MUONIUM REACTION-KINETICS WITH THE HYDROGEN HALIDE GASES

The reaction rates of the muonium (Mu) atom with HBr and HI in approximately 1 atm N2 moderator have been measured over the temperature range 160-490 K using the muSR technique. While both abstraction and exchange reactions are possible, only the abstraction reaction should be observable, being moderately exothermic. Comparisons with the corresponding H(D) reactions reveal small kinetic isotope effects in both reactions, which do not vary strongly with temperature (k(Mu)/k(H) almost-equal-to 3.5 near 300 K), consistent with the (classical) ratio of mean velocities. Surprisingly, quantum tunneling, normally facile for similarly exothermic reactions of the ultralight Mu atom (m(Mu)/M(H) almost-equal-to 1/9), appears to be of little importance here. This despite the fact that the (temperature-independent) experimental activation energies are much less than the expected vibrationally adiabatic barrier heights (estimated to be almost-equal-to 1. 5 kcal mol-1) and, particularly in the case of Mu + HI, much less than the corresponding H-atom activation energy: 0. 13 +/- 0.03 vs 0.70 +/- 0. 3 kcal mol-1. In the case of reactions with HBr, the experimental Mu- and H-atom activation energies are much more similar: 0. 51 +/- 0.03 and 0.74 +/- 0. 12 kcal mol-1, respectively, over comparable temperature ranges. These data pose a conundrum in which several compensating effects related to the much lighter Mu-atom mass seem to be involved. Theoretical calculations are urgently required. In our view the topography of the potential-energy surface(s) for H-2X is poorly known, particularly in the region of the barrier. It may be that the abstraction barriers for both Mu + HI and Mu + HBr are considerably later and even smaller than current calculations indicate, resulting in a cancellation of the effects of zero-point-energy shifts and quantum tunneling at the transition state. Differences in skewing angles between Mu and H + HX could favor a shorter tunneling path for the H-atom reaction, possibly compensating for its heavier mass. Steric or rebound effects from "bottlenecks" on the (mass-weighted) potential surfaces for Mu reactivity may also play some role. An upper limit for the 300 K reaction rate of Mu + HCI is given as well. In contrast to both HBr and HI, this reaction is quite endothermic and hence exhibits an inverse kinetic isotope effect (k(Mu) much less than k(H)).