Density functional calculations of the influence of substitution on singlet-triplet gaps in carbenes and vinylidenes

Density functional theory calculations of multiplet splittings are presented that agree closely with experimental measurements for six carbenes and six vinylidenes. The calculations are further analyzed to gauge the relative importance of different factors influencing the stabilities of the different spin and electronic states. In the carbene series, with halogen substituents, orbital rehybridization effects and charge redistribution effects are large. The magnitude of π-conjugation (back-bonding) is calculated to be only moderately larger (6-8 kcal mol^-1) for singlets than for triplets based on natural bond orbital-derived conjugation energies. In the vinylidene series, substituion effects are primarily associated with through-space and through-bond inductive stabilization effects, especially hyperconjugation; π-conjugation effects are found to be small