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  • 1985-1989  (1,095)
  • 1965-1969
  • 1986  (1,095)
  • Organic Chemistry  (570)
  • Computational Chemistry and Molecular Modeling  (411)
  • Physical Chemistry  (114)
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Years
  • 1985-1989  (1,095)
  • 1965-1969
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  • 101
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 1351-1364 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED. A detailed analysis of the electronic factors involved in this process is presented.
    Additional Material: 4 Ill.
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  • 102
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    International Journal of Quantum Chemistry 29 (1986), S. 1177-1180 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preferred conformations of the thyrotropin-releasing hormone (TRH) have been calculated by the global optimization method proposed earlier by us. [G. Subba Rao, R. S. Tyagi, and R. K. Mishra, J. Theor. Biol. 90, 377 (1981)]. The potential function used comprises the electrostatic, nonbonded, torsional and hydrogen-bonding terms. The results are in good agreement with the crystal structures of TRH. No intramolecular hydrogen bonding is found to occur.
    Additional Material: 1 Ill.
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  • 103
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    International Journal of Quantum Chemistry 29 (1986), S. 1373-1382 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several Fe(H2O)n2+ clusters, with n up to 20, have been studied, both by energy minimization in the pairwise approximation and by Monte Carlo simulation. In the last case the calculations have been carried out at three different temperatures in order to investigate the effect of thermal agitation. The most interesting result which can be deduced from this work is the existence of eight water molecules in the first hydration shell of the iron (II) ion. A microscopic analysis has shown that the minimum energy structure of the Fe(H2O)82+ cluster presents a D4d symmetry. This structure is slightly distorted as far as the temperature is increased. The validity of these theoretical predictions is discussed.
    Additional Material: 3 Ill.
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  • 104
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    International Journal of Quantum Chemistry 29 (1986), S. 1457-1462 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The appearance of the maximum linewidth by the vibrational predissociation of I2He van der Waals (vdW) molecule as a function of the initial vibrational quantum number of I2 is predicted by the dynamic perturbation theory. The kinetic energy perturbation is introduced in addition to perturbation potential. The linewidth agrees quite well with experiment.
    Additional Material: 1 Ill.
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  • 105
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    International Journal of Quantum Chemistry 29 (1986), S. 1561-1573 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aim of our work was to find an unambiguous connection between irreversible macroscopic dynamics and reversible microdynamics that makes it possible to manifest irreversibility on a submacroscopic level without the use of coarse graining arguments. On this level the state of a physical system can be described by a field amplitude Ψ and the time evolution of this system is determined by a field equation for Ψ. For conservative systems, this field equation is formally identical with the linear Schrödinger equation, which can be constructed with the help of the classic Hamiltonian function for the corresponding problem. Regarding irreversible phenomena like damping due to a frictional force, for those dissipative systems no classic Hamiltonian function exists. Therefore the corresponding field equation cannot be obtained in the usual way. Nevertheless, also for dissipative systems it is possible to obtain a field equation in an unambiguous way using only Newton's form of classic mechanics. The result of our method is a nonlinear Schrödinger-type field equation with a logarithmic nonlinearity. We discuss in more detail the properties of our logarithmic nonlinearity that corresponds to a macroscopic frictional force in a unique way. A figurative interpretation in terms of environment and interaction can be given. From a more formal point of view, the compatibility of our nonlinear operator with principles known from the theory of linear operators is investigated. One of the surprising results is the fact that although our nonlinear Hamiltonian HNL is “Hermitean” in the usual sense, in contrast to the linear theory an operator exp(i · HNL) is not unitary. Furthermore, in our theory the time-derivative of the mean value of an operator is not only essentially determined by (the mean value of) its commutator with the Hamiltonian. There also occurs an additional term that causes irreversibility of the evolution and is connected with the feature of our theory that (in general) time derivative and construction of the mean value are no longer commuting operations. This fact shows some similarity with coarse graining theories, but in our theory the reason can be traced back unambiguously to an irreversible physical phenomenon.
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  • 106
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    International Journal of Quantum Chemistry 29 (1986), S. 1651-1683 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A brief general survey of the current state of the art of quantum chemistry is given with some aspects also towards the future. It is emphasized that, if one wants to incorporate such concepts as temperature, entropy, and free energy into quantum chemistry, it is necessary to make a transition from pure quantum mechanics based on wavefunctions to the more general “quantum statistics” based on density matrices and system operators. In addition to the Schrödinger equations, one obtains the Liouville equations, and it is shown that both the time-dependent and the time-independent equations may be solved in both cases by using analogous Hilbert-space methods. Some of the methods for solving the time-independent eigenvalue problems are reviewed, and the need for giant “number crunchers” in this connection are discussed. It is shown that the resolvent methods combined with the “inner projection” technique for the calculations provide a powerful tool for handling the eigenvalue problems in the future in both the Hamiltonian and Liouvillian formalisms. It is stressed that, by going over to supercomputers, one may gain a factor of 100, and that one may gain another factor of 100 by going over to more powerful theoretical methods; however, for programming reasons, it will take a long time before one can reach the combined efficiency factor 100 ✗ 100.
    Additional Material: 2 Ill.
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  • 107
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    International Journal of Quantum Chemistry 29 (1986), S. 1775-1778 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Let f1, ⃛, fn be a basis of unit vectors and g1, ⃛, gn an orthonormal basis in a Hilbert space. We consider and solve the problem of finding an orthonormal basis e1, ⃛, en such that a weighted average of the distance of ej from fj and gj is minimized in the sense of least squares.
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  • 108
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    International Journal of Quantum Chemistry 29 (1986), S. 1815-1824 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A theory of electron transfer at low temperatures has been developed. The mechanism of electron transfer consists in the fact that donor ionization and electron capture by the acceptor take place in a tunnel manner.
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  • 109
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    International Journal of Quantum Chemistry 29 (1986), S. 1839-1839 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 110
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    International Journal of Quantum Chemistry 29 (1986), S. 1841-1841 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 111
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    International Journal of Quantum Chemistry 30 (1986), S. 213-224 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction pathway of fulminic acid (HCNO) and acetonitrile oxide (CH3CNO) with methanol as a nucleophile (RCNO + CH3OH → RC(OCH3)=NOH) and the formation of H-bonded complex with methanol have been studied using the MNDO method. MNDO-SCF calculations were performed with complete geometry optimization using the Davidon-Fletcher-Powell method. The reaction pathways were studied by varying all the bond lengths, the bond angles and the twist angles, using the distance C3—O2(R) between the carbon of the 1,3-dipoles and the oxygen of the methanol molecule as the reaction coordinate. The reaction is exothermic and proceeds in two steps. The first step is the formation of a five-centered hydrogen-bonded complex (INT) and is the rate-determining step of the reaction. The second step involves the rearrangement of the H-bonded complex to the product, and this step requires a very small amount of activation energy. Thus, there is an intermediate on the reaction pathway, and therefore, the reaction is stepwise. Acetonitrile oxide is less reactive (activation energy 34.59 kcal/mol) relative to fulminic acid (activation energy 28.91 kcal/mol).
    Additional Material: 4 Ill.
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  • 112
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    International Journal of Quantum Chemistry 29 (1986), S. 1843-1844 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 113
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    International Journal of Quantum Chemistry 30 (1986), S. 433-436 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this letter we show that the calculation of analytic second derivatives of variational potential energy surfaces with respect to nuclear coordinates is a minimization problem.
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  • 114
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    International Journal of Quantum Chemistry 30 (1986), S. 437-437 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 115
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 116
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    International Journal of Quantum Chemistry 30 (1986), S. 479-494 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient scheme for calculating one- and two-electron transition density matrices for two wave functions is described. The method applies to CAS (complete active space) wave functions and certain multireference CI expansions. The orbital sets of the two wave functions are not assumed to be equal. They are transformed to a biorthonormal basis, and the corresponding transformation of the CI coefficients is carried out directly, using the one-electron coupling coefficients.
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  • 117
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    International Journal of Quantum Chemistry 30 (1986), S. 567-569 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 2 Ill.
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  • 118
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 119
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    International Journal of Quantum Chemistry 30 (1986), S. 591-615 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Butadiene and benzene SCF ground states are analyzed using the bond orbital resonance theory (BORT) approach. The results obtained are compared with the analogous description of these states within the VB approach. It is shown that the structural weights of BORT Kekulé structures are much larger than the structural weights of the corresponding VB Kekulé structures. For example, in the butadiene case the structural weight of the BORT Kekulé structure is over 0.941, while the structural weight of the VB Kekulé structure is only 0.221. In addition, in the BORT approach one has to consider a much smaller number of resonance structures than in the VB approach. This suggests that the description of conjugated systems should be more successful in the BORT than in the VB basis, and that formal Kekulé structures should be interpreted in the BORT and not in the VB sense.
    Additional Material: 6 Ill.
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  • 120
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    International Journal of Quantum Chemistry 30 (1986), S. 647-661 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of the C2nH2n+2 trans-polyenes, n = 3-7, is calculated by the Discrete Variational Xα method (DVM-Xα). The valence ionization potentials (IP) calculated using the Clementi double zeta basis agree with the known experimental data within several tenths an electron volt. However, the DVM energies of the π → π* optical excitations are systematically underestimated by 0.8-1.0 eV. For polyenes with equal C - C bond lengths, the computed energies of the first optical transitions are smaller than those of polyenes with alternating C - C bond lengths. The charge distribution in polyenes is analyzed in the framework of a Mulliken scheme. The composition of the frontier molecular orbitals (MO) is analyzed.
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  • 121
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    International Journal of Quantum Chemistry 30 (1986), S. 695-711 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction energy of several conformations of the nitromethane dimer is investigated at the SCF level. The dispersion energy and counterpoise correction are computed for certain relative orientations of the monomers. Fourth-order many body perturbation theory SDQ-MBPT(4) energies are reported for selected points. Double zeta and double zeta plus polarization basis sets were used. All calculations were done with the monomer fixed at the isolated monomer geometry. Interaction energies as large as 6 kcal/mol are found at minima of hydrogen bonding orientations.
    Additional Material: 11 Ill.
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  • 122
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    International Journal of Quantum Chemistry 30 (1986), S. 751-762 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio calculations using a 4-21G basis set are reported for various possible metabolites of N,N'-dimethylnitrosamine. The relevance of these results to the nature of the alkylating agent is discussed. Although the calculations widen the range of possible alkylating agents that need to be considered, the diazonium ion appears to be the most likely candidate.
    Additional Material: 2 Ill.
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  • 123
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    International Journal of Quantum Chemistry 30 (1986), S. 783-790 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The matrix elements of the spin-free Hamiltonian between two atomic configuration state functions (CSF'S) in the L-S coupling scheme are expressed in terms of the atomic integrals Fk's and Gk's. Using these general expressions, the matrix elements have been obtained for all the atomic configurations with three valence electrons that have not been solved so far by earlier methods. The scope for applying this new approach to obtain the Auger line energies and the promotion energies of metals that involve more than two partially filled shells is indicated. The energy expressions for some of the relevant configurations are tabulated.
    Additional Material: 5 Tab.
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  • 124
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    International Journal of Quantum Chemistry 30 (1986), S. 821-830 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total (elastic plus inelastic) intensities of 51 keV electrons scattered by water molecules have been measured over a range of 1 ≦ K = (4π/λ) sin(θ/2) ≦ 12 Å-1. A computer program, ELIC, has been written for calculating the total intensities of electrons scattered by free molecules. The intensities can be calculated with self-consistent field and configuration interaction wavefunctions. The theoretical intensities based on a CI wavefunction are in good agreement with the observed intensities.
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  • 125
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    International Journal of Quantum Chemistry 30 (1986), S. 869-869 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 126
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 127
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    International Journal of Quantum Chemistry 30 (1986), S. 13-19 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 128
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    International Journal of Quantum Chemistry 30 (1986), S. 57-63 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Solutions to the many-dimensional analogue of the Schrödinger equation for the hydrogen atom are used as a starting point for solving the many-particle Schrödinger equation for systems with Coulomb interactions. It is shown that zeroth-order solutions can be improved by means of perturbation theory, which is simplified by means of a sum rule.
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  • 129
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    International Journal of Quantum Chemistry 30 (1986), S. 275-288 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The pseudo-polarization tensor mutually consistent field (PPT-MCF) method recently introduced [1] has been applied to study the stacking interactions between the nucleotide bases in large periodic B-DNA fragments. The effects on the global and local binding properties caused by replacing one base in the periodic sequence by another base are investigated.The increase in the stability for comparable fragments owing to this base substitution is further enforced in the case of periodic alternating helices. The most important results are that the stacking interaction between two bases is slowly converging with the interbase distance and that the average contribution per base to the binding energy is repulsive.Furthermore, the energetical properties of double helix models in B- and Z-DNA configurations, respectively, consisting of up to five base pairs have been compared. It turns out that the G C G C sequence in Z-DNA is significantly more stable than either in periodic or periodic alternating B-DNA. In these cases the average energy contribution of a single Watson-Crick-type base pair is predicted also to be positive. From the calculations it follows that the double helix is not stabilized owing to the hydrogen bonding between the bases belonging to both strands, in contradiction to most other investigations.
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  • 130
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 131
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    International Journal of Quantum Chemistry 30 (1986), S. 663-670 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An intermolecular potential for the Fe(II)-H2O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI-2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H2O)n2+ clusters and to Monte Carlo simulations are discussed.
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  • 132
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    International Journal of Quantum Chemistry 30 (1986), S. 713-713 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 133
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    International Journal of Quantum Chemistry 30 (1986), S. 717-735 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calculations of a wide range of position and momentum space properties of 23 orbital basis sets for atomic helium are performed to supplement our previous work. The combined results on 94 orbital basis sets are used to demonstrate that, as in the case of d-orbitals, the least squares error is not as discriminating as other information theoretic measures of overall quality for s-orbitals. Detailed comparisons suggest that constrained variation can improve the overall quality of the resulting orbital provided that the constraints are selected carefully to complement the bias inherent in the energy expectation value and the functional form of the basis functions.
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  • 134
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    International Journal of Quantum Chemistry 30 (1986), S. 769-782 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The degeneracy lifting in the pi bands, near the Fermi energy, for two- and three-dimensional graphites and boron nitride are qualitatively determined, without calculation, using first-order perturbation theory and the concept of transferability. The relationships between the band diagrams for several three-dimensional modifications of graphite and boron nitride are presented in a simple orbital context.
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  • 135
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    International Journal of Quantum Chemistry 30 (1986), S. 809-819 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A transformation decoupling the Dirac second-order equations for the large and small components of the radial atomic wavefunction has been derived. The decoupling procedure is exact for one-electron atoms and approximate, though very accurate, for a general case of an atomic spherical potential. Its connection with the quasirelativistic theory of atoms has been discussed. The transformation has been applied to deriving an optimized quasirelativistic equation and to establishing relations between the quasirelativistic and the Dirac wavefunctions. Formulas resulting from these considerations allow improvement of quasirelativistic expectation values of operators so that the error, relative to the Dirac values, is reduced by a factor of 10.
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  • 136
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    International Journal of Quantum Chemistry 30 (1986), S. 853-865 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comparative dynamical analysis of several quasi-exact wavefunctions for helium has been carried out by computing their corresponding correlation coefficients. The aim is to use the resulting information to study the correlation included in approximate wavefunctions within the factor correlation model. Information about the possibility of making simplifications in the approximate wavefunctions and about the flexibility of the quasi-exact wavefunctions is also obtained.
    Additional Material: 6 Ill.
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  • 137
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    International Journal of Quantum Chemistry 30 (1986), S. 737-750 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A graph theoretic approach to the representation of N particle systems involving arbitrary single-particle spin is presented. The method is a generalization of the Distinct Row Table (DRT) technique employed by Shavitt in developing the graphical unitary group approach (GUGA) to electronic spin-orbitals. A detailed analysis of the DRT and GUGA representations is presented based on several theoretical considerations and computer-tested implementations. The use of the representation to establish rules concerning the evaluation of the matrix elements of the group generators is also discussed.
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  • 138
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    International Journal of Quantum Chemistry 30 (1986), S. 845-852 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Making use of the local pair distribution function, h(r12+;r1, α), we have carried out a detailed study of the local mean values of r12n obtained from a CI wavefunction and from its different possible partitions for the helium isoelectronic series. Through comparison of these results with those provided by Hartree-Fock and Hylleraas-type explicitly correlated wavefunctions, we reached several conclusions concerning the relative importance of different partitions of the CI wavefunction in order to get a proper description of local correlation effects.
    Additional Material: 3 Tab.
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  • 139
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    International Journal of Quantum Chemistry 30 (1986), S. 867-867 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 140
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    International Journal of Quantum Chemistry 30 (1986), S. 1-2 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 141
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    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Lipkin's method is used to rigorously transpose field-theoretic, self-consistent descriptions of Cooper pairs and Bose condensation of Lewis structures and resonance. The effective one-body equations can be simplified according to models of valence. The resultant self-consistent problem may require not much more work than the Hartree-Fock problem. The effort required for MBPT remains comparable to the Hartree-Fock case.
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  • 142
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    International Journal of Quantum Chemistry 30 (1986), S. 119-127 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 143
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    International Journal of Quantum Chemistry 30 (1986), S. 153-171 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general computationally amenable chemico-graph-theoretic cluster expansion method is suggested as a paradigm for incorporation of chemical structure concepts in a systematic manner. The cluster expansion approach is presented in a formalism general enough to cover a variety of empirical, semiempirical, and even ab initio applications. Formally such approaches for the utilization of chemical structure-related concepts may be viewed as discrete analogues of Taylor series expansions. The efficacy of the chemical structure concepts then is simply bound up in the rate of convergence of the cluster expansions. In many empirical applications, e.g., boiling points, chromatographic separation coefficients, and biological activities, this rate of convergence has been observed to be quite rapid. More note will be made here of quantum chemical applications. Relations to questions concerning size extensivity of energies and size consistency of wave functions are addressed.
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  • 144
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    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A graph representation of terms in the gradient expansion of the kinetic energy density functional is presented. We briefly discuss the implications of the virial theorem for the graph structure and relations between possible graphs at a given order of expansion.
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  • 145
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    International Journal of Quantum Chemistry 30 (1986), S. 271-276 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Lieb and Thirring derived a lower bound to the electronic kinetic energy of an N particle system. Using an energy-density functional approach, and the Ne atom as an example, the present work investigates the tightness of the bound both in its original form and in its modified forms.
    Additional Material: 3 Tab.
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  • 146
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    International Journal of Quantum Chemistry 30 (1986), S. 313-323 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have developed a new version of the variational cellular method for the calculation of the electronic structure of polyatomic systems. It is a very attractive method for large systems, such as solids and large molecules, and capable of giving calculated results with a degree of precision at least as good as those obtained by Hartree-Fock calculations. We are now improving the method by the introduction of crystalline boundary conditions with the intention to apply it to the study of impurities in semiconductors. The results for the ZnS clusters simulating the pure crystal show that the method leads to a fairly consistent description of the bulk electronic structure.
    Additional Material: 6 Ill.
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  • 147
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    International Journal of Quantum Chemistry 30 (1986), S. 353-366 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-consistent local density theory is used to calculate the electronic structure associated with impurities and defects in transition metals and their oxides. An embedded molecular cluster scheme is used in which 15-30 atoms are treated explicitly by the LCAO discrete variational scheme, and the surrounding environment is represented by a potential field. One-electron spectroscopic properties are discussed in terms of densities of states and population analyses; emphasis is placed upon detectable features induced by the presence of defects or impurities. Hyperfine fields and local magnetic moments are examined for binary transition metal alloys, and the effects of local clustering are simulated for comparison with Mössbauer, NMR, and neutron magnetic scattering data. The influence of interstitials and vacancies on metal X-ray absorption near edge spectra (XANES) of the monoxides is evaluated and used to interpret features of the M1-xO K edge spectra. The energy of formation of isolated cation vacancies in Fe1-xO is calculated, and the binding energies of several plausible (m:n) vacancy-interstitial metal defect clusters are presented. We give some semiquantitative explanations for the relative stability of different defect configurations.
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  • 148
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    International Journal of Quantum Chemistry 30 (1986), S. 409-435 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, a method of generating separable forms of the wave-operator for incomplete model spaces is discussed. With a time-dependent access to the many-body perturbation and coupled-cluster theories, it is shown how one can extract the regular part of the wave-operator which consists of linked cluster-operators only in the adiabatic limit. The procedure naturally suggests a hierarchy of lower valence model spaces P(k). once a particular m-valence incomplete model space P(m) is specified. The wave-operator Ω and the effective Hamiltonian Heff are linked in this development and are valence-universal in the sense of being valid for all P(k)' s. 0 ≤ k ≤ m. We have derived two distinct forms for Ω: (i) Ω = {exp(S)}, with { } as normal order with respect to suitable vacuum, where S are open operators inducing transitions from P(m) to outside it; (ii) ΩN = {exp(S + X)}, where X are additional closed operators which are introduced to maintain isometry of ΩN: P(k)ΩN + ΩN P(k) = P(k). In neither of these choices do we have intermediate normalization. It is also possible to develop an alternative strategy with the complete model spaces, such that an effective valence-universal operator H may be found which generates roots, only a subset of which are equal to the eigenvalues of H. These subsets are the ones that Heff would have furnished. This may thus be viewed as a Fock-space realization of the intermediate Hamiltonian approach.
    Additional Material: 3 Ill.
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  • 149
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    International Journal of Quantum Chemistry 30 (1986), S. 455-469 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rates of intermolecular-to-intramolecular energy transfer and chemical reaction were determined by quantum mechanical calculations on systems of strongly interacting molecules at high shock pressure. The energy transfer rate was estimated by utilizing a theory of radiationless transitions which describes the coupling of a sparse set of internal mode energy levels with a quasicontinuum of intermolecular mode energy levels. An important quantity in this theory is the time tpv required for activation of the internal modes. Calculated values of tpv 〈 1 μsec are identified with the most efficient transfer of energy when the molecules interact and react bimolecularly in the head-to-tail configuration. These times are consistent with those found in shock experiments and those calculated by other techniques. A comparison of tpv with chemical reaction half-lives indicates that energy transfer and reaction alternate as rate-determining steps over large ranges of P and T.
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  • 150
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    International Journal of Quantum Chemistry 30 (1986), S. 529-540 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extensional elastic moduli have been calculated for several organic high polymers based on the modified neglect of diatomic overlap (MNDO) Hamiltonian. Standard semiempirical methods, by application of the Born-von Karman boundary conditions, can be used to calculate heats of formation of polymer chain sections, or computational unit cells, called clusters. Unit cell heats of formation at elongated translation vectors, combined with experimental or estimated densities, allow for the calculation of elastic moduli. Two potential sources of error were identified: (a) finite geometry optimization can result in pseudorandom errors in the calculated heat of formation, and (b) anharmonic distortion can become significant at large strains. Errors due to both causes can typically be minimized if strains between 3 and 10% are selected. As expected, the calculated moduli, although higher than those observed experimentally, agree with longitudinal values for perfectly oriented systems.
    Additional Material: 4 Ill.
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  • 151
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    International Journal of Quantum Chemistry 30 (1986), S. 563-572 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bonding in both CuOH and AgOH has a covalent component that leads to a bent structure. The larger electrostatic stabilization in CuOH leads to a larger De (2.83 eV) compared with AgOH (2.20 eV). Using Ni5OH to model chemisorption of OH on a Ni surface, we find that OH adsorption in the fourfold hollow of Ni(100) leads to an OH normal to the surface, while adsorption directly above a Ni atom leads to a tilted OH. These qualitative Ni5OH calculations allow us to speculate on the observed variation of OH on metal surfaces.
    Additional Material: 3 Ill.
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  • 152
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    International Journal of Quantum Chemistry 30 (1986), S. 613-625 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We review the progress which was made during the last 6 years in the attempts to use electron diffraction data to gain insights into the electronic charge densities of isolated molecules and their moments. In regard to theory, we investigated the sensitivity of the elastic differential high-energy cross sections to the basis sets in the wave function calculations. Extensive calculations on H2O showed that there is no significant change by the molecular vibrations on the static charge density deformation maps. Finally new relations could be derived which allow the determination of nuclear and electronic moments from the differential cross sections at small angles. In regard to experimental data, new sets of purely elastic cross sections are available for He, Ne, CO2, N2, and CH4, which are compared with Hartree-Fock and limited configuration interaction wave function results. The differences are discussed. New experimental values for some moments for N2 and CH4 are reported.
    Additional Material: 13 Ill.
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  • 153
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    International Journal of Quantum Chemistry 30 (1986), S. 445-453 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An open-shell coupled-cluster method for the direct calculation of excitation energies is presented and applied to excitations of atomic Mg and Ar. The so-called CCSD + T approximation involves the exact inclusion of T1 and T2 operators and the lowest-order inclusion of T3. Quasi-complete model spaces, built from determinants with one valence hole and one valence particle, are used. Two ionization potentials and 20 excitation energies are calculated. Good agreement with experiment (˜0.2 eV) is obtained. The T3 effects are less important than for the Be and Ne atoms.
    Additional Material: 1 Ill.
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  • 154
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    International Journal of Quantum Chemistry 30 (1986), S. 507-520 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The state-of-the-art in non-empirical calculations of potential energy surfaces for small molecules is discussed, as is the present position with respect to the analytic fitting of such surfaces. The results of some nonempirical vibration-rotation calculations performed on such analytic surfaces are presented and compared with experimental results. An attempt is made to assess the extent to which present methods of electronic structure calculation and present analytic fitting methods are adequate to produce surfaces for the interpretation of high-resolution spectral data.
    Additional Material: 3 Tab.
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  • 155
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    International Journal of Quantum Chemistry 30 (1986), S. 521-527 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We examine the effects of macroscopic solvation using the self-consistent reaction field technique. On the basis of ab initio, INDO SCF, and INDO/CI calculations we conclude that the introduction of a dielectric media seems to have little effect on calculated geometries. However, the charge distribution is effected: the presence of dielectric media allows greater local charge to develop. The dissociation of hydrogen fluoride is examined. With electron correlation, HF dissociates homolytically in the “gas” phase, but heterolytically in a “solvent” of dielectric constant of 80.
    Additional Material: 3 Ill.
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  • 156
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    International Journal of Quantum Chemistry 30 (1986), S. 547-554 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using a comprehensive set of electron cross sections, we have carried out a Monte Carlo study of the spatial and energetic aspects of electron degradation in the energy range from 30 eV to 1 MeV. The results are given in terms of a four-dimensional spatial yield spectral distribution. This is represented by a simple approximate analytic form which is convenient to use for calculating the spatial distribution of the initial species created due to electron energy degradation. We also investigate the sensitivity of these electron transport results to some of the input cross sections. We have specifically examined the influence of the assumed form of cross sections for vibrational states on the gross yields. We conclude that in transport calculations it is important to use approximate forms with reasonable asymptotic limits. Finally, we call attention to molecular processes which are important in microdosimetry but whose cross sections are particularly uncertain.
    Additional Material: 2 Ill.
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  • 157
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    International Journal of Quantum Chemistry 30 (1986), S. 743-748 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some basic features of the method of complex scaling are briefly reviewed. This method is associated with a similarity transformation H = UHU-1, in which the many-particle Hamiltonian H loses its self-adjoint character. In connection with the eigenvalue problems for H and H, one has formally the relations ψ = Uψ and E = E. However, since the proper boundary conditions have to be satisfied, the spectrum {E} may still be subject to change: even if some eigenvalues are persistent (E = E), others may be lost, and new eigenvalues may occur also in the complex plane. It is pointed out that these changes are related to the fact that the “dilatation operator” U is an unbounded operator, and that the eigenfunctions involved are transformed not only within the ordinary L2 Hilbert space, but also out of and into this space. Reference to a more complete treatment is given.
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  • 158
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    Helvetica Chimica Acta 69 (1986), S. 85-90 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Macrocyclic Benzolactones by Ring Enlargement of 2-NitrocycloalkanonesReaction of 2-nitroalkanones with 1,4-benzoquinone in the presence of 1,8-diazabicyclo[5.4.0]undec-7-en gave (hydroxybenzo)nitrolactones of general formula 6 (Scheme 1). The transformation involves a Michael reaction, aromatization, and ring enlargement via a five-membered intermediate. The (hydroxybenzo)nitrolactones were converted to the already known (acetoxybenzo)oxolactones 15-17. The characteristic mass spectral behavior of the ring-enlarged products is discussed.
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  • 159
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    Helvetica Chimica Acta 69 (1986), S. 136-140 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 15N-NMR spectra of cis- and trans-2,3-dialkyl-substituted triaziridines are reported. The assignment of the 15N-NMR resonances is described, and the chemical-shift and coupling data are compared with those of aziridines and diaziridines. The 1J(N,H) coupling constants allow one to deduce the relative configuration of the involved N-atom. In CDCl3, a 1J(N,H) value of 58.1 ± 0.5 Hz is assigned to an N—H bond cis to two lone-pairs of electrons at neighboring N-atoms, i.e. to the cis,trans-triaziridinesThe first stereodescriptor denotes the relative configuration of the two equal alkyl substituents, the second the relative configuration of one alkyl substituent and the H-atom., and a value of 51.7 ± 0.5 Hz is assigned to a N—H bond cis to only one lone-pair of electrons, i.e. to trans, cis-triaziridines.
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  • 160
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    Helvetica Chimica Acta 69 (1986), S. 156-162 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A Novel Ester Iridoid Glucoside from Sambucus ebulus L. (Caprifoliaceae)A novel ester iridoid glucoside, ebuloside (1,), which shows structure elements of the rare 7-Dehydrologanin derivatives combined with those of the Valerianaceae iridoids, was isolated from the roots of Sambucus ebulus L. Its structure was determined by MS, NMR, and CD spectroscopy as 7-oxo-8-deoxyvalerosidatum.
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  • 161
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    Helvetica Chimica Acta 69 (1986), S. 152-155 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions carried out on benzoyl (2) and hydroxybenzyl (10) derivatives of L-phenylalanine-cycloanhydride 1 lead to the conclusion that the structure 3, previously proposed for picroroccellin (a metabolite from the lichen Roccella fuciformis), should be revised to 6, While the FeCl3-catalyzed photo-oxidation of 1 gives 2, the N,N′-dimethyl derivative 7 leads to N,N′-dimethyl-2-benzylpiperazine-3,5,6-trione (9). N,N′ -diacetyl-L-phenylalanine-cycloanhydride 11 remains unchanged. It is established that N,N′ -substitutions of 2,5-dibenzylpiperazines-3,6-diones considerably influence their photo-oxidations under the reported conditions.
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  • 162
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    Helvetica Chimica Acta 69 (1986), S. 184-194 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that 2-allylated anilines (cf. Schemes 2-4, 7, and 8) on irradiation in protic solvents such as H2O. MeOH, and EtOH in the presence of H2SO4 undergo a novel photosolvolysis reaction to yield specifically trans-2-hydroxy- and trans-2-alkoxy-1-methylindanes. Intermediates are presumably tricyclo[4.3.0.01,8]nona-2,4-dienes formed in an intramolecular [2s + 2s] cycloaddition reaction (cf. Scheme 7). On the other hand, N,N,N-trimethyl-2-(1′-methylallyl)anilinium salts 18 (Scheme 6) and 2-(3′-butenyl)-N,N-dimethylaniline (17) lose on irradiation in MeOH or H2SO4/MeOH the ammonium group reductively to yield (1-methylallyl)benzene (19) and 1-methylindane (20), respectively.
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  • 163
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    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Polyacetylated 5,6,7,8-Tetrahydro-D- and L-neopterins. A Special Case of N(5)-Alkylation of 5,6,7,8-TetrahydroneopterinsImproved conditions are reported for the preparation of the earlier described (6R)- and (6S)-1′-O,2′-O,3′-O,2-N,5-pentaacetyl-5,6,7,8-tetrahydro-L-neopterins, one of which could be obtained as pure crystals. Its structure, determined by X-ray-diffraction analysis, corresponds to the (6R)-enantiomer. The method has also been used to make the corresponding D-diastereoisomers. Further acetylation of (6RS)-1′-O,2′-O,3′-O,2-N-tetraacetyl-5,6,7,8-tetrahydro-D-neopterin under drastic conditions yields a mixture of several polyacetylated D-neopterin derivatives and a polyacetylated ethyl-tetrahydro-D-neopterin which was isolated in crystalline form and established by X-ray-diffraction analysis to be (6R)-1′-O,2′-O,3′-O,4-O,2-N,2-N,8-heptaacetyl-5-ethyl-5,6,7,8-tetrahydro-D-neopterin.
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    Helvetica Chimica Acta 69 (1986), S. 615-620 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The α-hydroxylation of chiral esters of 3-phenylpropionic acid by MoO5·Py·HMPT was optimized to 98% de and 73% yield by systematic variation of the reaction conditions. The addition of at least 3 equiv. of K(sec-BuO) proved to be essential.
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  • 165
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparation and the CD spectra of optically pure (+)-trans-μ-[(1R,4S,5S,6R,7R,8S)-C,5,6,C -η : C,7,8,C-η-(5,6,7,8-tetramethylidene-2-bicyclo [2.2.2]octanone)]bis(tricarbonyliron) ((+)-7) and (+)-tricarbonyl[(1S,4S,5S,6R)-C-5,6,C-η-(5,6,7,8,-tetramethylidene-2-bicyclo[2.2.2]octanone)]iron ((+)-8), and of its 3-deuterated derivatives (+)-trans-μ-[(1R,3R,4S,5S,6R,7R,8S)-C,5,6,C-η : C,7,8,C-η-5,6,7,8-tetramethylidene(3-D)-2-bicyclo[2.2.2]-(octanone)]bis(tricarbonyliron) ((+)-11) and (+)-tricarbonyl[(1S,3R,4S,5S,6R)-C-5,6,C- η-(5,6,7,8-tetramethylidene(3-D)-2-bicyclo[2.2.2]octanone)]iron ((+)-12) are reported. The chirality in (+)-7 and (+)-8 is due to the Fe(CO)3 moieties uniquely. The signs of the Cotton effects observed for (+)-7 and (+)-8 obey the octant rule (ketone n→π*CO transition). Optically pure (-)-3R-5,6,7,8-tetramethylidene(3-D)-2-bicyclo[2.2.2]octanone ((-)-10) was prepared. Its CD spectrum showed an ‘anti-octant’ behaviour for the ketone n→π*CO transition of the deuterium substituent. The CD spectra of the alcoholic derivatives (-)-trans-μ-[(1R,2R,4S, 5S,6R,7R,8S)-C,5,6,C-η : C,7,8,C- η-(5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octanol)]bis(tricarbonyliron) ((-)-2) and (-)-tricarbonyl- [(1S,2R,4S,5S,6R)- C,5,6,C- η-(5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octanol)]iron ((-)-3) and of the 3-denterated derivatives (-)-5 and (-)-6 are also reported. The CD spectra of the complexes (-)-2, (-)-3, (+)-7, and (+)-8 were solvent and temperature dependent. The ‘endo’-configuration of the Fe(CO)3 moiety in (±)-8 was established by single-crystal X-ray diffraction.
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  • 166
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal and molecular structure of dinitrato(1,7,10,16-tetraoxa-4,13-diazacyclooctadecane)europium(III) nitrate, ([Eu(NO3)2(C12H26N2O4)]NO3) has been determined from single-crystal X- ray diffraction: a = 12.567(3), b = 11.585(3), c = 16.354(5) Å, β = 112.45(2)°, space group P21/n, Z = 4. The structure consists of discrete dinitrato complex cations and of nitrate anions. The Eu(III) ion is 10-coordinate, bonding to the six donor atoms of the macrocycle and to four O-atoms of the two bidentate nitrates. The mean distances are Eu—O(ether) = 2.60(2), Eu—O(NO3) = 2.47(3) and Eu—N = 2.62(2) Å. The metal site has an approximate C2 symmetry. The IR and Raman spectra show the presence of an ionic and of two bonded bidentate nitrates. These latter have a different v1-v4 splitting, which reflects their dissymmetrical bonding. Luminescence spectra have been recorded at 296, 77, and 4 K by laser-excitation techniques. One sharp 5D0←7F0 transition was observed and almost all the sublevels of the 7FJ manifold could be identified. The interaction between a sharp distribution of the phonon states (especially between 950 and 1200 cm-1) and the electronic 7F2 sublevels results in the presence of several satellite lines accompanying the 5D0→7F2 transition. In MeCN solutions, both luminescence and conductivity data point to the presence of the [Eu(NO3)2(2,2)]+ cation.
    Additional Material: 6 Ill.
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  • 167
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Friedel-Crafts mono and double acylations of trans-μ-[(1RS,2RS,3SR,5RS,6SR,7SR)-C,2,3,C-η:C,6,7,C-η-(2,3,6,7-tetramethylidenebicyclo[3.2.1]octane)]bis(tricarbonyliron) (4) are highly stereoselective and yield trans-μ-{(1RS,2RS,3SR,5RS,6SR,7RS)-C,2,3,C-η :C,6,7,C-η-[(Z)-1-(3,6,7-trimethylidenebicyclo[3.2.1]-oct-2-ylidene)-2-propanone]}bis(tricarbonyliron) (5) and trans-μ-{(1RS,2RS,3SR,5RS,6SR,7SR)-C,2,3,C-η :C,6,7,C-η-[(Z,Z)-1,1′-(3,7-dimethylidenebicyclo [3.2.1] octane-2,6-diylidene)di(2-propanone)]}bis(tricarbonyliron) (6) whose structure has been established by single-crystal X-ray diffraction.
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  • 168
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    Helvetica Chimica Acta 69 (1986), S. 329-332 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 14N- and 15N-NMR spectra have been recorded for 2-(aminomethyl)pyridine (1), tris[(2-pyridyl)methyl]amine (2), and some of their protonated forms. For 1, the most basic site is the aliphatic N-atom, whereas for 2 the pyridine N-atoms are more basic, in contrast to what might be expected for a tertiary aliphatic amine.
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  • 169
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Triafulvene Precursors for Retro-Diels-Alder ReactionsTriafulvene precursors exo-15 and endo-15 have been prepared by addition of dibromocarbene to benzobarrelene 12 followed by a lithium-halogen exchange, methylation, and elimination of HBr (12→13→14→15), (Scheme 2). Gas-phase pyrolysis of exo/endo-mixtures of 15 above 400° gave minor amounts of naphthalene (16), traces of a hydrocarbon C4H4 identified by MS (presumably triafulvene 1) and predominantly (36%) the isomerization product 17 (Scheme 3). In a second synthetic approach the well-known cycloheptatriene-norcaradiene equilibrium of type 26⇆27 has been utilised to prepare various endo-trans-3-(X-methyl) tricyclo[3.2.2.02,4]nona-6,8-dienes 31 (Scheme 5). However, numerous elimination experiments 31→9 failed so far. The structure of two rearrangement products 33 and 34 (Scheme 6) has been elucidated.
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  • 170
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically pure (-)-trans-μ-[(1R,2R,3S,4S,5S,6R)-C,2,3,C-η:C,5,6,C-η-(2,3,5,6,7-pentamethylidenebicyclo[2.2.2]octane)]bis(tricarbonyliron) ((-)-9), (-)-trans-μ-[(1R,2R,3S,4S,5S,6R,7S)-C,2,3,C-η:C,5,6,C-η-(7-methyl-2,3,5,6-tetramethylidenebicyclo[2.2.2]octane)]bis(tricarbonyliron) ((-)-10), and (-)-trans-μ-[(1R,2R,3S,4S,5S,6R,7R)-C,2,3,C-η:C,5,6,C-η-(2,3,5,6-tetramethylidene(7D)bicyclo[2.2.2]octane)]bis(tricarbonyliron) ((-)-16) have been prepared. Their CD spectra were solvent- and concentration-independent, but temperature-dependent, in accord with the existence of equilibria between rapidly interconverting diastereoisomeric species which can be interpreted as arising from distortions of the tricarbonyl(diene)iron units from the Cs symmetry.
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  • 171
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically Active 4,5-Epoxy-4,5-dihydro-α-ionones; Synthesis of the Stereoisomeric 4,5:4′,5′-Diepoxy-4,5,4′,5′-tetrahydro-∊,∊-carotenes and the Steric Course of their HydrolysisWe prove that epoxidation with peracid of α-ionone, contrary to a recently published statement, predominantly leads to the cis-epoxide. Acid hydrolysis affords a single 4,5-glycol whose structure, established by an X-ray analysis, shows that oxirane opening occurred with inversion at the least substituted position (C(4)). Stable cis-and trans-epoxides are prepared by epoxidation of the C15-phosphonates derived from α-ionone. Both the racemic and optically active form are used for the synthesis of the 4,5:4′,5′-diepoxy-4,5,4′,5′-tetrahydro-∊,∊-carotenes having the following configuration in the end groups: meso-cis/cis, meso-trans/trans, rac-cis/trans, rac- and (6R, 6′ R)-cis/cis, rac- and (6R, 6′R)-trans/trans, rac- and (6R, 6′R)-cis/trans, and (6R, 6′ R)-cis/∊. Acid hydrolysis of the cis/cis-epoxycarotenoids under relatively strong conditions occurs again with inversion at C(4)/C(4′) in case of the cis/cis-epoxycarotenoids, but at C(5)/C(5′) in case of the trans/trans-epoxycarotenoids. An independent synthesis of this 4,5,4′,5′-tetrahydro-∊,∊-carotene-4,5,4′,5′-tetrol is presented. The irregular results of the oxirane hydrolysis are explained by assumption of neighbouring effects of the lateral chain. 400-Mz-1H-NMR data are given for each of the stereoisomeric sets. In the visible range of the CD spectra, the (6R, 6R′)-epoxycarotenoids compared with (6R, 6R′)-∊,∊-carotene exhibit an inversion of the Cotton effects.
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  • 172
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    Helvetica Chimica Acta 69 (1986), S. 410-414 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The polymer-bound heptapeptide H-Glu-His-Pro-Gly-Ser-Gly-PEGM was designed as a ‘single-centre model’ for the active site of α-chymotrypsin. The peptide was synthesized according to the general principles of the liquid-phase method for peptide synthesis, and its conformational properties were investigated by CD and IR spectroscopy in solution and in the solid state. In harmony with empirical prediction codes, experimental and theoretical conformational considerations, the peptide adopts a β-turn conformation stabilized by H-bonds involving the side chains of Glu, His, and Ser. The development of a H-bonded system similar to the active site of α-chymotrypsin leads to implications with respect to a possible catalytic activity of the model peptide.
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  • 173
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    Helvetica Chimica Acta 69 (1986), S. 415-418 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel, mild, and highly stereoselective transformation of epoxides to azidohydrins by treatment with Et3Al/HN3 in toluene is described. As an example for the versatility of the reaction, a polyfunctional compound, 14,15-epoxy-14,15-dihydromilbemycin D (1), was transformed to 14-azido-14,15-dihydro-15-hydroxymilbemycin D (2) in 61% yield.
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  • 174
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    Helvetica Chimica Acta 69 (1986), S. 955-961 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Biphenylene dianion has been prepared by Li reduction of the parent hydrocarbon. It is stable below -30° and was characterized by its 1H- and 13C-NMR spectra. A comparison of these data with those of the dication indicates the existence of ion pairs in the dianion case which are responsible for a different charge distribution. The diamagnetic ring currents of both ions, however, are of comparable magnitude, and both are more diatropic than the parent hydrocarbon. Predictions of the π-charge-density effect on 1H chemical shifts are improved by calculations that use the linear and quadratic electric-field effect equation instead of the simple Spiesecke-Schneider relation.
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  • 175
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general procedure for the design of synthetic vaccines with the retained conformational features of protein antigenic determinants is described. This new concept emerges from detailed studies on the relationship between primary sequence and secondary structure formation of synthetic peptides and takes advantage of the amphiphilic nature of epitope-containing peptide segments in the native protein to accomplish structural modifications. These segments, for example amphiphilic helices or β-sheets, are stabilized by the insertion of secondary structure-inducing amino-acid residues on the hydrophobic part of the peptide without affecting the spatial arrangement of functional residues on the hydrophilic side. The availability of amphiphilic peptides with tailor-made conformational properties, e.g. helices, β-sheets, and, moreover, assemblies of these blocks to structures of higher order (‘folding units’), allows the presentation and stabilization of continuous as well as discontinuous epitopes by this approach. This strategy is exemplified for the case of two discontinuous epitopes taken from lysozyme, which are matched to host molecules with adequate conformational features by the help of computer-assisted molecular modelling. The implications of this new concept for the design of synthetic vaccines are discussed with special emphasis to the important role of peptide synthesis and chemical structure modification.
    Additional Material: 9 Ill.
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  • 176
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    Helvetica Chimica Acta 69 (1986), S. 1025-1033 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2-Aza-1,3-dienes. Synthesis and Properties of 1-Alkylpyrazone-imines and their TautomersTreatment of 5-dialkylamino-1,1-dicyano-2-aza-1,3-dienes (or their 1-methoxycarbonyl analogoues) with primary amines gives substituted 1-alkyl-3-cyano-(or methoxycarbonyl-)pyrazoneimines. Tautomeric behaviour, nucleophilic addition to imino function and spectroscopic data of these isomers are examined.
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  • 177
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound 1, {[Fe(H2pztc)(H2O)2] · 2 H2O}∞, has been prepared from pyrazinetetracarboxylic acid (H4pztc) and Fe(II) salt in H2O. It crystallizes in the monoclinic space group P21/a with cell parameters a = 13.56(1) Å, b = 7.17(1) Å, c = 6.48(1) Å, β = 101.0(2)°, V = 618.4 Å3. The molecules form infinite parallel chains with bis(bidentate) coordination (through N and O) of the bridging pyrazine ligands. Two trans-COOH groups of the ligand remain uncoordinated, and two trans-H2O ligands complete a pseudo-octahedral Fe(II) coordination which is distorted by the steric effect of the free COOH groups. The parallel polynuclear chains are linked through an extended network of H-bonds involving the COO functions of the bridging ligand and the coordinated and lattice H2O molecules. Mössbauer and magnetic-susceptibility data suggest normal paramagnetic behaviour of high-spin Fe2+. An observed drop below 15 K in effective magnetic moment for 1 was attributed to temperature-dependent population changes of the levels of the ground state 5Γ, and not to antiferromagnetic coupling (J 〈 1 cm-1). This result establishes that the obvious potential magnetic exchange pathway through the pyrazine bridges is inefficient, and this may be attributed, in part, to the high planarity of Fe(H2pztc) moieties.
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  • 178
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    Helvetica Chimica Acta 69 (1986), S. 1120-1131 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of voltammetric, potentiometric, chronoamperometric, ion transport, and extraction studies on neutral carrier based, plasticized poly(vinyl chloride) membranes are summarized. They unambiguously confirm that such bulk membranes dispose of immobile anionic sites. These fixed sites lead to a Donnan exclusion of other anions from the membrane and thus to a permselectivity for cations. The results are in perfect agreement with the predictions of earlier membrane models. A rigorous Poisson-Boltzmann analysis of macroscopic liquid membranes clearly indicates that space-charge at the membrane/solution interfaces does not influence the electrochemical properties and the ion-selectivity behavior at steady state.
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  • 179
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    Helvetica Chimica Acta 69 (1986), S. 1153-1162 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Peptide-Bond Formation with C-Terminal α,α-Disubstituted α - Amino Acids via Intermediate Oxazol-5(4H)-onesThe formation of peptide bonds between dipeptides 4 containing a C-terminalα,α-disubstituted α-amino acid and ethyl p-aminobenzoate (5) using DCC as coupling reagent proceeds via 4,4-disubstituted oxazol-5(4H)-ones 7 as intermediates (Scheme 3). The reaction yielding tripeptides 6 (Table 2) is catalyzed efficiently by camphor-10-sulfonic acid (Table 1). The main problem of this coupling reaction is the epimerization of the nonterminal amino acid in 4 via a mechanism shown in Scheme 1. CSA catalysis at 0° suppresses completely this troublesome side reaction. For the coupling of Z-Val-Aib-OH (11) and Fmoc-Pro-Aib-OH (14) with H-Gly-OBu1 (12) and H-Ala-Aib-NMe2 (15), respectively, the best results have been obtained using DCC in the presence of ZnCl2 (Table 3).
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  • 180
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    Helvetica Chimica Acta 69 (1986), S. 1163-1171 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (1RS, 5SR)-2,2,5-Trimethylcyclopentane-1-carboxylic acid (17) and (1r, 2RS, 5SR)-1,2,5-trimethylcyclopentane-1-carboxylic acid (19) are the starting materials for the α-alkynone routes to (±)-capnellene and for similar efforts towards ptychanolide. Since 17 and 19 have, so far, been available only by a branching reaction from the same precursor, the cyanohydrin mixture 2/3, a modified synthesis for 17 and a new one for 19 was developed (Scheme 1). The common precursor 2/3 was treated with POCl3 which effected normal dehydration to 6 (47%, major path) in competition with Me migration to 8 and 9 (17%). The minor path to 8 and 9 could be reduced to 3% when SOCl2 was used for the dehydration of 2/3. This reaction was the basis for an improved synthesis of 17 from 1, using the steps b, e, i, r, and v see Scheme 1 in an overall yield of 35%. The POCl3 reaction was also studied with the pure cyanohydrins 2 and 3, the configurations of which were determined by an X-ray analysis of 2. Me migration did not occur form 2 but only from 3 (25%), which has HO—C(1) and H—C(5) in a cis position. With SOCl2, 3 underwent only 5% Me migration. The new synthesis of 19 started with 4 using the steps h, n, p, and s (see Scheme 1) in an overall yield of 68%.
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  • 181
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    Helvetica Chimica Acta 69 (1986), S. 1205-1215 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A synthesis of N-acetylneuraminic acid (1) and of N-acetyl-4-epineuraminic acid (2, R = H) from 2-acetamido-4,6-O-benzylidene-1,2-dideoxy-1-nitro-D-mannopyranose (3) and 2-acetamido-1,2-dideoxy-4,6-O-isopropylidene-1-nitro-D-mannopyranose (4), respectively, is described. Michael addition of 3 and 4 to tert-butyl 2-(bromomethyl)prop-2-enoate (5) and subsequent hydrolytic removal of the NO2 group gave the 4-nonulosonate tautomers 6/7 and 8/9, respectively (Scheme). Stereoselective reduction of 6/7 and 8/9 with NaBH4/AcOH in dioxane/H2O yielded 12/13 (94:6) and 14/15 (92:8), respectively. Reduction of 6/7 and 8/9 in the absence of AcOH or in EtOH gave 12/13 (15:85) and 14/15 (15:85), respectively. Ozonolysis of 12 and 13 followed by hydrolysis gave tert-butyl neuraminate 22 and tert-butyl 4-epineuraminate 24, respectively. Ozonolysis of 14/15, separation of the products 20 and 21, and hydrolytic removal of the isopropylidene groups gave 22 and 24, respectively. The tert-butyl ester 22 was saponified to give 1, which was further characterized as the methyl ester 23. Saponification of 24 gave the crude 4-epimer of 1, which was converted into the stable Na salt 2 and also into the methyl ester 25.
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  • 182
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Preparation of Unprotected and Partially Protected 1-Deoxy-1-nitro-D-aldoses and Some Representative X-Ray Structure AnalysesThe unprotected and partially protected 1-deoxy-1-nitro derivatives of α-and β-D-glucopyranose (see 15 and 14), β-D-mannopyranose (see 16), N-acetyl-β-D-glucosamine (see 17), β-D-galactofuranose (see 19), β-D-ribofuranose (see 20), α-D-arabinofuranose (see 21), 4,6-O-benzylidene-β-D-glucose (see 40), N-acetyl-4,6-O-benzylidene-β-D-glucosamine (see 41), and 4,6-O-benzylidene-β-D-galactose (see 42) were prepared by ozonolysis of the corresponding nitrones which were obtained from the acid-catalyzed reaction of p-nitrobenzaldehyde with the hydroxylamine 4, the unprotected oximes 3 and 5-9 and the 4,6-O-benzylidene oximes 35-37, respectively (Schemes 1-3). The gluco- and manno-nitrones 10 and 12 were isolated, and their ring size and their anomeric and (E/Z) configurations were determined by NMR spectroscopy and by their transformation into their corresponding nitro derivatives. The structure of the deoxynitroaldoses were determined by NMR spectroscopy, polarimetry, and, in the case of 14, 16, and 17, by formation of the 4,6-O-benzylidene (14→40) or 4,6-O-isopropylidene (16→43, 17→23) derivatives (Scheme 3). Acetylation of the nitroglucopyranose 14, the 2-acetamido-nitroglucopyranose 17, and the nitrogalactofuranose 19 gave the crystalline peracetylated nitroaldoses 22, 24, and 45, respectively (Scheme 4, Figs. 1 and 3); acetylation of the nitromannopyranose 16 gave the nitro-arabino-glycal 44 (Scheme 4). The structure of the peracetylated nitroglucopyranose 22, the nitroglucosamine 25, the nitrogalactofuranose 45, and the nitroribofuranose 20 were confirmed by X-ray analysis (Figs. 1 4). In all cases, including the β-D-glucopyranose derivative 22, considerably shortening of the (endocyclic) C(1)-O bond was observed. Base-catalyzed anomerization of the β-D-configurated nitroglucopyranose 14, the nitromannopyranose 16, the benzylidene acetal 40 of nitroglucose, and the 2,3,4,6-tetraacetylated glucosamine derivative 24 gave the corresponding nitro-α-D-aldoses 15, 26, 47, and 25, respectively (Scheme 4).
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  • 183
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    Helvetica Chimica Acta 69 (1986), S. 1263-1270 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective Functionalisation of N,N-Diethyl-1,6-methano[10]annulene-2-carboxamidesLithiation of N,N-diethyl-1,6-methano[10]annulene-2-carboxamide with BuLi followed by quenching with different electrophiles yields 2,10-disubstituted-1,6-methano[10]annulenes.
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  • 184
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    Helvetica Chimica Acta 69 (1986), S. 704-707 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis, Reactions, and Photochemistry of Heterocyclic N-AlkylthioamidesVarious heterocyclic N-alkylthioamides of the pteridine (2, 7) and pyridine series (13) have been synthesized and investigated regarding their photooxidation. The compounds are very photolabil under aerobic conditions and are converted into the corresponding amides (1, 6 and 12, resp.) on irradiation.
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  • 185
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    Helvetica Chimica Acta 69 (1986), S. 718-725 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Changes in chemical shifts of olefinic protons in a number of α,β- and α,β,γ,δ-unsaturated carboxylic acids caused by ionization of the COOH group were investigated. The ionization shifts of α-H-atoms are -0.09 to 0.07 ppm, those of β-H-atoms are 0.32-0.47 ppm. The ionization shifts of δ-H-atoms are substantially larger than those of γ-H-atoms. The ionization shifts can be used for immediate determination of the esterification site in monoesters of (2E,4Z)-2,4-hexadienedioic (muconic) acid, which are of interest in connection with synthetic studies on verrucarins. Thus, isomerization by heating in aqueous solution of monoesters of (2Z,4Z)-2,4-hexadienedioic acid yields 1-monoesters rather than 6-monoesters of (2E,4Z)-2,4-hexadienedioic acid, in accordance with the isomerization mechanism involving anchimeric assistance of the free COOH group. Solutions of the ABXY spectra of olefinic protons of monomethyl (2E,4E)- and (2Z,4Z)-2,4-hexadienedioate are reported.
    Additional Material: 2 Ill.
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  • 186
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A highly regio- and stereoselective Diels-Alder reaction between dienophiles of type I and dienes of type II (Scheme 1) gives rise to Diels-Alder adducts of type III. Upon treatment with BF3.Et2O, these adducts are smoothly converted into the corresponding enones (Scheme 6). Under mild acidic conditions, enone (±)-33 gave bicyclic diketone (±)-34 via an intramolecular Michael-type addition. Diketone (±)-34 has the correct relative configuration and a suitable ketone function at C(6) for further conversion into the hexahydrobenzofuran portion of the avermectins and milbemycins.
    Additional Material: 1 Tab.
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  • 187
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of 2,3-dimethylidene- and 2,3,5,6-tetramethylidene-7-oxabicyclo[2.2.1]heptanes substituted in position C(1) are reported. The 1-dimethoxymethyl group in derivatives 2 and 6 controls the regioselectivity of the Lewis-acid-catalyzed Diels-Alder additions with methyl vinyl ketone and butynone. For the EtAlCl2-catalyzed addition of methyl vinyl ketone to 6, the regioselectivity can be reversed by a small solvent modification. The tetraene 2 is a versatile reagent for regioselective ‘tandem’ cycloadditions.
    Additional Material: 7 Ill.
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  • 188
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    Helvetica Chimica Acta 69 (1986), S. 1343-1348 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two-dimensional, high-field NMR methods are employed in confirming the relative configuration at the C(20) ethyl side-chain junction as S* in the natural alkaloid tubotaiwine (dihydrocondylocarpine).
    Additional Material: 2 Ill.
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  • 189
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    Helvetica Chimica Acta 69 (1986), S. 1323-1330 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A New Synthesis of (±)-DihydrorecifeiolideEthyl 1-(2′-formylethyl)-2-oxocyclooctane-1-carboxylate (2) prepared by Michael reaction of ethyl 2-oxocyclooctane-1-carboxylate (1) was regioselectively methylated at the aldehyde group with (CH3)2Ti[OCH(CH3)2]2 to give 3 (Scheme 1). The alcohol 3 was treated with Bu4NF to give the deethoxycarbonylated product 4 which by distillation gave the bicyclic enol ether 5. Oxidation (m-chloroperbenzoic acid) of 5 and reduction of the resulting oxolacton 6 yielded the title compound (±)-dihydrorecifeiolide (7) in an overall yield of nearly 50 %. Methylation of the aldehyde 2 with MeLi gave the ring-enlarged lacton 9 in poor yield (13 %). The deethoxycarbonylation reaction 3 → 4 was studied in more detail (Scheme 3).
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  • 190
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metabolites of Microorganisms. Sulfur-Containing Ansa Compounds of the Naphthomycin TypeFrom a strain of Streptomyces (Tü 2357) in addition to naphthomycin A, 4 new yellow pigments were isolated and their structures determined by spectroscopic comparison with naphthomycin A. Whereas the naphthomycins D and E are simple derivatives of naphthomycin A, having OH and H, respectively, instead of Cl, the naphthomycins F and G contain an N-acetylcysteine residue linked to the aromatic moiety by a thioether group. Degradations with O3 yielded identical products from the naphthomycins A, D, F, and G, showing coincident configurations in parts of the molecules. Naphthomycin F shows some biological activity against gram-positive bacteria and fungi, although much weaker than naphthomycin A. The naphthomycins D, E, and G are inactive against microorganisms.
    Additional Material: 4 Tab.
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  • 191
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    Helvetica Chimica Acta 69 (1986), S. 1418-1423 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The crystal structure of (±)-corynoline (1) has been determined by X-ray diffraction methods. The rings B and C form the half-chair and the twist-half-chair conformations, respectively. The B/C ring conjunction exists in an anti-cis conformation, with a N … H—O intramolecular H-bond. Conformational energy calculation by the CNDO/2 method show that the conformations of 1, (+)-chelidonine (2), and their acetates, observed in crystal structures, are all in the one of total energy minimum.
    Additional Material: 2 Ill.
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  • 192
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    Helvetica Chimica Acta 69 (1986), S. 1041-1051 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (2S,4S)-2-(tert-Butyl)-5-oxo-1,3-dioxolan-4-acetic acid (1) was dilithiated to the enolate 2 in toluene and THF with lithium diisopropylamide (LiN(i-Pr)2) under various conditions. Deuteration experiments with 2 showed that two different stable forms of the enolate could be produced, depending on whether the deprotonation of 1 had been carried out in toluene or THF. The generated (i-Pr)2NH was coordinated to the enolate and, hence, served as a strong lipophilic solvating agent. LiN(i-Pr)2 could substitute the amine ligands of the two species to a different extent. THF was partially able to displace (i-Pr)2NH that solvated the species produced in toluene, but the coordination of the amine to the one produced in THF was quantitive. Correlations with given explanations are made.
    Additional Material: 2 Ill.
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  • 193
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    Helvetica Chimica Acta 69 (1986), S. 1085-1087 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a mononuclear Rh(III) complex, as a representative example of a series of related species, containing two cyclometallating ligands ppy (2-phenylpyridine) and one diimine bpy (2,2′-bipyridine) from a binuclear Cl-bridged compound is described. The absorption spectrum shows a maximum at 364 nm (ε = 7000), with a very weak shoulder (ε = 10) at 454 nm. This band is tentatively assigned to a metal-to-ligand charge-transfer transition. A reversible, one-electron reduction appears in the cyclic voltammogram at E1/2 = -1.41 V (vs. NHE) and a irreversible oxydation at Ep = + 1.1 V. A detailed NMR analysis including 13C-NMR, NOE, SFORD as well as deuteration of the bpy ligand indicates the formation of only one isomer, having a C2 axis, bisecting the bpy ligand, with the two carbon ligands in cis-position. 103Rh, 13C and 103Rh,H couplings are observed.
    Additional Material: 2 Tab.
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  • 194
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    Helvetica Chimica Acta 69 (1986), S. 1095-1106 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis and Reactivity of Lumazine-7-sulfenic AcidsThe chemical synthesis of the pteridine-7-sulfenic acids 13-16 is described (Scheme 1). The 1,2,3,4-tetrahydro-1,3,6-trimethyl-2,4-dioxopteridine-7-sulfenic acid (14) was isolated as a stable crystalline solid. Its chemical reactivity was investigated and the physical properties determined. In the solid state, the S-oxide form 14A predominates, but in protic solvents the S-OH tautomer 14 exists most likely. In basic medium, the pteridine-7-sulfenate species is stable, whereas a low pH gives rise to a disproportionation to the disulfide 10 and the corresponding pteridine-7-sulfinic acid 37 (Scheme 4). Reaction of 14 with ethyl propiolate leads, under cis-addition, to 36, oxidation with KMnO4 forms 1,2,3,4-tetrahydro-1,3,6-trimethyl-2,4-dioxopteridine-7-sulfonic acid (38) and NaBH4 reduction the corresponding 7-mercaptopteridine dione 18 (Scheme 4).
    Additional Material: 2 Ill.
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  • 195
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: CO and H2 are photogenerated simultaneously by visible-light irradiation of systems containing a photosensitizer, the [Ru(bpy)3]2+ complex, Co(II) species as homogeneous catalysts, which mediate CO2 and H2O reduction by intermediate formation of Co(I), a tertiary amine as electron donor, which provides the electrons for the reduction, and an organic solvent which also facilitates dissolution of CO2. The efficiency of (CO + H2) gas production and the selectivity CO/H2 markedly depend upon the composition of the medium, the nature of the tertiary amine, the solvent, and the ligand of the Co ions. 2,9-Dimethyl-1,10-phenanthroline is particularly effective in promoting CO and H2 formation, giving a quantum yield of 7.7% in (CO + H2) (1.2% for CO and 6.5% for H2). The process consists of two catalytic cycles: a photocatalytic cycle for the Ru complex and a double dark reaction pathway for the Co species; oxidative and/or reductive quenching of the excited state of the photosensitizer lead to the formation of Co(I) species which reduce either CO2 or H2O to CO or H2, respectively.
    Additional Material: 10 Ill.
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  • 196
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Estimation of preferred conformation, orientation, and accumulation of adrenocorticotropin (1-24)-peptide at an aqueous-hydrophobic interface produced a model that agreed with that developed from experimental observations with lipid membranes. Thus, the N-terminal message segment (residues 1-11) was incorporated into the hydrophobic phase as an α-helical, perpendicularly oriented domain with an apparent dissociation constant of ca. 5 · 10-5 M. The C-terminal address segment (residues 12-24) remained in the aqueous phase as a random-coil domain. Three parameters proved sufficient to define the model: the Gibbs free energy of hydrophobic association, the molecular amphiphilic moment, and the molecular electric dipole moment. For estimating interactions with biologic membranes (that carry a net negative charge), the Boltzmann distribution of charged peptides was also considered. The estimations were extended to adrenocorticotropin (1-10)-peptide and α-melanotropin. In the first case, the prediction agreed with the earlier observations, in the second, it awaits its experimental proof. The estimated membrane structures were compared with available biologic data. As for opioid peptides, it appears that the amphiphilic moment is an important new parameter for determining quantitative structure-activity relationships (QSAR) in receptor selection and biologic potency.
    Additional Material: 3 Ill.
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  • 197
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of 2,3,5-trimethylidenebicyclo[2.2.1]heptane (1) and 2,3,5,6,7-pentamethylidenebicyclo[2.2.2]-octane (2) are reported. The Diels-Alder additions of the diene moieties of these polyenes can be regioselective, probably because of a possible transannular interaction between the homoconjugated methylidene and s-cis-buta-diene groups.
    Additional Material: 1 Tab.
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  • 198
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    Helvetica Chimica Acta 69 (1986), S. 1602-1613 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The N(1)- and N(2)-(2′-deoxyribofuranosides) 1 and 2, respectively, of 8-aza-7-deazaguanine were prepared via phase-transfer glycosylation in the presence or absence of Bu4NHSO4 as catalyst of 6-amino-4-methoxy-lH-pyrazolo[3,4-d]pyrimidine (7c) with 2-deoxy-3,5-di-O-(p-toluoyl)-α-D-erythro-pentofuranosyl chloride (10). On a similar route, but without catalyst and employing THF as organic phase, the 6-amino-4-chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)-and N(2)-substituted 4-thioxo analogues 17a and 18a, respectively. The ratio of N(1)- to N(2)-glycosylation was 2:1 for 7c and 1:1 for 7a, viz. depending on the nucleobase structure. The rate of the H+-catalyzed N-glycosyl hydrolysis was strongly decreased for the N(2)-(β-D-2′-deoxyribofuranosides) as compared to the N(1)-compounds. However, the N(1)-nucleoside 1, which is an isostere of 2′-deoxyguanosine, is sufficiently stable to be employed later in solid-phase oligonucleotide synthesis.
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  • 199
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    Helvetica Chimica Acta 69 (1986), S. 1588-1596 
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Betenamine and of Betalaine Model SubstancesFor comparisons of color, spectroscopic properties, pKa values, and stabilities, a number of model substances containing the 1,7-diazaheptamethinium chromophore 8 of the yellow and red betalaine plant pigments were prepared by the thermal or photolytic ring opening of simple pyridine derivatives (such as 2 and 14-16), followed by the introduction of amines. Among the novel compounds prepared were betenamine perchlorate (5), the ‘naked’ ring system of the beet-pigment betanine (C) as well as two 1,7-diazaheptamethinium salts 25 and 27 with terminal amono acids. The synthesis of 5 started with 4-(2-aminoethyl)pyridine (1) and proceeded via 2, ring opening with indoline to 4, saponification, and intramolecular amine replacement (Scheme 1). The syntheses of 25 and 27 involved only one step, namely ring opening of γ-picoline using (S)-cyclodopa (24) and (S)-proline (26), respectively (Scheme 3).
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  • 200
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) ChlorideStarting with a nearly quantitative coupling of cyclononatetraenide 7 to 1, 1′-dihydrononafulvalene 8 by means of AgBF4, a simple general synthetic concept for fulvalenes is outlined (Scheme 2), consisting in an oxidative coupling of ‘Hückel anions’ like 2 and 7 to 1, 1′-dihydrofulvalenes 10 with Ag(I) or Cu(II) salts, followed by deprotonation (→11) and oxidation (→12); it has been realised in the case of pentafulvalene (1; overall yield 61%; Scheme 3) and 1,2:5,6-dibenzopentafulvalene (18; overall yield 66%, Scheme 4). NMR-spectroscopic investigations show that 1 is a non-aromatic compound with strongly alternating bond-lengths, its π-system being even more localised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above -50°. Besides of polymerisations, Diels-Alder dimerisation 1→19 followed by a rearrangement 19→20 takes place (Scheme 5).
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