Influence of the collision energy on the O(1D) + RH --> OH(X2Pi) + R (RH=CH4, C2H6, C3H8) Reaction Dynamics. A laser Induced Fluorescence and Quasiclassical Trajectory Study

Resum

The influence of the collision energy (ET) on the O(¹D) + RH → OH(X²H) + R (RH = CH4, C2H6, and C3H8) reaction dynamics has been studied, using the N2O photodissociation at 193 nm as O(¹D) precursor (〈ET〉 = 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 O3 photodissociation is used to produce O(1D) (〈ET〉 = 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(¹D), 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 A-doublet populations. The reaction takes place near exclusively through the insertion of the O(¹D) 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 ET, the abstraction mechanism also contributes appreciably to reactivity. © 2000 American Chemical Society.