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

Abstract

Ab initio quantum chemistry is used to generate a three-dimensional reactive potential-energy surface for the collision of 1 S Al + ions with 1 ∑ g + H 2 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 + + H 2 f AlH + + H reactions with initial conditions appropriate to a thermal H 2 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-C 2V insertive path which spontaneously breaks C 2V 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 H 2 and, at shorter range, by favorable overlap between the H 2 σ u 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 H 2 approach, (2) transfer significant energy to the H-H stretching coordinate, thus weakening the H-H bond, (3) convert initial H 2 rotational motion as well as Al + to H 2 collisional angular momentum into rotational angular momentum of the HAlH + complex, "locking" the H 2 moiety into the insertive near-C 2V 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 H 2 , D 2 , and HD, allow us to explain certain unusual threshold and isotope effects seen in the experimental reaction cross-section data on these reactions.

The Al+–H2 cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

“…10 Conversion of the Al + -H 2 complex to the inserted HAlH + form is predicted to proceed by breaking first the H 2 bond, followed by consecutive formation of two equivalent AlH bonds with a calculated activation energy of 85.0 kcal/ mol. 10 Related experimental 14 and theoretical [15][16][17][18] studies have focused on the Al + ͑ 1 S͒ +H 2 ͑ 1 ⌺ g + ͒ → AlH + ͑ 2 ⌺ + ͒ +H͑ 2 S͒ reaction which is 3.98 eV endothermic. 12,13 Although the insertion barrier is also predicted to be lowered for the larger Al + -͑H 2 ͒ n clusters, it remains appreciable ͑53.2 kcal/ mol for n =3͒.…”

Molecular Dynamics of Complex Gas-Phase Reactive Systems by Time-Dependent Groups

“…This difficulty is also being worked on by storing the group potential energy data to fast-access databases and combining these databases with accurate and computationally facile general methods of interpolation so that eventually the computationally expensive calculation of the ab initio intragroup interactions will yield to fast methods of interpolation. Interpolants similar to previous work − are being generalized for any dimensionality, and studies are being performed on methods of optimizing the interpolants so as to obtain general, accurate, and fast methods of PES interpolation.…”

Global potential energy surfaces for the Al+(1S)+H2 system

“…The work of Gutowski et al,13 Chacon-Taylor and Simons,14 and Bell and Simons15 have each attempted to understand the unusual results. The work of Gutowski et al,13 Chacon-Taylor and Simons,14 and Bell and Simons15 have each attempted to understand the unusual results.…”