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The availability for the first time of detailed rate constants k(V′, R′, T′) (where V′, R′ and T′ are product vibrational, rotational and translational excitation) for the highly exothermic reaction H + F2 → HF(V′, R′) + F has prompted the 3D classical-trajectory study reported here. The potential-energy surface is found to be predominantly repulsive (⊥ ≈ 42%, R⊥ ≈ 58%) corresponding to the rather low fractional conversion of reaction energy into vibration ((f′V) = 0.58 from experiment, and 0.56 from theory). In the homologous series of reactions H + X2 (X F, Cl, Br, I) the percentage of repulsive energy-release decreases for X Cl, Br, I, but increases from X F to Cl. It is shown that this cannot be due to charge in mass-combination, but can plausibly be explained by the anomolously short range of interaction between the separating X atoms in the case X F. It is predicted that the more-forward scattered HF will be more rotationally excited. The form of the cross section function Sr(T) (where T is reagent translation) is analysed. In accordance with the expectation for a strongly exothermic reaction, it is found that Sr(T) rises more steeply than Sr(V) (where V is reagent vibrational energy). The effect on the product energy distribution conforms qualitatively to the “adiabatic” behaviour noted in previous work: ΔT → ΔT′ + ΔR′; ΔV → ΔV′. The explanation is to be found in reaction through more-compressed or more-extended intermediate configurations than are characteristic of room temperature reaction. We note the existence of an amplification effect: (ΔT′ + ΔR′)/ΔT ≈ 2, and ΔV′/ΔV ≈ 2. 相似文献
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LoSecco JM Bionta RM Biewitt G Bratton CB Casper D Chrysicopoulou P Claus R Cortez BG Errede S Foster GW Gajewski W Ganezer KS Goldhaber M Haines TJ Jones TW Kielczewska D Kropp WR Learned JG Lehmann E Park HS Reines F Schultz J Seidel S Shumard E Sinclair D Sobel HW Stone JL Sulak L Svoboda R van der Velde JC Wuest C 《Physical review letters》1985,54(21):2299-2301
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Infrared emission has been recorded from a heated seeded supersonic primary beam of HCl or HF (1) prior to collision with a target beam, and (2) subsequent to that collision. Mean collision energy and collision partner were varied systematically. After correction for elastic scattering, the net population change due to inelastic scattering in a translation—rotation (T ? R) energy-transfer encounter was obtained for specific J states ranging from J = 0–16 of vibrational level υ = 1 of the primary-beam molecule. The broad picture is that a net transfer into low-J states out of higher-J states takes place at low collision energies, and the converse at high collision energies. These observations are interpreted in terms of the “exponential model” for the relative cross sections of T ? R inelastic collisions, SR (Ji → Jf), proposed earlier [J.C. Polanyi and K.B. Woodall, J. Chem. Phys. 56 (1972) 1563], modified here to satisfy microscopic reversibility. The constant C in the model, which governs the exponential decrease in SR with increasing energy difference ΔEJ between Jf and Ji, can be derived, as a function of collision energy T, from the present experimental data; C decreases as T increases, i.e. larger ΔJ become more probable. In order to check the validity of the model, it was compared with 3D trajectory results; according to this criterion it was found to give a very good representation of SR(Ji → Jf) with a single value for C, within a limited range of Ji. The collision partners HCl + HF exhibit anomalously efficient rotational deactivation; evidence is presented which indicates that at low collision energies this is due to resonant R → R transfer. Very efficient deactivation of HCl by HCl, at low collision energy, is likely to be due to V — V transfer. 相似文献
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The first test of the information-theoretic approach to branching ratios has been made for the reaction: F + HD å HF2 (V′, R′) + D å DF3(V′, R′) + H.The vibrational (V′) and rotational (R′) product energy-distributions for both branches of this reaction have been obtained by the infrared chemiluminescence technique, and have been used in the calculation of an information-theoretic branching ratio, ΓHF/DF = 1.41 ± 0.18. This is in excellent agreement with the experimentally measured branching ratio of 1.45. However, results from classical trajectory calculations raise a question as to the significance of this agreement. Classical trajectory calculations (on various energy-surfaces) predict an increase in Γ with reagent J. The information-theoretic analysis applied to the product energy-distributions from these trajectory calculations leads to a qualitatively different result. As a possible alternative to the information-theoretic view, simple kinematic features are noted which could account for Γ > 1, as well as for the significant differences in product energy-distribution. On this alternative view, the two features are not indisolubly linked — the extent to which they appear in conjunction will depend on the nature of the energy surface. 相似文献
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The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction: For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?′ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH3Cl to CH3Br to CH3I, viz <?′ν> = 0.50 (1a), <?′ν> = 0.58 (1b), <?′ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?′ν> = 0.50 (1a), <?′ν> = 0.23 (2a), <?′ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?′ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH2Cl, CHCl2, CCl3, due to increase in the number of secondary encounters between HF and the departing radical. 相似文献
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