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Panchenko, Yurii N.; Krasnoshchiokov, Sergei V.; Bock, Charles W.
doi: 10.1002/jcc.540090502pmid: N/A
The results of complete Hartree‐Fock (HF)/6–31G geometry optimization and HF/6–31G//HF/6–31G force fields evaluations of three rotamers of the H2CCH–HCNH molecule (s‐trans–d‐trans–, s‐trans–d‐cis‐ and s‐cis–d‐trans‐2‐propen‐1‐imine) and the H2C=N–H molecule are reported. All three conformers were found to be planar with the s‐trans–d‐trans conformer lowest in energy. This was corroborated by further complete optimizations of the geometries of the s‐trans–d‐trans and s‐trans–d‐cis conformers at the HF/6–31G*(5D) level as well as single‐point MP4/6–31G*//HF/6–31G*(5D) calculations. The assignment of the vibrational frequencies of the propenimine rotamers and some isotopomers of methanimine are also reported. The majority of the experimental frequencies of propenimine in the gaseous phase are found to belong to the s‐trans–d‐trans form, but a few frequencies are attributed to the high‐energy s‐trans–d‐cis conformer.
doi: 10.1002/jcc.540090503pmid: N/A
An algorithm for finding rings in graphs is presented. The algorithm is based on the Welch‐Assembly‐Gibbs algorithm of Wipke and Dyott but using the homeomorphically reduced pruned graph (the extension of HRG of Balaban et al). The algorithm is able to generate both the fundamental set of rings and all possible rings in a given graph. The time and storage needs are superior to both underlying algorithms. The CPU times of the old and new algorithms are given.
Schmitz, Lawrence R.; Allinger, Norman L.; Profeta, Salvatore
doi: 10.1002/jcc.540090504pmid: N/A
Ab initio Hartree‐Fock calculations at the 6–31G*//6–31G* and MP3/6–31G*//6–31G* levels of theory are reported for propylamine. All ten stationary points needed in a description of the rotation around the C1–C2 bond have been located on the 6–31G* surface and each of these points has been examined at the MP3 level.
doi: 10.1002/jcc.540090505pmid: N/A
A branching point is a point on a reaction path leading from reactants to products (via a transition state) at which it is energetically favorable for the system to break symmetry. Such a point can be defined in terms of normal modes along the reaction path and corresponds to zero curvature (a zero Hessian eigenvalue) along a symmetry‐breaking mode. An effective method for the location of such points is presented and realized in an efficient, practical algorithm designed for use in the ab initio program package Gaussian 82.
doi: 10.1002/jcc.540090506pmid: N/A
Electrical interactions between molecules are important effects in weak and long‐range attractions. With quantum mechanical techniques now capable of yielding values of all multipole polarizabilities, hyperpolariabilities, and beyond, exhaustive treatment of electrical interaction is no longer out of the question. An efficient computational strategy is presented for the evaluation of electrical interaction energies to any desired level for small, medium, and large (ca. 100 molecules) clusters. With incorporation of repulsive, hard‐wall potentials, global surfaces may be examined. Example calculations are presented.
Wiberg, Kenneth B.; Murcko, Mark A.
doi: 10.1002/jcc.540090507pmid: N/A
The optimized geometries for the rotamers of propanal, 2‐butanone, isobutyraldehyde, methyl isopropyl ketone, and isobutyric acid obtained using the 3–21G and 6–31G* basis sets are compared, and systematic changes are noted. The relative 6–31G* energies using the 3–21G and 6–31G* geometries are generally the same within 0.1 kcal/mol. The effect of electron correlation on the relative energies is generally small. These and related data show that 6–31G* relative energies obtained using 3–21G geometries are generally satisfactory when studying rotation about CC bonds. However, this is not the case for CO bonds. The calculated relative energies of isomeric compounds are reproduced only with the full MP4 correction for electron correlation.
Michalska, D.; Schaad, L. J.; C̆arsky, P.; Andes Hess, B.; Ewig, C. S.
doi: 10.1002/jcc.540090508pmid: N/A
Tatewaki and Huzinaga's (J. Comput. Chem. 1, 205 (1980)) basis sets, constructed to minimize superposition error, were used to calculate infrared (IR) frequencies and intensities. They were found inferior to Pople bases such as 3–21G and 6–31G*. The question of whether a theoretical vibrational spectrum should be computed at experimental or theoretical bond lengths was also investigated. If the magnitude of the correlation energy increases with bond length, Hartree‐Fock bond lengths are expected to be shorter than experimental, and frequencies computed there will be higher than those computed at experimental lengths. Conversely, if this magnitude decreases with R, computed lengths should be longer than experimental and should give lower computed frequencies.
doi: 10.1002/jcc.540090509pmid: N/A
A general algorithm of calculating the eigenstates of a rigid molecule trapped in an external potential is reported. The wave function and the potential are expanded about a common reference configuration. The expansion coefficients of the wave function are variationally determined. Contracted basis functions may be used to restrict the number of expansion coefficients. The use of the algorithm is illustrated by calculations of intermolecular eigenstates of benzene–water complexes.
Barone, Vincenzo; Minichino, Camilla; Lelj, Francesco; Russo, Nino
doi: 10.1002/jcc.540090510pmid: N/A
The relative stabilities of the bidentate and tridentate configurations of the complex hydrides NaBH4, AlH2BH4, and GaH2BH4 have been computed both at the Hartree‐Fock and post‐Hartree‐Fock levels using the ab initio pseudopotential method. For both compounds correlation effects favor the configurations with the highest coordination of the metal. The changes with respect to HF results are not very large, so that second‐order perturbative computation of correlation energy is sufficient to give accurate results.
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