The torsional barrier for the HCCH group |
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| Affiliation: | 1. Department of Chemistry, Chemical Theory Center, Nanoporous Materials Genome Center and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455-0431, USA;1. Departamento de Farmacia, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, Gto., 36050, Mexico;2. Departamento de Química, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, Gto., 36050, Mexico;3. Departamento de Ingeniería Química, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, Gto., 36050, Mexico;4. Overseas Office GIGAKU Techno Park Network, Nagaoka University of Technology, Noria Alta s/n, Guanajuato, Gto., 36050, Mexico;5. Centro de Investigación en micro y nanotecnología, Universidad Veracruzana, Boca del Río 94294, Veracruz, Mexico;6. Red de Estudios Moleculares Avanzados, Clúster Científico y Tecnológico Biomimic, Campus III, Instituto de Ecología A. C., Carretera Antigua a Coatepec 351, 91073, Xalapa Veracruz, Mexico |
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| Abstract: | Ab initio calculations have been carried out to examine the torsional barrier for non-linear HCCH fragments. At the SCF level the most stable configuration is skew for all geometries studied, but this is not found when correlation is included. For acetylene bond lengths and ∠ HCC = 121.2 °, the Møller-Plesset (MP) perturbation method gives trans as the most stable structure and the resulting torsion potential agrees with that calculated from an analytic many-body-expansion (MBE) potential of Halonen, Child and Carter. When the CC bond length is increased to 1.34 Å (as in ethene) or 1.54 Å (as in ethane), the MP method is poorly convergent and unreliable. Variational (CI) calculations at 1.34 A give a minimum at the trans structure, but at 1.54 A and ∠ HCC = 110.9° a skew structure is slightly more stable than trans. The MBE potential is in poor agreement with these calculations at the longer bond lengths, and is probably in error. |
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