Theoretical Studies of the Low-Lying States of Vinylidenela

The ground and excited states of vinylidene, H2C=C:, were studied using generalized valence bond (GVB) and configuration interaction (GVB-CI) wave functions. We find that the ground state is the singlet state [1A1 (σ2)] with the methylene-like triplet [3B2 (σπ)} lying 2 eV = 46 kcal higher. These states lead to CC bond lengths of 1.35 and 1.36 A, respectively, slightly longer than a normal CC double bond. The ionization potential for singlet vinylidene (1A1) is calculated to be 10.8 eV (out of the π orbital). With extensive CI calculations we find a CC bond energy of De(H2C=C) = 160 kcal/mol or including zero-point effects and estimates of additional correlation effects D0(H2C=C) = 162 kcal/mol. This leads to ΔH298(H2CC:) = 100 kcal/mol. The dipole moment for the ground state is calculated to be 2.23 D (the methylene end negative), while the dipole moment for the 3B2 excited state is 0.55 D.