Valence-Rydberg banding in bimolecular R-Ca+center dot NH2-R ' complexes

Ab initio molecular orbital calculations were performed on species associat ed with the bimolecular reaction of protonated methylamine (CH3NH3+) with m ethylcalcium (CaCH3) to form the valence-Rydberg bonded complex H3CH3N.+CaC H3. Gradient geometry optimizations and frequency calculations were perform ed at levels of theory up to and including UMP2(full)/6-311 G(d,p) augmente d by diffuse functions on the nitrogen atom. The complex H3CH3N.+CaCH3 is b ound by 9.77 kcal mol(-1) relative to reactants at the projected second-ord er Moller-Plesset perturbation theory (PMP2) level, and both the charge and unpaired spin densities are delocalized between calcium and the adjacent a mino group, the region of the valence-Rydberg bond. A rearranged product. [ H3CH2N:-->CaH ... CH3](+.), lies 12.38 kcal mol(-1) below the valence-Rydbe rg complex, and is bound by 9.66 kcal mol(-1) relative to dissociation prod ucts [H3CH2N:-->CaH](+) + . CH3. The rearranged species is a distonic ion, with the positive charge mainly on calcium, and the unpaired electron on th e carbon atom of the quasi-planar CH3 moiety. A transition structure interc onverting the two isomers lies ca. 21 kcal mol(-1) above the higher-energy isomer, but is far above the dissociation limits of either species. Thus, i n the gas phase at thermal energies, the valence-Rydberg complex and the lo wer-energy isomer [H3CH2N:-->CaH . .. CH3](+.) will not interconvert throug h this transition state.