A quantum dynamical treatment of symmetry-induced kinetic isotope effects in the formation of He₂ ⁺

Kinetic isotope effects for He₂ ⁺ formation are calculated quantum dynamically using high-quality Born-Oppenheimer (BO) potentials for two electronic states of He₂ ⁺ and an accurate treatment of all nonadiabatic BO corrections. The two potentials are coupled only when the helium isotopes are different, and the calculations reveal that this coupling is sufficient to allow the two sets of distinguishable reactants, ⁴He⁺ + ³He or ³He ⁺ + ⁴He, to yield He₂ ⁺ with comparable efficiency over a wide temperature range. Consequently, the potential coupling provides a significant formation rate enhancement for the low isotopic symmetry reactants, as compared to the symmetrical cases (e.g., ⁴He⁺ + ⁴He or ³He⁺ + ³He). The computed symmetry-induced kinetic isotope effects (SIKIEs) are in substantial agreement with the available experimental results and represent the first theoretical demonstration of this unusual kinetic phenomenon. Possible application of SIKIE to ozone formation and other chemical systems is discussed.