Sigma Bond Activation by Cooperative Interaction with ns2 Atoms: B+ + nH2

Abstract

The reactions of B+ + nH2 to produce BH2+(H2)n-1 have been studied by high-level ab initio techniques. The reaction mechanism and associated activation energy is found to depend dramatically on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise: first breaking the H2 bond and forming one BH bond followed by forming the second BH bond. This process has an activation energy of about 57 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism though a planar cyclic transition state where two H2 bonds are broken while simultaneously two BH bonds and one new H2 bond are formed. The activation energy for this process decreases dramatically from the n =1 value to only about 11 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism; however, the actual insertion occurs late in the reaction after over 75% of the exothermicity has been realized. The addition of the third H2 molecule decreases the activation energy to only about 3.4 kcal/mol. For n = 4,...