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

Abstract

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 σ 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.

The Al+–H2 cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

“…Kemper et al established dissociation energies for the Al + -H 2 and Al + -͑H 2 ͒ 2 complexes of 470± 50 and 385± 50 cm −1 through measurements of the clustering equilibrium constants for the Al + -͑H 2 ͒ n−1 +H 2 Al + -͑H 2 ͒ n reactions. 9,10 One important aspect of the theoretical studies has been investigation of bond activation and formation of the inserted HAlH + species. [9][10][11] Due to the long-range charge-quadrupole interaction, the Al + -H 2 complex preferentially adopts a T-shaped structure, a bonding motif that is preserved in the larger Al + -͑H 2 ͒ n clusters.…”

“…[9][10][11] Due to the long-range charge-quadrupole interaction, the Al + -H 2 complex preferentially adopts a T-shaped structure, a bonding motif that is preserved in the larger Al + -͑H 2 ͒ n clusters. 10 Conversion of the Al + -H 2 complex to the inserted HAlH + form is predicted to proceed by breaking first the H 2 bond, followed by consecutive formation of two equivalent AlH bonds with a calculated activation energy of 85.0 kcal/ mol. Sharp et al found that the electrostatically bound Al + -H 2 complex lies 13.1 kcal/ mol lower in energy than the inserted HAlH + structure.…”

“…9,10 One important aspect of the theoretical studies has been investigation of bond activation and formation of the inserted HAlH + species. 10 The corresponding reaction for the isovalent B + ion, where the inserted HBH + form is 55.9 kcal/ mol more stable than the B + -H 2 complex, was found to be expedited in B + -͑H 2 ͒ n clusters, with a significant lowering in the insertion barrier for n =3 compared to n =1 ͑3 kcal/ mol vs 56 kcal/ mol͒. 10 Conversion of the Al + -H 2 complex to the inserted HAlH + form is predicted to proceed by breaking first the H 2 bond, followed by consecutive formation of two equivalent AlH bonds with a calculated activation energy of 85.0 kcal/ mol.…”

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

“…A similar argument was invoked to explain an analogous effect observed for the Al + /(H 2 ) 1-3 and AlH 2 + /(H 2 ) 1,2 systems. 13 Transition State Formation. One of the most interesting results of this study is the variation in transition state structure and properties with the number of H 2 molecules present.…”

“…Some evidence that 3c-2e bonding is important was inferred from the results of theoretical study of the isovalent systems, 13 Al + + nH 2 , where similar reaction mechanisms were found only for the n ) 1 and n ) 2 cases. Additional support for the important role of 3c-2e bonding was provided by a theoretical study of the isoelectronic systems, 14 Li -+ nH 2 , where a novel 3c-2e bonding scheme was identified in the transition state of the n ) 3 case.…”