Second‐order Jahn–Teller instability and the activation energy for Al⁺(¹S) + H₂ → AlH⁺(²∑⁺) + H

The interaction of Al⁺ (¹S) ions with H₂ 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 H₂ 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 ¹B₂ electronic state. In addition to these primary findings, a weakly bound T‐shaped Al⁺ ——— H₂ C_2v van der Waals complex is found that lies only 242 cm⁻¹ below the Al⁺ and H₂ asymptote, with HH internuclear separation only slightly distorted from the equilibrium bond distance of H₂ and AlH 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.