MU+NO - KINETIC ISOTOPE EFFECTS IN UNIMOLECULAR DISSOCIATION

The thermal addition reaction Mu + NO + M reversible arrow MuNO* + M has been studied by a longitudinal magnetic field muon spin relaxation (mu SR) technique, at room temperature, in the presence of up to 60 atm of N-2 as inert moderator. The pressure dependence of the effective rate constant for Mu addition (k(eff)) demonstrates that the system remains well in the low-pressure regime in this pressure range. The termolecular rate constant, k(0)(Mu) = (8.76 +/- 0.46) x 10(-33) cm(6) molecule(-2) s(-1), is about 5 times smaller than that reported for the corresponding H-atom reaction (Tsang, W.; Herron, J. T. J. Phys. Chem. Ref. Data 1991, 20, 609), the largest isotope effect of its kind yet reported for reactions of this nature and most likely due to the increased rate of MuNO* dissociation resulting from the much lighter mass of the Mu atom (m(Mu)/m(H) similar to 1/9). This result should provide an important test of theoretical calculations for dissociating molecules involving few degrees of freedom. Quantum tunneling, normally facile for the much lighter Mu atom, is unlikely to play a major role in establishing the isotope effect seen in k(0). In the present instance, the mu SR technique relies on measuring the relaxation rate for the chemical process of addition (lambda(c)) in competition with that for spin exchange (lambda(se)), with the NO unpaired electron. The pressure-independent average value for the spin exchange rate constant found, k(se) = (3.00 +/- 0.12) x 10(-10) cm(3) molecule(-1) s(-1), is in good agreement with previous values obtained by transverse field mu SR (Fleming, D. G., et al. J. Chem. Phys. 1980, 73, 2751).