σ bond activation by cooperative interaction with ns2 atoms:: Be+nH2, n=1-3

Ab initio investigations at the MP2, CCSD(T), and MRCISD levels of theory with augmented triple-xi basis sets have identified and characterized various stationary points on the Be/(H-2)(n), n = 1-3, hype;surfaces. The van der Waals complexes, Be(H2)(n), are very weakly bound (D-e = 0.08-0.32 kcal/mol with respect to H-2 loss) with H-2/H-2 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 + H-2,in agreement with previous calculations. BeH2 interacts weakly with additional H-2 molecules (D-e < 0.75 kcal/mol) which are positioned parallel to the near-linear BeH2 moiety in the equilibrium structures of the BeH2(H-2)(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 H-2 molecules present. For n = 1, the reaction proceeds stepwise: first breaking the HZ 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 H-2 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 H-2d molecule, decreasing the activation energy to about 33 kcal/mol. The results are discussed in comparison to the isoelectronic B+/nH(2) systems where significant a bond activation through a cooperative interaction mechanism has been identified.