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

Stephanie B. Sharp, Blake Lemoine, Gregory I. Gellene

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The reactions of Al+ + nH2 to produce AlH2+(H2)n-1 have been studied by high-level ab initio electronic structure techniques motivated by the ρ bond activation by cooperative interaction observed experimentally and theoretically for the isovalent B+ + nH2 reaction systems. For n = 1, the reaction proceeds stepwise: first breaking the H2 bond and forming one AlH bond followed by the formation of the second AlH bond. This process has an activation energy of 85.0 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism through a planar, cyclic transition state where two H2 bonds are broken simultaneously while two AlH bonds and one new H2 bond are formed. The activation energy for this process decreases from the n = 1 value to about 55.0 kcal/mol. These two cases are qualitatively very similar to what was observed for B+ + nH2 with the major quantitative differences being that corresponding activation energies were 30-40 kcal/mol lower and reaction energetics were 60-80 kcal/mol more exothermic in the boron systems. For n = 3, no additional activation energy lowering was observed with Al+, which contrasts significantly with the behavior observed with B+. This difference is rationalized in terms of the special ability of boron to form strong three center-two electron bonds.

Original languageEnglish
Pages (from-to)8309-8316
Number of pages8
JournalJournal of Physical Chemistry A
Issue number41
StatePublished - Oct 14 1999


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