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, E_c, 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.