2ND-ORDER JAHN-TELLER INSTABILITY AND THE ACTIVATION-ENERGY FOR AL-1)+H-2-]ALH+((2)SIGMA(+))+H((S)

The interaction of Al+ (S-1) ions with H-2 on the lowest electronic en ergy surface is studied using ab initio electronic structure methods. A C-S symmetry transition state is located and found to have the geome try of a product AlH+ ion loosely bound to a H atom, consistent with t he Hammond postulate for this endothermic reaction. Locating this tran sition state, beginning at geometries that characterize vibrationally cold H-2 and translationally hot Al+, posed special challenges to the commonly used ''hill-climbing'' algorithm because of regions of geomet rical instability along the path thus generated. This instability was found to be a result of second-order Jahn-Teller coupling with a low-l ying B-1(2) electronic state. In addition to these primary findings, a weakly bound T-shaped Al+---H-2 C-2v van der Waals complex is found t hat lies only 242 cm(-1) below the Al+ and H-2 asymptote, with H-H int ernuclear separation only slightly distorted from the equilibrium bond distance of H-2 and an Al-H distance (3.5 Angstrom) much longer than the covalent bond length in AlH+ (1.6 Angstrom). The locally stable bu t thermodynamically unstable linear HAlH+ ((1) Sigma(g)(+)) species an d, of course, the H + AlH+((2) Sigma(+)) reaction products have also b een identified as critical points on the ground-state surface. Where k nown, the geometries and energies that we calculate agree well with ex perimental data. (C) 1993 John Wiley & Sons, Inc.