Monte Carlo trajectory study of the family of reactions H + X2? HX + X(X ? F, Cl, Br, I)

Abstract

The 3D classical trajectory method has been used to investigate the dynamics of the H + X2→ HX + X(X F, Cl, Br, I) exchange reactions using LEPS (London, Eyring, Polanyi, Sato) potential energy hypersurfaces without any ad hoc adjustment of the parameters to fit experiment. A collision energy T= 10 kcal mol–1 was used throughout. Where possible, computed dynamical reaction properties are compared with experimental data. The principal findings are as follows : (a) For the series X F, Cl, Br the computed average fractions of available reaction energy entering product vibration, 〈ƒ′V〉, and rotation, 〈ƒ′R〉, are in fair accord with experimental results, the largest discrepancy being observed for X F (significantly, this case is also marked by having its barrier-height, Ec, out of sequence). (b) Along the series X Cl, Br, I the computed average fractions of available reaction energy entering product translation 〈ƒ′T〉, and hence the fraction becoming product internal excitation 〈ƒ′int〉=(〈ƒ′V〉+〈ƒ′R〉), correctly mirror the experimental trend of progressive diminution in 〈ƒ′T〉 and, enhancement in 〈ƒ′int〉. (c) For the series X Cl, Br, I the computations predict increasing forward scattering of the molecular product, with the calculated angular distributions passing from predominantly backward scattering to sideways-peaked scattering. This behaviour is in qualitative accord with experiment. Increasing internal excitation along the series H + Cl2, Br2, I2 correlates with increased attractive energy release; the kinematic effect of alteration in the reacting masses was shown to be a negligible factor.