1,1-DILITHIOETHYLENE - GROUND-STATE TRIPLET OLEFIN WITH NEARLY FREE ROTATION ABOUT THE DOUBLE-BOND

Molecular electronic structure theory has been applied to the CLi2=CH2 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 ζ quality: C(9s 5p 1d/4s 2p Id), 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 Å (twisted triplet). The analogous Li-C-Li bond angles are 133.6,104.1, 73.9, and 75.5°, while the corresponding C-Li bond distances are 2.000, 1.866, 2.106, and 2.064 Å. 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 CLi2CH2 electronic structures are discussed in terms of orbital energies, Mulliken populations, and predicted dipole moments. © 1979, American Chemical Society. All rights reserved.