1,1-Dilithioethylene. A ground-state triplet olefin with nearly free rotation about the double bond

Abstract

Molecular electronic structure theory has been applied to the CLi/sub 2/==CH/sub 2/ molecule 1,1-dilithioethylene. Both planar and triplet structures were considered for each of the lowest singlet and triplet electronic states. Geometry optimizations were carried out at the self-consistent-field (SCF) level of theory using a basis set of better than double zeta quality: C(9s 5p 1d/4s 2p 1d), Li(9s 4p/4s 2p), H(4s/2s). The predicted C==C bond distances are 1.356 (planar singlet), 1.334 (twisted singlet), 1.322 (planar triplet), and 1.323 A (twisted triplet). The analogous Li-C-Li bond angles are 133.6, 104.1, 73.9, and 75.5/sup 0/, while the corresponding C-Li bond distances are 2.000, 1.866, 2.106, and 2.064 A. SCF theory predicts the twisted triplet to be the ground state, followed energetically by the planar triplet (1.2 kcal), twisted singlet (28.4 kcal), and planar singlet (29.3 kcal). The effects of electron correlation were investigated by configuration interaction (CI) including single and double excitations. The ordering of states is unchanged, with the relative energies being 0.0, 1.4, 14.0, and 15.5 kcal. After Davidson's correction for the effects of unlinked clusters, the same relative energies become 0.0, 1.4, 10.5, and 12.5 kcal. Qualitative features of the CLi/sub 2/CH/sub 2/ electronic structures are discussed inmore » terms of orbital energies, Mulliken populations, and predicted dipole moments. 3 figures, 4 tables.« less