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The lowest singlet and triplet electronic states of methylene, HzC:. vinylidene, HzCC:, and vinylidenecarbene, HzCCC:, are studied theoretically at the spin-unrestricted Hartree-Fock (H F) and spin-unrestricted many-body perturbation theory (MBPT) levels of approximation. Calculations at both levels predict a triplet ground state for methylene and singlet ground states for vinylidene and vinylidenecarbene. MBPT singlet-triplet energy splittings are found to be - 13.1 kcal/mol for methylene, 51 .I kcal/mol for vinylidene, and 48.7 kcal/mol for vinylidenecarbene. A comparison of the effects of correla- tion on electronic structure is made between the parent carbene, methylene, and each of these unsaturated carbenes. From electronic structure trends observed in HzC:, HzCC:, and H2CCC:, singlet ground states are predicted for the extended unsatu- rated carbenes HzCCC:, and H2CCCC:, the next members of the series. The importance of d-type carbon polarization func- tions in basis sets used for the calculation of electronic structure properties in unsaturated carbenes is demonstrated by com- parison with results in which d functions are not included in the basis.

Low-lying electronic states of unsaturated carbenes. Comparison with methylene