Influence of the collision energy on the O(1D)+RH→OH(X2Π)+R (RH = CH4, C2H6, C3H8) reaction dynamics:: A laser-induced fluorescence and quasiclassical trajectory study

The influence of the collision energy (E-T) on the O(D-1) + RH --> OH(X(2)Pi) + R (RH = CH4, C2H6, and C3H8) reaction dynamics has been studied, using the N2O photodissociation at 193 nm as O(D-1) precursor ([E-T] = 0.403 eV) and probing the OH v " = 0 and 1 levels by LIF. A triatomic QCT study of the reaction with CH4 on a fully ab initio based analytical PES has also been performed, and a quite good agreement with the experimental OH rovibrational distributions has been obtained. Our experimental results are similar to those obtained when the O-3 photodissociation is used to produce O(D-1) ([E-T] = 0.212 eV), as expected on the basis of the available energy in products and also from the QCT calculations. The P(v " = 0)/P(v " = 1) populations ratio values reported for C2H6 and C3H8 in a very recent work (Wada and Obi, J. Phys. Chem. A 1998, 102, 3481), where the N2O was also used to generate O(D-1), are probably largely underestimated. The rotational distributions obtained are similar to those obtained in other experiments, and a quite good agreement has been obtained for the spin-orbit and Lambda-doublet populations. The reaction takes place near exclusively through the insertion of the O(D-1) atom into a C-H bond below 0.6 eV, and the mechanism may be direct or nondirect (mainly through short-lived (CH3)OH collision complexes) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect one leads to a noninverted distribution. At higher E-T, the abstraction mechanism also contributes appreciably to reactivity.