Sigma bond activation by cooperative interaction with s(2) 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 th e B+/nCH(4), n = 1, 2 hypersurfaces. The electrostatic complexes show a str ong variation in the sequential binding energy with D-e 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 geometry and to be strongly bound by 84.0 kcal mol(-1 ) with respect to B+ + CH4. The interaction of CH4 with CH(3)BH(+)is qualit atively very similar to the interaction of CH4 with HBH+, however, the bind ing is only about 50% as strong due to the electron donating characteristic of the methyl group. Of particular interest are the insertion transition s tates 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 th e insertion activation energy by about 25 kcal mol(-1) from that expected f or 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 le ading to an additional activation energy decrease of about 15.7 kcal mol(-1 ) relative to B+ + nCH(4).