Generalized valence bond study of rotational singlet structures and π bond energies for systems containing C=C, Si=Si, and C=Si double bonds

Abstract

Ab initio GVB(6/12)/6-31G^(**) calculations were performed on A_2X=YB_2 (A, B = H, F; X, Y = C, Si) to obtain the optimized geometries for planar and twisted singlet structures, and to also calculate π bond energies (rotational barriers). The nature of C-C, Si-Si, and C-Si π bonds has been investigated. The results show that the C-C π bond energy (E_π (ethene) = 65.4 kcal/mol) decreases with increasing fluorine substitution. The pyramidalization at the carbon or silicon center for the twisted structures decreases the π bond energies in the substituted ethenes and their silicon counterparts. The Si-Si (E_π (disilene) = 23.2 kcal/mol) and C-Si (E_π (silaethene) = 31.6 kcal/mol) π bonds become much weaker. Fluorine substitution stabilizes both the diradical and the dipolar twisted singlet structures.