σ Bond Activation by Cooperative Interaction with ns2 Atoms: Be + nH2, n = 1−3

Abstract

Ab initio investigations at the MP2, CCSD(T), and MRCISD levels of theory with augmented triple-ζ basis sets have identified and characterized various stationary points on the Be/(H2)n, n = 1−3, hypersurfaces. The van der Waals complexes, Be(H2)n, are very weakly bound (De = 0.08−0.32 kcal/mol with respect to H2 loss) with H2/H2 interactions playing an important role in determining equilibrium structures which can be understood in terms of the various relevant long-range potentials. The covalent molecule, BeH2, is found to have a linear, centrosymmetric structure and to be strongly bound with respect to Be + H2, in agreement with previous calculations. BeH2 interacts weakly with additional H2 molecules (De < 0.75 kcal/mol) which are positioned parallel to the near-linear BeH2 moiety in the equilibrium structures of the BeH2(H2)n-1 complexes. Of particular interest is the dramatic change in the nature of the transition state for BeH2 production depending on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise: first breaking the H2 bond and forming one BeH bond followed by forming the second BeH bond. This process has an activation energy of about 56 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism through a planar cyclic transition state where two H2 bonds are broken while simultaneously two BeH bonds and one new H2 bond are formed. The activation energy for this process decreases from the n = 1 value to about 38 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism with the addition of the third H2 molecule, decreasing the activation energy to about 33 kcal/mol. The results are discussed in comparison to the isoelectronic B+/nH2 systems where significant σ bond activation through a cooperative interaction mechanism has been identified.