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

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 De for the loss of one CH4 molecule 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 C3 nu 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.