Energy Distribution Among Reaction Products. V. H + X2 (X ≡ Cl, Br), D + Cl2

Abstract

The ``arrested relaxation'' infrared chemiluminescence technique has been used to obtain the distribution of vibrational, rotational, and translational energies (V′, R′, and T′) in the products of the exothermic reactions (i) H + Cl2 → HCl + Cl, (ii) its isotopic analog D + Cl2 → DCl + Cl, and (iii) H + Br2 → HBr + Br. Detailed rate constants k(V′, R′, T′) are reported in the form of contour plots for these three reactions. The total detailed rate constants into specified vibrational quantum states (summed over the rotational levels of each v′ level) are: (i) for H + Cl2, k(v′ = 1) = 0.28, [k(v′ = 2) = 1.00], k(v′ = 3) = 0.92, k(v′ = 4) = 0.1, k(v′ = 5) = 0.05, k(v′ = 6) = 0.005 [the values for v′ = 5 and 6 are from P. D. Pacey and J. C. Polanyi, J. Appl. Opt. 10, 1725 (1971)]; (ii) for D + Cl2, [k(v′ = 1) ∼ 0.1], k(v′ = 2) ≈ 0.3, [k(v′ = 3) = 1.00], k(v′ = 4) = 0.9, k(v′ = 5) = 0.3, k(v′ = 6) = 0.06; (iii) for H + Br2, [k(v′ = 1) = 0.03], k(v′ = 2) = 0.18, [k(v′ = 3) = 1.00], k(v′ = 4) = 0.99, k(v′ = 5) = 0.2, k(v′ = 6) ≤ 0.002. All of these reactions exhibit a comparatively low fractional conversion of the total available energy into internal excitation of the new molecule. The mean fractions entering vibration plus rotation are (i) for H + Cl2, f̄V′ + f̄R′ ( = 0.39 + 0.07) = 0.46; (ii) for D + Cl2, f̄V′ + f̄R′ ( = 0.39 + 0.10) = 0.49; (iii) for H + Br2, f̄V′ + f̄R′ ( = 0.55 + 0.04) = 0.59. The relatively inefficient conversion of reaction energy into vibration is thought to arise from the combined effect of ``repulsive'' energy release and a light attacking atom. As in the case of other isotopic pairs of reactions there is a close parallelism in k(V′) though not in k(v′) between the members of the pair (H+Cl2 and D+Cl2 in the present case). The increased fractional conversion of the available energy into vibration for H+Br2 as compared with H+Cl2 is indicative of a less-repulsive potential-energy surface. This is in accord with expectation, based on the change in barrier height and consequent change in barrier location on the energy surface. These reactions exhibit only a very small increase in product rotational excitation with decreasing vibrational excitation. It follows that the translational energy of the products is markedly greater for successively lower v′ states.