Probing the Unusual Thresholds of AlH+/AlD+ formation by Molecular Dynamic Simulations on MRCI Potential Energy Surfaces

In an experiment performed by P. Armentrout (Int. Rev. Phys. Chem. \textbf{1990}, $9$, 115), the Al$^{+}$ cation was accelerated into the various isotopic combinations of H$_{2}$ to form AlH$^{+}$ and AlD$^{+}$. It was found that the product-forming reactions proceed very inefficiently. The experiments also showed a reduction of $\sim $29{\%} in the threshold for the formation of AlD$^{+}$ from the HD reactant whereas all other AlH$^{+}$ and AlD$^{+}$ products formed at the same energetic threshold. Four previous theoretical attempts at capturing this unusual phenomenon have not been successful. The lowest energy singlet surfaces for the reaction of Al$^{+}$ with H$_{2}$ have been calculated at the multi-reference configuration interaction (MRCI) level of theory. The real/imaginary boundary of the symmetry-breaking b$_{2}$ vibrational mode was examined in three dimensions using Hessian matrices computed at a multi-configurational self-consistent field (MCSCF) level of theory. Molecular dynamic simulations numbering on the order of 10$^{7}$ were performed, sampling initial conditions reflective of the experiments. The simulations were run until they reached the location where the b$_{2}$ vibrational mode became unbound. A dissociation model was applied at these greatly compressed geometries to model the dissociation into AlH$^{+}$ and AlD$^{+}$ products.