The torsional barrier for the HCCH group

• Published in: Chemical physics letters 1987-02, Vol.134 (1), p.12-16
• Main Authors: Marks, A., Murrell, J.N., Pedley, J.B.
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Electron correlation calculations for atoms and molecules; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Physics
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/0009-2614(87)80004-0

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.