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  • Computational Chemistry and Molecular Modeling  (20)
  • formimidol  (1)
  • 1
    Digitale Medien
    Digitale Medien
    Behavior of interaction energy and intramolecular bond stretch in linear and bifurcated hydrogen bonds (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 181-190 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: It is first demonstrated that a simple Morse potential can fit very well the ab initio interaction energy for both the linear H bond of the water dimer and the bifurcated arrangement in HOH‥HOCHCHOH, using simple physical properties as the parameters in the function. This energy correlates linearly with the stretching observed in the internal OH bond of the proton donor water, leading to the formulation of an analogous Morse function that can describe the latter stretch. Although simple physical parameters can be used for the linear H bond, a good fit is achieved in the bifurcated case only after the parameters are adjusted empirically. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560480719
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  • 2
    Digitale Medien
    Digitale Medien
    Modeling proton transfer potentials in angularly deformed hydrogen bonds (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 77-87 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Earlier work demonstrated that either a simple fourth-order polynomial or a pair of Morse functions could be fit with high accuracy to model proton transfers across H-bonds. The work is extended here to a systematic set of angular distortions in the H2O‥H+‥OH2 and H3N‥H+‥NH3 systems. So long as the deformation does not impose a left-right asymmetry into the system, either of these types of functions can reproduce ab initio transfer potentials well. But the Morse potentials are superior in that neither stretches nor bends of the H-bonds cause large variation in the parameters. For those modes of angular distortion which introduce asymmetry into the transfer potential, the ab initio data can be accommodated by permitting small variations in two of the parameters in the Morse functions. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560480711
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  • 3
    Digitale Medien
    Digitale Medien
    Proton transfer in H5O2+ and H3O2- with an external restraining force (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 567-575 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A comparative study between proton transfer in H5O2+ and H3O2- has been carried out using the 6-31 + G** basis set at the MP level. An external harmonic force is imposed between the two terminal hydrogens (one on either end of the complex) to restrain the H bond lengths to a range where two minima exist in the potential energy surface, while providing the OH2 and OH- groups appropriate flexibility to approach one another during the course of transfer. The H bond length in the anion is found to be longer and more linear than that in the cation. Geometries and orientations of the subunits play an important role in determining the H bond length and the nonlinearity of the bond. Similar trends are noted for both the ions as the spring stiffens: The barrier reaches its asymptotic maximum for intermolecular force constants larger than about 7 mdyn/Å, as do the equilibrium and transition-state values of R(OO). The energy barrier for the anion is higher than that of the cation. For both systems, the intrinsic reaction coordinate divides the transfer process into two consecutive steps: The approach of the two O atoms is followed by the actual motion of the proton. In fact, the hydrogen-bonded hydrogen moves upward, perpendicular to the line joining the O atoms, during the first step. The first steps account for about 30% of the energy required for proton transfer in the cation, whereas for the anion it is about 45%. © 1995 John Wiley & Sons, Inc.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560560860
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  • 4
    Digitale Medien
    Digitale Medien
    Effect of proton transfer on neighboring hydrogen-bond strength (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 37-48 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The transfer of a proton from A to B in AH+‥B‥C to form A‥+HB‥C is expected to enhance the binding energy of C to the remainder of the H-bonded chain since C is now adjacent to a charged ion rather than to a neutral molecule. But since the actual proton motion between A and B is typically only 1 Å or so, the interaction should be considerably less than that in isolated +HB‥C. Ab initio calculations with a variety of basis sets are used to quantitatively measure the enhancement of this binding energy. Systems investigated include the homotrimers of water and of ammonia, ammonium-formate-water, and formic acid-imidazole-water.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560400709
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  • 5
    Digitale Medien
    Digitale Medien
    Modeling of coupled proton transfers by analytic functions (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 109-124 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Potential energy surfaces are generated by ab initio calculations for the transfer of two protons in the pentameric chains (H2O ·· H2O ·· H2OH+ ·· OH2 ·· OH2) and (H3N ·· H3N ·· H3NH+ NH3 ·· NH3). These surfaces are then fit by the sum of (a) two functions, each of which reproduces the transfer potential of a single proton plus (b) a function that models the coupling between the two protons. Several candidates are tested for the latter coupling function. A simple linear dependence upon the motion of the two protons reproduces the full two-dimensional surface to an accuracy of some 1 kcal/mol. The popular quadratic function provides a poorer approximation as also do exponential and Coulombic-type functions. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560440713
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  • 6
    Digitale Medien
    Digitale Medien
    Calculation of barriers to proton transfer using a variety of electron correlation methods (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 817-835 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The usefulness of various combinations of MCSCF and CI methods in computing correlated proton transfer potentials is investigated for the systems, HF2-, H7N2+, H3O2-, and H5O2+. MCSCF calculations can accurately determine proton transfer barriers, provided the correlation is limited to the proton transfer process. The proper correlated space can be obtained more easily if the canonical occupied MOs are first subjected to a localization. Various means are tested of including additional electron correlation into the MCSCF methods. CIS and CISD calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The MCSCF + CISD results are excellent, being fairly independent of choice of virtual Mos, although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied MOS prior to the MCSCF part of the calculation again results in a further improvement. These results are compared to CI computations using the canonical orbitals (and which are not preceded by MCSCF preparation of the wave function) and to Møller-Plesset results. © 1992 John Wiley & Sons, Inc.
    Zusätzliches Material: 8 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560440872
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  • 7
    Digitale Medien
    Digitale Medien
    Comparison of ground and triplet state geometries of malonaldehyde (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 48 (1993), S. 419-429 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The geometries of the ground and first triplet excited state of malonaldehyde are compared for two distinct configurations of the molecule. The first represents the equilibrium geometry and the second structure corresponds to the transition state for the interamolecular hydrogen transfer. The ground state computations utilize both self-consistent field and Moller-Plesset second-order perturbation theory. The excited state computations employ several different theoretical methods; unrestricted Hartree-Fock (UHF), unrestricted second-order Moller-Plesset perturbation theory (UMP2), CI Singles (CIS), and complete active space self-consistent field (CAS). The geometric parameters obtained for the two configurations are contrasted in both the ground and excited state. The structural differences are related to the extent of hydrogen bonding present, which is compared to the calculated proton transfer barrier. © 1993 John Wiley & Sons, Inc.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560480842
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  • 8
    Digitale Medien
    Digitale Medien
    Ab initio study of proton transfers including effects of electron correlation (1983)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 739-751 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Proton transfers in a number of systems are investigated using ab initio molecular orbital methods. Calculations are carried out with several different basis sets ranging in size from 4-31G to 6-311G**. Electron correlation is included using Møller-Plesset (MP) perturbation theory to second and third orders. Enlargements of the basis set invariably lead to higher energy barriers to proton transfer, while substantial reductions result from inclusion of correlation effects. Application to (HOHOH)- of third-order MP theory with a triple-valence basis set augmented by polarization functions on oxygens and the central proton, denoted MP3/6-311G*(*), leads to excellent agreement with the results of Roos et al. whose calculations involved an extensive CI treatment with a large basis set. For equivalent hydrogen bond lengths, the transfer barrier in the cation (H2OHOH2)+ is nearly identical to that for the (HOHOH)- anion while the barrier in (H3NHNH3)+ is somewhat smaller. The reduction of the SCF barrier height resulting from inclusion of correlation is greater for (O2H3)- than for the above cations. The lowest energy structure of (O2H5)+ contains a symmetric hydrogen bond in which the proton is located midway between the two oxygens whereas asymmetric H bonds are found in the equilibrium geometries of (N2H7)+ and (S2H5)+. The difference in energy between the symmetric and asymmetric configurations of (O2H3)- is extremely small.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560230243
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  • 9
    Digitale Medien
    Digitale Medien
    Interactions involving aromatic systems: Benzene + acetylene (1984)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 201-208 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Various configurations of the system composed of benzene and acetylene are investigated in an effort to identify the most stable. The total interaction energy is computed as the sum of the dispersion energy ED and ΔESCF, the interaction energy calculated by ab initio molecular orbital methods. Pairwise summation schemes of both the atom-atom and bond-bond types are used to evaluate ED. The most stable structure is found to be that in which the acetylene approaches benzene from above and is oriented such that it lies directly along the C6 axis of benzene. Although the contribution of dispersion to the total interaction energy is dominant, the smaller electrostatic component plays a crucial role in controlling the relative orientations of the two molecules. Indeed, it is possible to select the most stable configuration solely on grounds of the most favorable quadrupole-quadrupole interaction. The likelihood of observation of this configuration is further confirmed by consideration of basis set extension and entropic effects.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560260721
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  • 10
    Digitale Medien
    Digitale Medien
    The basis set dependence of structures and energies of various states of cyclodisiloxane (1986)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 1191-1208 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Several common basis sets, ranging from minimal to double-zeta, are applied to study the neutral singlet and triplet as well as positive- and negative-ion doublet states of cyclodisiloxane. The effect of d-polarization function exponents on the equilibrium geometries and energies is analyzed. The d-type functions seem to be essential in the basis set of silicon, whereas their presence on oxygen is less critical. The optimum exponents (with respect to SCF energy) are determined to be 0.45 for Si and 0.60 for O, very close to those recommended for the 6-31G** basis set. The best structural predictions are obtained with the 6-31G(2d, p) basis set, which contains two sets of d functions on the heavy atoms. The predicted Si—O bond length is 166 pm; the Si—Si and O—O distances are 237 and 232 pm, respectively, which correspond to an O - Si—O angle of 88.6°. The ground state is found to be a singlet. All higher states have longer Si—O bonds and Si - Si distances, whereas O - O distances are shorter. The energy separation between the singlet and other states is modified by electron correlation (MP treatment) by only a few kcal/mol.
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560290518
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