Stabilization of methyl anions by first-row substituents. The superiority of diffuse function-augmented basis sets for anion calculations

The entire set of methyl anions, XCH₂^?, substituted by first-row substituents, Li, BeH, BH₂, CH₃, NH₂, OH, and F, was examined at various ab initio levels. Diffuse orbital-augmented basis sets, such as 4?31+G and 6?31+G *, are needed to describe the energies of these anions adequately. Estimates of proton affinities are further improved by second-order Møller–Plesset (MP 2) electron correlation corrections, but relative energies are less affected. The methyl group in the ethyl anion is destabilizing, the amino substituent is borderline, but all other groups are stabilizing. Very large π effects are exhibited by BH₂ and BeH groups; inductive stabilization by the electronegative F and OH groups is less effective. Lithium also is stabilizing, but the best singlet geometry of CH₂Li^? is not planar. A planar CH₂Li^? triplet with a π¹ configuration may be lower in energy.