 895-906 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper presents boundary integral equation to two-dimensional elasticity with the stress component σijtitj as one of the boundary values, where ti are direction cosines of the tangent on the boundary. This form of BEM has an advantage in that the stress component σijtitj on the boundary can be calculated directly from the numerical solution. The present formulation for planar problems uses two kernels, one of which is logarithmic singular and the other is 1/r singular. The effectiveness of the approach is also discussed through some test examples.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101105
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    Numerical solution to a stochastic interception problem (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 907-911 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A finite difference scheme is derived to solve for the probability density that a moving point, whose velocity is continually pertubed by Gaussian white noise, reaches a given target within a given time period. The numerical scheme is applied to the problem of finding the probability that two moving spheres collide.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101106
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  • 139
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    Higher-order numerical methods for third-order non-linear boundary value problems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 943-948 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A sixth-order and a ninth-order method are developed for the numerical solution of special non-linear third-order boundary value problems y‴ = ø(x,y), a〈 x 〈 b, y(a) = A1, y″(a) = B2 and y(b) = A2. The method arise from a four-point recurrence relation. Convergence analysis of the sixth-order method is discussed. The methods are tested on a problem. Modification to these methods are obtained for problems with boundary conditions of the form y(a) = A1, y'(a) = B1 and y(b) = A2.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/cnm.1640101110
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  • 140
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    Numerical viscosities of difference schemes (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 927-931 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Numerical viscosities of finite-difference schemes are usually obtained from truncation-error analyses based on Taylor series expansions. Here we observe that numerical viscosities can also be obtained very simply and directly from the growth factor ξ in a conventional Fourier stability analysis. A general formula is derived for the numerical viscosity in terms of the first and second derivatives of ξ with respect to the wavenumber k, evaluated at k = 0. A single Fourier analysis therefore suffices to determine both stability limits and numerical viscosities.
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    URL: http://dx.doi.org/10.1002/cnm.1640101108
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    A new algorithm for finding a pseudoperipheral vertex or the endpoints of a pseudodiameter in a graph (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 913-926 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Based on the concept of the Laplacian matrix of a graph, this paper presents the SGPD (spectral graph pseudoperipheral and pseudodiameter) algorithm for finding a pseudoperipheral vertex or the end-points of a pseudodiameter in a graph. This algorithm is compared with the ones by Grimes et al. (1990), George and Liu (1979), and Gibbs et al. (1976). Numerical results from a collection of benchmark test problems show the effectiveness of the proposed algorithm. Moreover, it is shown that this algorithm can be efficiently used in conjunction with heuristic algorithms for ordering sparse matrix equations. Such heuristic algorithms, of course, must be the ones which use the pseudoperipheral vertex or pseudodiameter concepts.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101107
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    Local-global analysis on localized non-linear shells of revolution (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 933-941 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A finite element program is developed as a tool to analyse shells of revolution with local non-linearities. In reality, shells of revolution often exhibit local deviations, like a cut-out, a junction and/or an imperfection. The stress concentration around a local deviation may produce plasticity and/or geometric non-linearities in the surrounding region. The analytical model consists of three different types of elements: rotational, transitional and general. The rotational shell elements are used in the region where the shell is axisymmetrical and linear, while the two-dimensional general shell elements are deployed in the deviation region where non-linearities may occur. Transitional shell elements connect the two distinctively different types of elements to achieve displacement field continuities. The solution using the local-global system with appropriate condensation and a predicted stress incremental procedure is suggested. It is shown that the technique is a very attractive alternative to the entirely general element style analysis for axisymetric shell structures with local deviations.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101109
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    Comments on ‘equivalent versions of the quick scheme for finite-difference and finite-volume numerical methods’ (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 949-953 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/cnm.1640101111
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    Conference diary (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 955-956 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101112
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    Announcements (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 957-959 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101113
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994) 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101201
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    Further developments towards a new virtual-pulse time integral methodology for general non-linear transient thermal analysis (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 961-970 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: New developments of describing the theoretical basis towards an effective virtual-pulse (VIP) time integral methodology are proposed for general non-linear transient heat transfer problems. Primarily to validate the proposed methodology of computation, simple numerical test cases are provided and comparisons are also drawn with the implicit second-order-accurate Crank-Nicolson method. For the models tested, the proposed method has comparable or improved accuracy and stability characteristics. The VIP methodology introduced here for tackling non-linear thermal problems offers attractive features and is a viable alternative to traditional time-stepping practices. Efforts are underway to demonstrate the practical applicability to multi-dimensional thermal analysis.
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    URL: http://dx.doi.org/10.1002/cnm.1640101202
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  • 148
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    An optimized implementation of the Newmark/Newton-Raphson algorithm for the time integration of non-linear problems (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 983-992 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper describes an optimized computational implementation of a basic ‘building block’ for non-linear structural dynamic analysis programs: the combination of the modified Newton-Raphson iterative technique with an implicit time integration operator (in this case a member of the Newmark family), working in an incremental--iterative formulation for the equations of motion. The objective of this implementation is to attain improved computational efficiency, regarding both CPU time and memory requirements. The basic formulation and derivation are presented, along with the implementation details; the positive aspects related to the computational optimization are highlighted.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101204
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    An improved FE expansion for micropolar localization analysis (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1005-1012 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A mixed variational principle is presented for the geometrically linear micropolar continuum. Then a suitable discretization and its implementation are discussed, resulting in an improved element behaviour for micropolar localization analysis. The intriguing element performance is demonstrated for the case of localization within a compression problem.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101206
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  • 150
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    Coupled discrete-layer finite elements for laminated piezoelectric platess (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 971-981 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: Finite element models based on discrete-layer theories are presented for the coupled-field analysis of laminated plates containing piezoelectric layers. The three displacements and the electrostatic potential are treated as unknowns in this formulation, which allows for piece-wise approximations of the variables through the thickness of each layer. Two specific models are demonstrated in which the transverse displacement is either variable or constant, and the in-plane displacements and potential take piece-wise linear approximations through the thickness. The models are applied to example problems with applied surface tractions and specified surface potentials. Good agreement is found with exact solutions.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/cnm.1640101203
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    Comparison of explicit and implicit finite element methods in the quasistatic simulation of uniaxial tension (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 993-1003 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: On the basis of simple one-dimensional finite element analyses, comparison in simulations of uniaxial tension using explicit dynamic and implicit static tonnulations has been made. Results show that for materials having a Hollomon-type constitutive law with power-law strain rate sensitivity, the explicit dynamic method can be employed for the quasistatic simulations of tensile tests only below the critical test velocities or critical normalized material densities. These critical values are determined numerically as a function of material parameters.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101205
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    A new type of B3-spline interpolation (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1013-1020 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A new type of B3-spline interpolation is presented. This new formulation allows the user to introduce interior point and line supports in much the same way as in a conventional finite element formulation. The bandedness and efficiency of the standard B3-spline interpolation are retained. The new formulation can be incorporated into existing B3-spline formulations with very little effort. Several examples are presented to demonstrate the versatility of the new approach.
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    URL: http://dx.doi.org/10.1002/cnm.1640101207
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    Conference diary (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1045-1046 
    Details
    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101210
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    Announcements (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1047-1048 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cnm.1640101211
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    A note on the calculation of consistent tangent operators for Von Mises and Drucker-Prager plasticity (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1021-1025 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: A comparison is made of implementations of consistent tangent operators that arise in implicit integration of Von Mises and Drucker-Prager yield criteria. When computing the consistent tangent operator a matrix inversion has to be performed at integration point level. The consequences of different formulations of the consistent tangent operator on the numerical accuracy are assessed.
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    URL: http://dx.doi.org/10.1002/cnm.1640101208
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    Boundary characteristic orthogonal polynomials in numerical approximation (1994)
    Chichester : Wiley-Blackwell
    Communications in Numerical Methods in Engineering 10 (1994), S. 1027-1043 
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    ISSN: 1069-8299
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: The paper describes a procedure to generate boundary characteristic orthogonal polynomials (BCOPs) over a domain in Rn. The method is based upon the Gram-Schmidt process of orthogonalization. The orthogonal polynomial sequence (OPS) is generated from a set of linearly independent functions which satisfy the given boundary conditions. The procedure is illustrated by taking several examples in R2, the most important case from the application point of view. Extensive numerical work has been carried out for simple domains like the circle, square and isosceles right-angled triangle. The following cases have been considered: (a) when the BCOPs vanish on the boundary; (b) when they and their normal derivatives vanish on the boundary; (c) when no conditions have been imposed on the boundary. The results for other geometries such as an ellipse, a parallelogram and an arbitrary triangle follow by some simple transformations. The procedure is also applicable to one-dimensional problems and can be easily extended to R3.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/cnm.1640101209
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  • 157
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    The influence of solvents on the basicity of dipolar amines (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 63-67 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative basicities of six monosubstituted dipolar trimethylamines (XCH2NMe2) were determined in nine different solvents from potentiometric titration. Solvation effects on basicity variations were analyzed by the solvatochromic parameters (π*, α and β). Non-specific solvation of the substituted dimethylammonium ions is not reflected adequately by the solvent's dipolarity-polarizability constant, π*. A better representation of non-specific interaction between solvents and the monosubstituted dipolar trimethylammonium ions is gained from the product of π* and the solvent dipole moment (μ). For these ions where the dipolar substituents are separated from the reaction center by one methylene unit, there is an interaction between the charge of the ammonium ion and the dipolar substituent. As a result, solvent molecules must not only solvate the dipolar substituents, but must also gain access between the lines of force of the dipolar substituent and the charge. Bulky solvents, such as nitrobenzene, cannot access adequately the region between the dipolar substituents and the reaction center to accomplish similar solvation as less bulky solvents.
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/poc.610070203
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  • 158
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    Kinetic study of the formation and rupture of stable tetrahedral intermediates. C—O, C—N and C—S bond formation (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 80-89 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Measured pseudo-first-order rate constants for intramolecular formation of tetrahedral intermediates from N-2-hydroxyethylphthalimide, N-2-aminoethylphthalimide and N-2-thioethylphthalimide at pH 〉 6 are reported. The reaction is specific and general base catalysed, with β(Brønsted) values 0·44, 0·52 and 0·52 respectively. From a plot of log kb (general base rate constants) vs γ′ [the affinities of EtXH (X = O, NH, S) toward the carbonylic carbon], β′nuc values of 0·01 (with OH- as specific base), 0·25 (with imdidazole as general base) and 0·27 (with HPO42- as general base were obtained). The observed relationships pxy = ϑβ′/ - ϑpKa = -ϑ(Brønsted)/ϑγ′ = 0·03 is supported by the predictions of an energy contour diagram, which, on extrapolation to a non-stable tetrahedral intermediate, predicts a late and slightly protonated transition state for the cleavage process. At pH 〈 3, these intermediates cleave to yield only the corresponding diacylimides. These reactions are general base and acid catalysed with β 〉 0·3 and α 〈 0·1. A fast equilibrium between the intermediate and its N-protonated (amide) form is reached. The general base rupture of the latter is faster than that of the corresponding non-protonated intermediate by a factor of ca 109 - 1010-fold.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070205
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    Single-parameter versus dual-parameter correlation for radical reactions. Addition of bromine atoms to α-methylstyrenes (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 96-104 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α-methylstyrenes (1-Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual-parameter equation and not by a single-parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070207
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  • 160
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    Methyl transfer from MeCo(III)Pc to thiophenoxide (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 90-95 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transfer of the cobalt-bound methyl in MeCo(III)Pc to thiophenoxide ion was studied (H2Pc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane). In dimethylacetamide solution, the reaction is rapid, requiring stopped flow for the kinetics, and yielding MeSPh and Co(I)Pc- in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS-], attributed to the reversible formation of an inert complex with PhS- occupying the vacant octahedral site in MeCo(III)Pc, on the other side of the phthalocyanine plane from the methyl group. The kinetics allow the estimation of the equilibrium constant, K, and the SN2 rate constant, k, which at 25°C have values of ca. 9·4 × 103 l mol-1 and 1·8 × 104 l mol-1, respectively. Although these values are rough, the ratio k/K is firm at 1·91 ± 0·02 s-1; this is the limit of the rate at high [PhS-]. An alternative mechanism, which is entirely consistent with the kinetics, involves a rate-determining homolysis of the Co—S bond of the same complex. The mechanism is not favored because the product yields are high for a radical combination process and alternative chain processes are kinetically unacceptable. Further, the rate constant is about what would be expected from the reactivity of other nucleophiles in SN2 reactions. Further arguments in favor of the SN2 mechanism are presented. This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B12-promoted methionine synthesis in nature. The other step in the biological, enzymatic process is the transfer of methyl from the nitrogen of N-methyltetrahydrofolate to cobalt. An attempt to model this with the very reactive N-methyl-2,6-dichloropyridinium ion was unsuccessful; the reaction took an entirely different course, presumably initiated by electron transfer, but leading to substantial loss of Cl- from the pyridine. No more than 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070206
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    Announcements (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 116-116 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070209
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070301
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  • 163
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    Protonation studies on isomeric tetrafluoro-2,11-dithia[3·3]cyclophanes, tetrafluoro[2·2]metaparacyclophane and their corresponding non-fluorinated analogs. Comparison with fluorinated and non-fluorinated[2·2]cyclophanes: A stable ion and semi-empirical (AM1, PM3) investigation (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 105-115 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Protonation of tetrafluoro-2,11-dithia [3·3]paracyclophane and tetrafluoro-2,11-dithia [3·3]metaparacyclophane in high-acidity super acid media, namely FSO3H.SbF5 (1:1) ‘magic acid’-SO2CIF, gave their corresponding acidic disulfonium ions. Additional ring protonation to give a disulfonium-monoarenium trication was not observed. With the non-fluorinated 2,11-dithia [3·3] cyclophanes, the disulfonium ions can be ring protonated in equilibrium to give a dynamic disulfonium-monoarenium trication. Tetrafluoro [2·2]-metaparacyclophane is monoprotonated at the meta ring and gives a complex mixture of conformational isomers. Multinuclear magnetic resonance data on the cyclophane precursors and their derived cations are compared and analysed. The energies, conformations and charge distributions of the isometric fluorinated and non-fluorinated [2·2] - and dithia [3·3] cyclophanes were calculated by the AM1 and PM3 methods, respectively. In all but one case the cyclophane arenium ions predicted by theory to be energetically most favoured are those observed in solution under stable ion conditions. In agreement with experiment, the instabilities of S,S,C-cyclophane trications are also theoretically predicted.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070208
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    Announcement (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. I 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070302
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  • 165
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    Kinetics and mechanism of electron-transfer reaction of aquothallium(III) and coordinated thallium(III). Oxidation of metol with thallium(III) in acid perchlorate medium (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 117-121 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of oxidation of metol (N-methyl-p-aminophenol) by thallium(III) have been studied in a perchloric acid medium. This organic compound is employed extensively in photographic work. The stoichiometry of the reaction is represented by: C6H4(OH)NHCH3 + TIIII → C6H4(O)NCH3 + TII + 2H+The kinetics indicate the rate law: \documentclass{article}\pagestyle{empty}\begin{document}$$k' = \frac{{Kk\left[{metal} \right]}}{{1 + K\left[{metal} \right]}}$$\end{document} where k′ is the pseudo-first-order rate constant. The rate constants and thermodynamic parameters for the rate-limiting step have been evaluated.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070303
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    The volume profile for a cheletropic reaction (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 178-180 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The volumes of activation and of reaction were measured for the reaction between sulphur dioxide and 2,3-dimethylbuta-1,3-diene to form 3,4-dimethylsulpholene. Values of ΔV and ΔV* are -33 and -35 cm3 mol-1, respectively in accordance with a concerted reaction having a very product-like transition state.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070403
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  • 167
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    Secondary α-deuterium isotpe effect on the hydration of p-nitrobenzaldehyde (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 192-195 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The secondary α-deuterium isotpe effect on the equiliburium constant (KD/KH) for the hydration of p-nitrobenzaldehyde is 1·39 ± 0·05. The Kinetic secondary α-deuterium isotope effect (KD/KH) for the addition of water to p-nitrobenzaldehyde is 1·18 ± 0·07. This result demonstrates that there is extensive carbon-oxygen bond formation in the transition state for addition of water to the carbonyl group of p-nitrobezaldehyde.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/poc.610070405
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070501
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  • 169
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    Mechanism of the hydride transfer reaction of leuco crystal violet with cyanomethylene acceptors (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 218-220 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the hydride transfer of Leuco Crystal Violet to form the Crystal Violet, cation, the role of cyanomethylene acceptors was found to be essentially different from that of p-benzoquinones, both previously believed to act as π-acceptors in the same manner.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070408
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  • 170
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    Competition between decarboxylation and isomerization in the C3H5O2+ energy surface. Justification of the experimental results by molecular orbital calculations on the solvated ions (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 221-226 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In constrast with recent molecular orbital calculations on the decarboxylation of O-protonated 2-oxetanone, this experimental work indicates that no decarboxylation of this cation occurs in sulphuric acid solution up to 150°C, but instead a clean isomerization to protonated acrylic acid takes place. Parallel theoretical work shows that the gas-phase model is too crude to account successfully for the experimental facts obtained in acidic media. However, the latter are well reproduced when the effect of the solvent is taken into account. The present findings do not necessarily invalidate the reaction mechanism currently accepted to explain the rate enhancement and change of stereochemistry accompanying the decarboxylation of 3,4-disubstituted 2-oxetanones under acid catalysis.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070502
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  • 171
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    Enolimine and geminaldiamine forms in the reaction of pyridoxal phosphate with ethylenediamine. An electrochemical and spectroscopic contribution (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 227-233 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The enol-keto tautomerism of the Schiff bases formed by pyridoxal 5'-phosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was studied by electrochemical and spectrophotometric methods. The spectroscopic results revealed differences between the two reactions, i.e. the enolimine/ketoenamine ratio observed in PLP-Etd mixture is higher than that of PLP-Et. The differences observed in the electroreduction mechanism and stability of the Schiff bases formed provide additional evidence for an unusually high concentration of enolimine in PLP-Etd in buffered aqueous solutions.The results are consistent with a cyclic structure such as that previously proposed by Robitaille et al. [J. Am. Chem. Soc. 111, 3034-3047 (1989)] on the basis of spectroscopic data. Protonation of the terminal amino group of the Etd moiety in the Schiff base involves the formation of the cyclic species. The low basicity of ethylenediamine favours the formation of similar concentrations of the enol and keto tautomers of the Schiff base at neutral pH. This behaviour, also observed in the Schiff base formed by PLP and polylysine, may be involved in coenzyme-protein linkages.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070503
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  • 172
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    Number and structure of solvolysis intermediates. Part 3.For Part 2, see Ref. 1.SN1 solvolysis of 2,2-Dimethyl-1-(p-Methoxyphenyl)Propyl p-Nitrobenzoate: Mechanism of the common ion salt effects arising at the stage of the second ion-pair intermediate (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 234-243 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2,2-Dimethyl-1-(p-methoxyphenyl)propyl p-nitrobenzoate (ROPNB) was subjected to solvolysis in phenol in the presence of tetrabutylammonium [carboxy-13C]-p-nitrobenzoate, in which the ‘common ion rate depression’ was confirmed to arise at the stage of the second ion-pair intermediate (Int-2). The unchanged substrate recovered at 46% reaction contained the isotopically labelled leaving group, indicating the occurrence of common ion exchange to the extent of 41-46%. In the solvolysis of the optically active substrate under identical conditions, the unchanged substrate was recovered with 51·4% racemization and ROPh was produced with slightly (1·56%) retained configuration, similarly to the solvolysis in the absence of the common ion salt. These isotope-tracer and stereochemical outcomes indicate that the common ion exchange in this solvolysis system should be attributable to the retentive nucleophilic attack on Int-2 by the common ion salt via a quadrupole (four-centre ion pair) transition state, accompanying the common ion rate depression, and they suggest that the special salt effect also should proceed by an analogous anion-exchange mechanism to that for the common ion effects.
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    Relative thermodynamic stabilities of the isomeric propenylbenzenes (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 256-258 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative thermodynamic stabilities of 2-propenylbenzene (allylbenzene), and the E and Z forms of 1-propenylbenzene were determined over the temperature range 50-170°C by chemical equilibration in DMSO solution with t-BuOK as catalyst. The values of the thermodynamic parameters ΔG⊖, ΔH⊖ and ΔS⊖ at 298·15 K for each isomerization reaction between the title compounds were evaluated.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/poc.610070507
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  • 174
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    Hydride transfer reactions of Michler's hydride with different π-accetors (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 309-315 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The hydride transfer reactions of 4,4′-bis(dimethylaminophenyl)methane (Michler's hydride) with p-benzoquinones were studied. The rate of formation of Michler's Hydrol Blue was followed spectrophotometrically. The second-order rate constants and the activation parameters were estimated. The formation of a charge-transfer complex was observed at low temperatures. Stable and unstable paramagnetic species formed during the reactions were assigned by ESR spectroscopy. The observed kinetic behaviour and the stoichiometry were in line with those previously obtained for the systems involving Leuco Crystal Violet, Leuco Malachite Green and Leuco Bindschedler's Green. Hence the reaction is considered to proceed according to the so-far accepted multi-step mechanism. However, when tetracyano-p-quinodimethane or tetracyanoethylene was used as a π-acceptor, a comparatively stable radical ion pair was formed as a result of a one-electron transfer, followed by the gradual formation of Michler's Hydrol Blue. The kinetic behaviour and the stoichiometry of the reaction were examined, together with the enhanced kinetic effects of added triethylamine. A modified mechanism for these systems is proposed. The role of cyanomethylenes was found to be essentially different from that of p-benzoquinones, and therefore π-acceptors are divided into two groups of cyanomethylenes and p-benzoquinones.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070607
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  • 175
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    Vinyl ether hydrolysis. 30. Effect of β-carboxy and β-carbomethoxy substitution (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 316-322 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of hydrolysis of the vinyl ether functional groups of (Z)- and (E)-β-methoxyacrylic acid and (Z)-and (E)-β-methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)-β-methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β-carboxy and β-carbomethoxy substituents produce strong rate retardations, ranging from 2000- to 25 000-fold, for both Z- and E-isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)-β-methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non-ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β-lactone formation as suggested in an earlier observation of the same phenomenon in a different system.
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    URL: http://dx.doi.org/10.1002/poc.610070608
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070701
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    Editorial (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. ii 
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    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070702
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    Newer aspects of electron transfer in electrophilic Aromatic Nitration. Radical pairs as reactive intermediates by time-resolved spectroscopy (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 325-351 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Aromatic nitration is effectively carried out by the deliberate irradiation of the charge-transfer absorption band (hvCT) of the transient electron donor-acceptor complex [ArH, NO2Y+], where NO2Y+ represents common nitrating agents. Time-resolved spectroscopy demonstrates that the charge-transfer nitration effected in this way proceeds via an electron-transfer mechanism, in which the collapse of the reactive triad, [ArH+ ·, NO2·, Y] is critical to the formation of the Wheland intermediate. Comparative product analyses of toluene and anisole nitrations together with the ‘non-conventional’ products arising from NO+ catalysis, ipso attack and addition/elimination show that the activation process leading to charge-transfer (photochemical) nitration is indistinguishable from that leading to electrophilic (thermal) aromatic nitration.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070703
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  • 179
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    Stereochemistry and kinetics of methoxide ion substitution in (Z)- and (E)-O-methylbenzohydroximoyl cyanides (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 352-358 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The stereochemistry and kinetics of the methoxide ion substitution reactions (Z)- and (E)-methylbenzohydroximoyl cyanidc [PhC(CN)=NOCH3] were investigated. The reaction of the (Z)-hydroximoyl cyanide with sodium methoxide in DMSO-methanol (9:1) solution at 44·8°C gives a mixture of methyl (Z)-O-methylbenzohydroximate [PhC(OCH3)=NOCH3] and the O-methyloxime of α-ketophenylacetamide PhC(CONH2)=NOCH3. The (E)-hydroximoyl cyanide undergoes methoxide ion-catalyzed isomerization to the E-isomer faster than it undergoes nucleophilic substitution. These observations were interpreted in terms of an addition-elimination mechanism in which the rate-limiting step is elimination of the nucleofuge (AN + DN#).
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070704
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  • 180
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    Kinetic studies on the aminolysis of 1-(trimethylsilyl)ethyl arenesulphonates in acetonitrile and methanol (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 359-363 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Nucleophilic substitution reactions of 1-(trimethylsilyl)ethyl arenesulphonates with anilines and benzylamines in acetonitrile and methanol at 65·0°C were studied. The cross-interaction constants, ρXZ, between substituents in the nucleophile (X) and leaving group (Z) are relatively small (0·10 for XC6H4NH2 in MeCN) but similar to those for other SN2 processes at a secondary carbon atom. This provides further evidence for an approximately constant, loose SN2 transition state at a secondary carbon regardless of the size of the Cα substituent. The transition-state variations with substituents X and Z are in accord with that expected from the positive ρXZ value observed: a stronger nucleophile and/or nucleofuge leads to an earlier transition state, i.e. a lower degree of bond making and breaking.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070705
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  • 181
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    Molecular mechanics calculations (MM3) on alkyl iodides (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 420-430 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 force field has been extended to cover alkyl iodides. Structures, vibrational spectra and heats of formation have been well fitted to available experimental data. The experimental and ab initio conformational relationships are also satisfactorily reproduced.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070806
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  • 182
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    Kinetics and mechanism of the imidazole-catalysed hydrolysis of substituted N-benzoylimidazoles (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 431-436 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Imidazole (Imz)-catalysed hydrolysis of benzoate esters proceeds via the intermediate formation of N-benzoylimidazoles. This paper considers the second step of this reaction, viz., Imz-catalysed hydrolysis of N-(4-X-benzoyl)imidazoles, X = CH3, H, Cl, CN and NO2, and N-(disubstituted benzoyl)imidazoles, 2-chloro-4-nitro, 2, 4-dinitro and 3,5-dinitro, in water-acetonitrile mixtures (10% or 14%, v/v, in organic solvent). On the basis of catalytic rate constants and the kinetic solvent isotope effect, it is shown that catalysis by Imz is of the general-base type. Unexpectedly, the hydrolysis of N-(2,4-dinitrobenzoyl)imidazole was found to be slower than that of N-(4-nitrobenzoyl)imidazole. It is shown that this reactivity order is due to a combination of a steric effect and stabilization of the reactant state due to a donor-acceptor interaction between the Imz moiety and the 2,4-dinitrophenyl ring.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070807
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  • 183
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    Micellar catalysis of organic reactions. Part 35. Kinetic determination of the critical micelle concentration of cationic micelles in the presence of additives (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 437-441 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The critical micelle concentration of solutions of cetyltrimethylammonium bromide and of tetradecyltrimethylammonium bromides were determined by a kinetic method. This involved the determination of the rates of the hydroxydehalogenation of some activated aromatic substrates over a wide range of detergent concentrations. Measurements were made in solutions containing significant quantities of added hydroxyl ion and substrates which were themselves amphiphilic. Conventional methods cannot be applied with confidence to such systems. The effects of changing hydroxyl ion concentrations, added sodium bromide, changing the nature of the aromatic substrate (whether neutral or charged), the identity of the micellar counterion and the temperature were investigated. It was wound that added bromide or hydroxyl ions resulted in a lower CMC whereas increased temperature led to an increase in the CMC. The nature of the micellar counterion (Br, F, OH, SO4) had little effect on the CMC. The presence of a charged aromatic substrate led to a considerable lowering of the CMC, whereas the neutral aromatic substrate used showed very little effect.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070808
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  • 184
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    Nucleophilic substitution reaction of benzyl bromide with N, N-Dimethylaniline: Significance of equilibrium cross-interaction constant (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 555-560 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Kinetic studies on the reversible reactions of benzyl bromides with N, N-dimethylanilines were carried out for both the forward (kf) and reverse (kr) directions. The equilibrium constants, K were calculated using the ratio kf/kr and the equilibrium cross-interaction constant, ρeXY was determined. The ρeXY value was shown to represent a maximum intensity of interactions between substituents X and Y through covalent bonds within a molecule. The normalized values of ρX (ρnuc) and ρXY indicate that in the transition state the fractional development or loss of polar and resonance interactions in the benzyl system are imbalanced or non-perfectly synchronized. In the forward reaction the fractional loss of resonance interaction becomes enhanced, whereas in the reverse reaction the fractional development of resonance interaction lags behind the corresponding changes of polar interactions by ca 45%.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071006
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  • 185
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    Competing solvolytic substitution and elimination reactions via a common irreversibly formed ion-molecule pair intermediate (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 578-584 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The acid-catalysed solvolysis of 9-(2-phenoxy-2-propyl)fluorene in mixtures of water with acetonitrile or methanol at 25°C provides 9-(2-hydroxy-2-propyl)fluorene, 9-(2-propenyl)fluorene, and 9-(2-acetamido-2-propyl)fluorene or 9-(2-methoxy-2-propyl)fluorene, respectively. The overall kinetic deuterium isotope effects for the reactions of the hexadeuterated analogue 9-(1,1,1,3,3,3-2H6)-9-(2-phenoxy-2-propyl)fluorene in 90 vol.% acetonitrile in water were measured as (kEH + kSH)/(kED6 + kSD6) = 1·54 ± 0·05, which is composed of the isotope effect kSH/kSD6 = 1·4 ± 0·1 for formation of the substitution products and kEH/kED6 = 4·0 ± 0·2 for production of 9-(2-methoxy-2-propyl)fluorene. Similar isotope effects were measured in other solvent mixtures. The results strongly indicate a branched mechanism involving rate-limiting formation of a common carbocation-molecule pair (with a secondary isotope effect of 1·54), which either undergoes nucleophilic attack by a solvent molecule (with a secondary isotope effect of ca 1), or is dehydronated (isotope effect ca 2·8) by the leaving group or by the solvent. The ion-molecule pair shows very low selectivities. Thus, in 50 vol.% acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion-molecule pair, kMeCN/kHOH ≥ 1 (ratio of second-order rate constants). The discrimination between methanol and water is anomalously small, kMeOH/kHOH = 0·7.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071009
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  • 186
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    Directional hydrogen bonding in the MM3 force field. I (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 591-609 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071103
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  • 187
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    Gas phase thermolysis of N-(tert-butylthio)allylamine and N-(tert-butylthio)diethylamine (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 585-590 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title sulphenamides were pyrolysed in a stirred-flow reactor at temperatures of 310-410°C, pressures of 8-15 Torr and residence times of 0·4-2 s using toluene as the carrier gas. N-(tert-Butylthio)allylamine formed 73 ± 4% isobutene, 23 ± 3% propene and N-allylthiohydroxylamine. The first-order rate coefficients for the formation of isobutene and propene, respectively, followed the Arrhenius equations kC4(s-1) = 1012·52 ± 0·36 exp(-163 ± 5 kJ mol-1 RT) and kC3(s-1) = 1010·99 ± 0·29 exp(-151 ± 4 kJ mol-1 RT) N-(tert-Butylthio)diethylamine gave 97 ± 1% isobutene, 1·9 ± 0·4% isobutane and N,N-diethylthiohydroxylamine. The first-order rate coefficients for isobutene elimination followed the Arrhenius equation k(s-1) = 1013·45 ± 0·24 exp(-164 ± 3 kJ mol-1 RT). The formation of the products is interpreted in terms of an elimination reaction with a unimolecular, four-centered, cyclic transition state. The reactivity of these sulphenamides was found to be much higher than that of previously studied alkyl or aryl tert-butyl sulphides and disulphides.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071102
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  • 188
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    Reactions of 1,3-dihaloadamantanes with diphenylphosphide ions by the SRN1 mechanism. Competition between intermolecular and intramolecular electron transfer reactions (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 610-614 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reactivity of 1,3-dihaloadamantanes with diphenylphosphide ions (Ph2P-) in liquid ammonia was studied. 1,3-Dichloroadamantane (1a), 1-bromo-3-chloroadamantane (1b) and 1,3-dibromoadamantane (1c) reacted with Ph2P- ions under photostimulation by the SRN1 mechanism. Irradiation of 1c without Ph2P- ions gave no reaction (〈5%). Three products were found: 1,3-bis(diphenyphosphinyl)adamantane (2), (3-X-1-adamantyl)diphenylphosphine (X = Cl, 3a; X = Br, 3b) and 1-adamantyldiphenylphosphine (4). Compounds 2 and 4 were formed by intramolecular electron transfer (ET) of the radical anion 3a-· (3b-·); whereas 3a (3b) were formed by intermolecular ET of this radical anion to the substrates. It was observed that the product distribution depends on the substrate and reaction conditions.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/poc.610071104
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  • 189
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    AB initio study of Diels-Alder reactions of 1,3,4-oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile and 1,1-dicyanoethylene. Inverse order and ratio of endo/exo reactivity (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 634-640 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition structures for the Diels-Alder reactions of 1,3,4-oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile, and 1,1-dicyanoethylene were located with ab initio molecular orbital calculations with the RHF/3-21G theoretical model. The activation energies were calculated by single-point calculations with the RHF/6-31G* and MP2/6-31G* theory levels on the RHF/3-21G geometries. Geometries of two transition structures and the corresponding reactants were generated at the MP2/6-31G* and the energies evaluated at the MP3/6-31G* level. The values obtained are comparable to those generated by MP2/6-31G*//3-21G. The asynchronicity of the transition structures follow the same trend as the corresponding all-carbon Diels-Alder reactions with buta-1,3-diene and cyclopentadiene, but the activation energies are almost in opposite correlation. This behavior is explained by the oxadiazole nitrogen lone pair repulsion with the π-orbitals of the nitrile groups.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071108
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  • 190
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071201
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  • 191
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    Effect of cyclic acetal-type cationic surfactants on the basic dehydrobromination reaction (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 646-651 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction of 2-(p-nitrophenyl)ethyl bromide with hydroxide ion was studied at 50 and 25°C in the presence of cationic chemodegradable surfactants, i.e. [2-alkyl(1,3-dioxolan-4-yl)methyl] trimethylammonium bromides Ia-c (alkyl: a = n-C9H19, b = n-C11H23, c = n-C13H27). The kinetic data were interpreted with the pseudo-phase ion-exchange (PIE) formalizm at both temperatures. The results indicate that the major source of rate enhancement is the increased reactant concentration in the small micellar reaction volume. The surfactant stability in micellar conditions was probed by means of a hydrolysis reaction of the surfactant 1,3-dioxolane ring. The kinetics of acid hydrolysis of Ia-c micelles, as a result of specific hydronium ion concentration, is one order of magnitude smaller than that of unaggregated systems.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071110
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  • 192
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    AB initio calculations of Diels-Alder transition structures for hetero-dienophile additions to cyclopentadiene (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 641-645 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Transition states for the hetero-Diels-Alder addition of formaldehyde, formaldimine, diazene and nitrosyl hydride to cyclopentadiene were located with ab initio molecular orbital calculations. Asynchronous transition structures were located for the asymmetric hetero-dienophiles, whereas the symmetrical hetero-dienophile, cis-diazene, produced a synchronous transition structure. The relative reactivities are in good agreement with the LUMO energies of the hetero-dienophiles, and the transition states with the exo oxygen or nitrogen lone pairs have lower energies than the corresponding endo lone electron pair transition states. Endo/exo energy differences are much higher than in the addition of ethylene derivatives to cyclopentadiene.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610071109
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  • 193
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    Professor Robert W. Taft (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 655-656 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071202
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  • 194
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    Equilibrium acidities and homolytic bond dissociation enthalpies of the acidic C—H bonds in dialkyl malonates and related compounds (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 751-756 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The equilibrium acidities, pKHA, of 18 dialkyl malonates, five alkyl 2-cyanoacetates and nine malononitriles and the oxidation potentials of their conjugate anions, EOX(A-), were measured in dimethyl sulfoxide solution. The homolytic bond dissociation enthalpies (BDEs) of their acidic C-H were estimated by combining their pKHA and EOX(A-) values. The pKHA values of the dialkyl malonates were found to increase from 15·9 to 16·4 to 18·4 as the dialkyl groups were changed from dimethyl to diethyl to di-tert-butyl, but the BDEs of the acidic C-H bonds remained constant [95·3±0·3 kcal mol-1] (1 kcal = 4·184 kJ). Introduction of methyl, ethyl, isopropyl and tert-butyl groups into the 2-position of diethyl malonate caused the equilibrium acidities to increase by 2·0, 2·4, 3·8 and 8·0 pKHA units, respectively, and the BDE values to decrease by 4·4, 3·7, 2·5 and 0·8 kcal mol-1, respectively. Introduction of a phenyl group into the 2-position of diethyl malonate had no effect on the acidity, but weakened the acidic C-H bond by 10 kcal mol-1. The effects on acidity and BDE of introducing 3,4,5- (MeO)3C6H2CO, CF3, Me3N+, c-C5H10N, p-MeC6H4, p-NO2C6H4, PhO, F and c-C5H5N+ groups into the 2-position of diethyl malonate were also examined.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071214
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  • 195
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    Study of the hydrogen bond donor acidity of binary aqueous mixtures and their role in reversed-phase liquid chromatography (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 757-769 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The solvatochromic hydrogen bond donor (HBD) acidity parameter (αmix) of aqueous mixtures of methanol, acetonitrile, propan-2-ol and tetrahydrofuran were determined spectrophotometrically. The study was carried out at 25deg;C as a function of composition. The indicators used were 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)phenolate, 2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)phenolate and Fe(LL)2(CN)2 (LL = N-(2-pyridylbenzylidene)-3,4-dimethylaniline). The HBD acidity of the aqueous organic mixtures was related to retention in reversed-phase liquid chromatography.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610071215
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  • 196
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    Masthead (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994) 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070101
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  • 197
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    Semi-empirical calculations on heterocumulene-heterodiene cycloaddition reactions: Ketene + 1-azabutadiene (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 1-8 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Semi-empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene-1-azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4-iminobenzylfuran-2,3-diones are presented. A [4 + 2] type of reaction of the ketene C=C double bond leading to a six-membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon-nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two-step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa-1,3-dienes are discussed.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/poc.610070102
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    N-alkylation and N-arylation of anilines starting from a mild N—Mg reagent: Its activation causing the ‘N=C’ coupling to extend the unified structure-reactivity relationshipAryliminodimagnesium reagents, part XXIV. For part XXIII, see ref. 3b. (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 9-17 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: New N-alkylation and N-arylation procedures starting from anilinomagnesium (ArNHMgBr) are reported. For N-alkylation with alkyl bromides, addition of hexamethylphosphoramide to an ArNHMgBr solution in tetrahydrofuran (THF) is effective. After heating at 55°C, N-monoalkylation product was obtained in 60-90% yield, slight dialkylation taking place. The combined use of aryliminodimagnesium [ArN(MgBr)2] with α,ω-dibromoalkanes led to N-arylazacycloalkanes. For N-arylation with iodobenzene, replacement of THF with pyridine and additional use of copper(I) iodide are effective. After heating at 115°C with iodobenzene, mono- and diarylation products were obtained, the former being predominant. The combined use of ArNHMgBr and N,N,N′,N′-tetramethylethylenediamine as the ligand of copper species is effective for elimination of diarylation and other undesired products, and leads to diarylamines in excellent yield. The method is of advantage over the conventional Ullmann and Chapman methods. The polar solvents and copper salt are effective additives for inducing ‘inert combinations’ of ArNHMgBr or ArN(MgBr)2 with alkyl and aryl halides into N—C coupling, to extend the unified view proposed for the reactivity of magnesium reagents. The difference in the roles of N—Mg and N—Cu species is discussed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070103
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  • 199
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    Exploratory photochemistry of alkylbromo- and alkylfluorodiazirines. Excited-state hydrogen migration and carbene formation (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 24-27 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Laser flash photolysis of a series of alklylbromo- and alkylfluorodiazirines in pentane at ambient temperature generates alkylhalocarbenes by decomposition of the diazirine excited states. The halocarbenes can be intercepted with pyridine to form ylides. The ylides absorb intensely between 350 and 400 nm and are fairly long lived (τ » 10 μs), making them convenient probes of the yield and dynamics of the carbene. The yield of the ylides increases with increasing pyridine concentration up to 1·5 M. At pyridine concentrations 〉 1·5 M the yield of ylide is saturated, signifying that every carbene generated in a laser pulse is captured by pyridine prior to reaction with solvent or intramolecular rearrangement. The yield of trappable carbene generated from alkylbromodiazirines closely tracks the bond dissociation energy of the C—H bond adjacent to the diazirine moiety. The data indicate that the excited states of the alkylbromodiazirines suffer C—H migration (or C—C migration with cyclobutylbromodiazirine) and nitrogen extrusion in competition with carbene formation. The yield of trappable carbene derived from the alkylfluorodiazirines is independent of the bond dissociation energy of the adjacent C—H bond. This is probably a consequence of the great thermodynamic stability of α-fluorocarbenes.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070105
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  • 200
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    Low temperature enol-enol association of stable 2,2-diarylethenolsstable simple enols, part 34. For part 33, see ref. 1. (1994)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 28-30 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The low temperature 1H NMR spectra of 2,2-bis(3,5-dibromomesityl)ethenol in CS2-CD2Cl2 (3:7) display new signals which indicate the presence of four enol species whose OH is hydrogen bonded. Oligomerization to intermolecularly hydrogen-bonded enol dimers or tetramers is suggested.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610070106
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