The addition reaction of X to O2 (X = Mu, H, D): isotope effects in intra- and intermolecular energy transfer

Abstract

Rate constants are calculated for the addition reaction of the light hydrogen isotope muonium (Mu) to oxygen and compared with the analogous additions of H and D and with experimental results. The origin and magnitude of kinetic isotope effects is discussed. Both the pressure dependence and the temperature dependence in different pressure regimes are analysed with the goal to check the applicability of statistical reaction theories to these systems. Two different theories are applied. One is based on approximations as introduced by Troe (J.Chem.Phys., 1977, 66, 4758; J.Phys.Chem., 1979, 83, 114; and J.Phys.Chem., 1981, 75, 226), while in the second one, the master equation is solved numerically. The microcanonical rate constants going into the master equation are calculated using RRKM theory, and for the activation/deactivation as a consequence of collisions with moderator (N₂), an exponential energy transfer mechanism is assumed. Comparison with low pressure experiments leads to the conclusion that collisions of the moderator with highly excited MuO₂^* molecules are much less efficient than those with HO₂^*, as far as activation/deactivation is concerned. This result may be explained by the larger vibrational frequencies of MuO₂^*. The linear dependence with the moderator concentration of the Mu rate constants up to 300 bar, as observed in the experiment, could partly be simulated, but some discrepancy remains. In this context, the possibility of non-RRKM behaviour is discussed. Finally, the major influence of tunneling, especially for the light Mu atom, is analysed. It is suggested that the effect of tunneling near the low pressure limit may exceed significantly its high pressure limiting value.