Multiconfiguration Self-Consistent Field Study on Formonitrile Imine and N-Substituted Nitrile Imines HCN₂-R: Energy...

Stable geometrical structures for formonitrile imine (1) and N-substituted nitrile imines HCN₂-R (R = Li, BeH, BH₂, CH₃, CN, CCH, C₆H₅, NH₂, OH, and F) (2-11) were examined by using the multiconfiguration self-consistent-field (MCSCF) method followed by second-order configuration interaction (SOCI) calculations and second-order multiconfiguration quasi-degenerate perturbation theory (MCQDPT2) calculations, together with the aug-cc-pVTZ basis sets. The results show that 1 suffers a pseudo-Jahn-Teller (JT) distortion from a linear C_∞v structure to a C₁ structure via a planar bent C_s structure. Each of the others is found to undergo pseudo-JT distortion from a symmetrical structure to a planar bent C_s structure for 2, 3, and 7 and to a C₁ structure for 4, 5, 6, 8, 9, 10, and 11. At the stationary structures of 1-11, the structural characteristics were briefly discussed in terms of allenic and propargylic. To elucidate the nature of pseudo-JT distortions, energy component analyses were carried out at the MCSCF+SOCI level of theory at all of the stationary structures for the relevant molecules. In most of the molecules examined, pseudo-JT stabilizations were classified into two groups, one in which the stability arises from a lowering of the energy of the attractive term V_en and the other in which the stability results from a lowering of the energy of the repulsive terms V_nn and V_ee. In addition to the above two groups, it was also found that the following three groups are responsible for the pseudo-JT stabilizations in a certain stage of the structural changes. Namely, one is a lowering of the energy of the term V_ee observed in 6, another is a lowering of the energy of the terms V_ee and V_en observed in 9-11, and the other is a lowering of the energy of the terms V_en and V_nn observed in 10. These energetic behaviors were accounted in terms of an elongation or a contraction of the molecular skeleton, a migration of electrons from one part of the molecule to other parts, and the combined effects arising from these two factors.