The addition reaction of X to O-2 (X = Mn, H, D): isotope effects in intra- and intermolecular energy transfer

Rate constants are calculated for the addition reaction of the light hydrog en isotope muonium (Mu) to oxygen and compared with the analogous additions of H and D and with experimental results. The origin and magnitude of kine tic isotope effects is discussed. Both the pressure dependence and the temp erature dependence in different pressure regimes are analysed with the goal to check the applicability of statistical reaction theories to these syste ms. Two different theories are applied. One is based on approximations as i ntroduced by Tree (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 mas ter equation is solved numerically. The microcanonical rate constants going into the master equation are calculated using RRKM theory, and for the act ivation/deactivation as a consequence of collisions with moderator (N-2), a n exponential energy transfer mechanism is assumed. Comparison with low pre ssure experiments leads to the conclusion that collisions of the moderator with highly excited MuO(2)* molecules are much less efficient than those wi th HO2*, as far as activation/deactivation is concerned. This result may be explained by the larger vibrational frequencies of MuO(2)*. The Linear dep endence with the moderator concentration of the Mu rate constants up to 300 bar, as observed in the experiment, could partly be simulated, but some di screpancy remains. In this context, the possibility of non-RRKM behaviour i s discussed. Finally, the major influence of tunneling, especially for the Light Mu atom, is analysed. It is suggested that the effect of tunneling ne ar the low pressure limit may exceed significantly its high pressure limiti ng value.