Volume 127, number 6 CHEMICAL PHYSICS LETTERS 4 July 1986 OSCILLATING REACTIVITY IN QUASICLASSICAL TRAJECTORY CALCULATIONS FOR THE REACTION O(3P) + HCI + OH + Cl Avigdor PERSKY and Haya KORNWEITZ Department of Chemutty, Bar-Ilan University, Ramat-Gan 52100, Israel Received 18 April 1986 Strong oscillations m the reactivity as a function of collision energy were found in collinear quasiclassical trajectory calculations for the reaction 0( 3 P)+ HCl( v = 0) + OH + Cl. Oscillations were also observed m 3D calculations for IICl( v = 0, J = 0) under the special conditions of near-zero impact parameters b and small and specific values of the azimuthal orientation angle &, the angle between the molecular axis of HCI and the mittal direction of the relative velocity vector. These oscillations were washed out when t9, and b were randomly selected from the appropriate ranges, and were not observed even for the reactive scattering in the angular ranges 170 to 180” and 175 to 180”. 1. Introduction Reactions of the type H t LH’ -+ HL + H’ (a light atom L is transferred between two heavy atoms H and H’) have become, in recent years, the subject of numerous theoretical and experimental studies (see for example refs. [l-3] and many other references therein). An interesting feature of such reactions is the oscillation in reaction probability as a function of collision energy observed in collinear quantum- mechanical and quasiclassical calculations. For ex- ample, oscillations were observed in quantum- mechanical calculations for the reactions Cl + HCl [1,4], I + HI [5,6], Br t HCl [7] and F t DBr [2] and in quasiclassical trajectory calculations for the reactions Cl t HCI [ 1,8] and I t HI [5]. Even the os- cillations found in the quantum-mechanical calculations were described as being of classical origin [ 3,5,9]. The possibility of observing oscillations in three- dimensional (3D) calculations was discussed in several publications. Pollak et al. [3] constructed a model for symmetric light-atom transfer reactions which predicted oscillations in the 3D cross sections for reactions which occur on a collinearly dominated potential energy surface with a low barrier. Hiller et al. [6] expect to find oscillations in the energy de- pendence of the cross section at a fixed scattering angle, rather than for the total cross sections. Bear and Last [8] performed 3D quasiclassical trajectory calculations for the reaction Cl t zyxwvutsrqponmlkjihgfedcbaZY HCl + ClH t Cl. The cross sections for scattering in the angular range 160- 180’ revealed strong oscillations which seemed to follow rather well the oscillations observed in the collinear calculations for the same system. We carried out collinear and 3D quasiclassical trajectory calculations for the heavy-light-heavy reaction 0(3P) + HCl -+ OH + Cl. The purpose of these calculations was to examine the copditions under which oscillations in the reactivity occur. 0 t HCl has a mass combination similar to that of Cl t HCI and very similar energetics. It is nearly thermo- neutral (A@ = 0.9 kcal/mole) and has an activation energy of about 6.6 kcal/mole [IO] * which is com- parable to the activation energy of about 5.5 kcal/mol ** for Cl t HCl which is thermoneutral. The po- tential barriers for these two reactions, for the col- linear configurations, are also comparable, 8.1 kcal/mol for 0 + HCI (present study) and 8.3 kcal/mol for Cl + HCl [8,14]. * See also table 4 of ref. [ 1 l] in which a summary of the available results is given. ** Evaluated by Garrett et al. [ 12) from relative rates mea- sured by Klein et al. [ 131 and other appropriate rate data. 0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 609