Energy distribution among reaction products. Part 2.—H + X2 and X + HY

Abstract

Two methods are described for obtaining information from infra-red chemiluminescent spectra regarding vibrational, rotational and electronic excitation in the products of some simple reactions. In method I, vibrational relaxation is measured. In method II, the objective is to arrest gas-phase relaxation. This paper reports on the present status of experiments by both methods, as they relate to the reactions H + X₂→HX + X and X + HY→XH + Y, where X and Y are halogen atoms. The main findings are the following. (i) The two methods yield k(v)(rate constants into specified vibrational states) in satisfactory agreement, for H + Cl₂→HCl + Cl. Method I yields approximate values for probabilities of vibrational deactivation of HCl, v= 1–5, by H₂. (ii)k(v) against v for H + Cl₂, reading downwards from the highest v(v_max= 6), first rises steeply (k(v= 3)≈ 300×k(v= 6)) and then falls (k(v= 1)≈ 0·2×k(v= 3)). The function corresponds to a mean fraction of the available energy entering vibration of k(ƒ_v)≈ 0·45 according to methods I and II. (iii) For the reaction H + Br₂→HBr + Br, k(ƒ_v)≈ 0·55, by method II. (iv) For Cl + HI→ClH + I, k(ƒ_v)≈ 0·65, by method II. Laser emission was observed in this system. Some results are reported for Cl + HBr and Br + HI. (v) First results are presented for the fractional conversion of available energy into rotation, k(ƒ_J). For Cl + HI, k(ƒ_v)+k(ƒ_J)≈ 0·8, leaving little energy for relative translational motion of the products, k(ƒ_T) 1. (vi) There is evidence from the experiments on H + Br₂ and X + HY that the reaction path with which we are concerned here is the one leading to the formation of the halogen atom in its ground electronic state, ²P.

Three-dimensional classical computations on these systems, using London-Eyring-Polanyi-Sato (L.E.P.S.) potential-energy hypersurfaces are reported. They tend to confirm the earlier hypothesis that these reactions proceed by direct interaction, with a major portion of the energy released as the products separate (“repulsive” energy release).