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.

Rotational and Vibrational State Distributions of HNC(0 0) from the Hot H Atom Reaction: H + (CN)2 → HNC + CN

“…Using the estimates for the energy partitioning summarized in Table and the observed vibrational state distribution, the global energy disposition for each degree of freedom was calculated, expressed as a fraction of the 〈 E avail 〉 that appears in a particular degree of freedom summed over the vibrational population. The results are summarized in Table along with similar results for related triatomic reaction systems: H + F 2 → HF + F and H + Cl 2 → HCl + Cl 6 Summary of the Global Fraction of 〈 E A vail 〉 that Appeared as Product Translation, Rotation, and Vibration for Reaction 2bl reaction〈 f T 〉〈 f R 〉〈 f V 〉 H +(CN) 2 →HNC + CN (E avail ≈ 19 kJ mol -1 ) 0.56 0.32 0.13 H + F 2 →HF + F b (E avail =427 kJ mol -1 ) 0.39 0.03 0.58 H + Cl 2 →HCl + Cl c (E avail =203 kJ mol -1 ) 0.54 0.09 0.37 a For comparison, the energy disposition for the related H + X 2 reactions are included, where X is F or C. b Ref .…”

“…The results are summarized in Table 6 along with similar results for related triatomic reaction systems: H + F 2 f HF + F 40 and H + Cl 2 f HCl + Cl. 41 Both the mass and electron affinity 42 of the CN radical lie between those of the F and Cl atom; however, the global dynamical features appear to be quite similar to the H + Cl 2 reaction system (Table 6). It appears that the global features of energy partitioning for a polyatomic reaction system can be rationalized from the observations of triatomic systems of similar mass combinations, 1 i.e., L + H-H reaction systems.…”

“… a For comparison, the energy disposition for the related H + X 2 reactions are included, where X is F or C. b Ref . c Ref . …”

Infrared chemiluminescence studies of the hydrogen + chlorine fluoride reaction. Energy disposal and branching ratios

“…Except for a slight difference in the = 1 and 2 populations of HFf, the vibrational distributions for both HCI and HF in the two experiments are in good agreement. The variation in HF(u = 1) and HF(u = 2) may be a consequence of secondary reaction between F atoms and H2 (see below), collisional relaxation from higher v levels, or experimental uncertainty related The GIF was metered from the storage tank to the reactor using clean glass lines for experiment 1 and was transferred to a stainless reservoir and metered through stainless lines for experiment 2.…”

Channeling of Products in the Hot Atom Reaction H + (CN)2 → HCN/HNC + CN and in the Reaction of CN with CH3SH

“…As expected, the estimates for HCN are very close to the results for CN. The disposition of reaction energy in the analogous thermal triatomic reaction, H + Cl 2 → HCl( V ‘ , R ‘ , T ‘ ) + Cl, is included in Table for comparison. Whereas collinear and near-collinear trajectories contribute most of the reactive flux in the H + Cl 2 reaction, the experimental data accumulated for the H + (CN) 2 reaction (refs and and the present work) support the expectation, based on theoretical calculations for the related H + ClCN 10,11 and H + HCN 12 systems, that collinear and near-collinear H−NCCN geometries are highly repulsive, and therefore trajectories that are transverse relative to the NCCN molecular axis contribute most of the reactive flux.…”