Theoretical Study of the Validity of the Born-Oppenheimer Approximation in the Cl + H ₂ → HCl + H Reaction

Abstract

Reactivity of the excited spin-orbit state of Cl with H2 to yield ground-state HCl products is forbidden by the Born-Oppenheimer (BO) approximation. We used new ab initio potential energy surfaces and exact quantum scattering calculations to explore the extent of electronic nonadiabaticity in this reaction. In direct contrast to recent experiments, we predict that the BO-allowed reaction of the ground spin-orbit state will be much more efficient than the BO-forbidden reaction of the excited spin-orbit state. Also, Coriolis coupling opens up an electronically nonadiabatic inelastic channel, which competes substantially with reaction.

Spin-orbit relaxation of Cl(P1∕22) and F(P1∕22) in a gas of H2

“…This demonstrates that the interaction potential surfaces computed by Klos et al 48 and the theoretical approach based on the diabatic transformation of Baer, 69 Rebentrost and Lester, 21 and Alexander et al 1,70,71 are accurate for the simulations of nonadiabatic dynamics in the entrance channel of the Cl͑ 2 P͒ +H 2 chemical reaction. Alexander et al 3 used the same approach to study the effects of the spin-orbit interaction in the the Cl͑ 2 P͒ +H 2 chemical reaction and the agreement of our results with the experimental data for nonreactive interactions thus supports their conclusions.…”

“…3 The same interactions determine the dynamics of spin-orbit transitions in nonreactive collisions of halogen atoms with H 2 . The analysis of fine structure relaxation in prereactive F͑ 2 P͒ -H 2 and Cl͑ 2 P͒ -H 2 complexes may therefore elucidate the mechanisms of the chemical reactions and help resolve the disagreement with the experiment.…”

“…The reaction of F with H 2 is a paradigm for exothermic abstraction reactions of atoms with diatomic molecules 1,2 and the reaction of Cl with H 2 is a prototype of chemical reactions of chlorine with hydrocarbons, 2,3 which play an important role in many atmospheric phenomena. The reactions of F and Cl with H 2 have therefore been studied theoretically and experimentally by many authors.…”

Extraction of state-to-state reactive scattering attributes from wave packet in reactant Jacobi coordinates

“…An extensive body of work, both theoretical [63][64][65][66][67][68] and experimental, 69-74 also exists for the Cl+ H 2 system and its isotopic variations. This system has played a central role in chemical kinetics and has served as a test case for bimolecular reaction rate theory.…”

“…77,66 However, the recent focus has been on the disagreement between theory and experiment for the Cl͑ 2 P 1 2 / 3 2 ͒ +H 2 reaction including spin-orbit effects. 64,72,[78][79][80][81] In the following numerical calculations, the benchmark BW PES 68 is used.…”