Isotopic substitution of muonium for hydrogen provides an unparalleled opportunity to deepen our understanding of quantum mass effects on chemical reactions. A recent topical review in this journal of the thermal and vibrationally state-selected reaction of Mu with H2 raises a number of issues that are addressed here. We show that some earlier quantum mechanical calculations of the Mu + H2 reaction, which are highlighted in this review, and which have been used to benchmark approximate methods, are in error by as much as 19% in the low-temperature limit. We demonstrate that an approximate treatment of the Born-Oppenheimer diagonal correction that was used in some recent studies is not valid for treating the vibrationally state-selected reaction. We also discuss why vibrationally adiabatic potentials that neglect bend zero-point energy are not a useful analytical tool for understanding reaction rates, and why vibrationally non-adiabatic transitions cannot be understood by considering tunnelling through vibrationally adiabatic potentials. Finally, we present calculations on a hierarchical family of potential energy surfaces to assess the sensitivity of rate constants to the quality of the potential surface.
Bibliographical noteFunding Information:
The US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences supported work at the University of Minnesota [grant number DE-FG02-86ER13579] and Pacific Northwest National Laboratory (PNNL) [contract number DE-AC05-76RL01830]. PNNL is a multiprogram national laboratory operated for DOE by Battelle Memorial Institute.
- BornOppenheimer diagonal correction
- cumulative reaction probability
- density of reactive states
- kinetic isotope effects
- quantum mechanical reactive scattering
- transition state
- vibrational adiabaticity
- vibrational coupling
- zero-point energy