Second‐order Jahn–Teller instability and the activation energy for Al + ( 1 S ) + H 2 → AlH + ( 2 ∑ + ) + H

The interaction of Al + ( 1 S ) ions with H 2 on the lowest electronic energy surface is studied using ab initio electronic structure methods. A C s symmetry transition state is located and found to have the geometry of a product AlH + ion loosely bound to a H atom, consistent with the Hammond postulate for this endothermic reaction. Locating this transition 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 geometrical instability along the path thus generated. This instability was found to be a result of second‐order Jahn–Teller coupling with a low‐lying 1 B 2 electronic state. In addition to these primary findings, a weakly bound T‐shaped Al + ——— H 2 C 2 v van der Waals complex is found that lies only 242 cm −1 below the Al + and H 2 asymptote, with HH internuclear separation only slightly distorted from the equilibrium bond distance of H 2 and AlH distance (3.5 Å) much longer than the covalent bond length in AlH + (1.6 Å). The locally stable but thermodynamically unstable linear HAlH + ( 1 ∑   g + ) species and, of course, the H + AlH + ( 2 ∑ + ) reaction products have also been identified as critical points on the ground‐state surface. Where known, the geometries and energies that we calculte agree well with experimental data. © 1993 John Wiley & Sons, Inc.