Sigma bond activation by cooperative interaction with s2 atoms: B+ + nCH4, n = 1, 2

Abstract

Ab initio investigations at the MP2 and CCSD(T) level with augmented double and triple zeta basis sets have identified various stationary points on the B⁺/nCH₄, n = 1, 2 hypersurfaces. The electrostatic complexes show a strong variation in the sequential binding energy with D_e for the loss of one CH₄ moleucle calculated to be 16.5 and 6.8 kcal mol⁻¹ for the n = 1 and n = 2 complexes, respectively. The covalent molecular ion, CH₃BH⁺, is found to have the expected C_3v geometry and to be strongly bound by 84.0 kcal mol⁻¹ with respect to B⁺ + CH₄. The interaction of CH₄ with CH₃BH⁺ is qualitatively very similar to the interaction of CH₄ with HBH⁺, however, the binding is only about 50% as strong due to the electron donating characteristic of the methyl group. Of particular interest are the insertion transition states which adopt geometries allowing the B⁺ ion to interact with multiple sigma bonds. In the n = 1 case, the interaction with two CH bonds lowers the insertion activation energy by about 25 kcal mol⁻¹ from that expected for a mechanism involving only one sigma bond. For n = 2, B⁺ interacts with two CH sigma bonds from one CH₄ and one CH sigma bond from the other CH₄ leading to an additional activation energy decrease of about 15.7 kcal mol⁻¹ relative to B⁺ + nCH₄.