J. Phys. Chem. 1995,99, 17160-17168
17160
Mu
+ NO:
Kinetic Isotope Effects in Unimolecular Dissociation James J. Pan, Masayoshi Senba, Donald J. Arseneau, Alicia C. Gonzalez,’ James R. Kempton: and Donald G. Fleming* TRIUMF and Department of Chemistry, University of British Columbia, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3 Received: May 23, 1995@
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The thermal addition reaction Mu NO M ==MUNO* M has been studied by a longitudinal magnetic field muon spin relaxation @SR) technique, at room temperature, in the presence of up to 60 atm of N2 as inert moderator. The pressure dependence of the effective rate constant for Mu addition (k& demonstrates that the system remains well in the low-pressure regime in this pressure range. The termolecular rate constant, kyu = (8.76 f 0.46) x cm6 molecule-2 s-I, is about 5 times smaller than that reported for the corresponding H-atom reaction (Tsang, W.; Herron, J. T. 1.Phys. Chem. Ref. Datu 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 (mMu/mH I/g). 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 ko. In the present instance, the pSR technique relies on measuring the relaxation rate for the chemical process of addition (Ac) in competition with that for spin exchange (Ase),with the NO unpaired electron. The pressure-independent average value for the spin exchange rate constant found, k,, = (3.00 f 0.12) x lO-’O cm3 molecule-’ s-l, is in good agreement with previous values obtained by transverse field pSR (Fleming, D. G., et al. J . Chem. Phys. 1980, 73, 2751).
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I. Introduction The thermal (re)combination reaction of H atoms with NO, involving the unstable intermediate HNO*, is important in the field of combustion kinetic^^-^ and can be thought of as well as a type of radical-radical r e a ~ t i o n . ~It- ~has been extensively investigated over a range of pressures, and the kinetic data for thermal addition has been reviewed by Tsang and Herron.IoThis reaction has also been studied at much higher energies, where the abstraction reaction becomes important,’ but such an energy regime is not of interest here. The mechanism of thermal kinetics for the H NO combinatiodaddition reaction is well established and has the form
+
H
4 + NO + M z H N O * + M-HNO +M ka
(1)
where k,, kd, and k, are rate constants for addition, (unimolecular) dissociation and stabilization with moderator “M”, respectively. Here it is the formation of HNO* by H-atom addition to the N atom in NO that is important, not the formation of NOH*. The latter has a large activation barrier, -13 kcallmol, in contrast to the zero (electronic) barrier for HNO* formation. I 3 - l 5 Invoking the steady-state assumption, the kinetic scheme in reaction 1 can be described in terms of an effective overall rate constant,
which exhibits the usual limits: at low pressures, when kd >> k , [ M ] , keff,o= ko[M], where k~ = k,k,/kd is the low-pressure termolecular rate constant; at high pressures, when kd
