Unusual Thresholds and Isotope Effects in Al+ + H2/D2/HD Reactions

Ab initio quantum chemistry is used to generate a three-dimensional reactive potential-energy surface for the collision of 1S Al+ ions with ∑g H2 molecules. This surface, in a tessellated and locally interpolated form, is used to generate forces for classical trajectory simulations of the 3.98 eV endothermic Al+ + H2 f AlH+ + H reactions with initial conditions appropriate to a thermal H2 sample and an Al+ beam of specified center of mass collision kinetic energies in the 3-20 eV range. Our findings indicate that the reaction occurs not on (or near) the collinear path, which has no barrier above the reaction endothermicity, but via a near-C2V insertive path which spontaneously breaks C2V symmetry via second-order Jahn-Teller distortion to permit flux to evolve to AlH+ + H products. The strong propensity to “avoid” the collinear path and to follow a higherenergy route is caused, at long range, by the ion-quadrupole interaction between Al+ and H2 and, at shorter range, by favorable overlap between the H2 σu and Al+ 3p obitals. Examination of a large number of trajectories shows clearly that reactive collisions (1) lose much of their initial kinetic energy to the repulsive ionmolecule interfragment potential as the closed-shell Al+ and H2 approach, (2) transfer significant energy to the H-H stretching coordinate, thus weakening the H-H bond, (3) convert initial H2 rotational motion as well as Al+ to H2 collisional angular momentum into rotational angular momentum of the HAlH+ complex, “locking” the H2 moiety into the insertive near-C2V geometry about which twisting motion occurs, and (4) allow the Al+ ion to form a new bond with whichever H atom is nearest it when the system crosses into regions of the energy surface where the H-Al-H asymmetric stretch mode becomes second-order JahnTeller unstable, thus allowing fragmentation into AlH+ + H. These findings, combined with considerations of kinematic factors that distinguish among H2, D2, and HD, allow us to explain certain unusual threshold and isotope effects seen in the experimental reaction cross-section data on these reactions.