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Rate constants for the Cl+H2 and D2 reactions have been measured at room temperature by the laser photolysis-resonance absorption (LP-RA) technique. Measurements were also performed at higher temperatures using two shock tube techniques: laser photolysis-shock tube (LP-ST) technique with Cl-atom atomic resonance absorption spectrometric (ARAS) detection, over the temperature range 699–1224 K; and higher temperature rates were obtained using both Cl-atom and H-atom ARAS techniques with the thermal decomposition of COCl2 as the Cl-atom source. The combined experimental results are expressed in three parameter form as kH2( ± 15%) = 4.78 × 10−16 T1.58 exp(−1610 K/T) and kD2( ± 20%) = 9.71 × 10−17 T1.75 exp(−2092 K/T) cm3 molecule−1 s−1 for the 296–3000 K range. The present results are compared to earlier direct studies which encompass the temperature ranges 199–1283 (H2) and 255–500 K (D2). These data including the present are then used to evaluate the rate behavior for each reaction over the entire experimental temperature range. In these evaluations the present data above 1300 K was given two times more weight than the earlier determinations. The evaluated rate constants are: kH2( ±14%)=2.52×10−11 exp(−2214 K/T) (199≤T<354 K), kH2(±17%)=1.57×10−16 T1.72 exp(−1544 K/T) (354≤T≤2939 K), and kD2(±5%)=2.77×10−16 T1.62 exp(−2162 K/T) (255≤T≤3020 K), in molecular units. The ratio then gives the experimental kinetic isotope effect, KIE ≡ (kH2/kD2). Using 11 previous models for the potential energy surface (PES), conventional transition state theoretical (CTST) calculations, with Wigner or Eckart tunneling correction, are compared to experiment. At this level of theory, the Eckart method agrees better with experiment; however, none of the previous PES’s reproduce the experimental results. The saddle point properties were then systematically varied resulting in an excellent model that explains all of the direct data. The theoretical results can be expressed to within ±2% as kH2th = 4.59 × 10−16 T1.588 exp(−1682 K/ T) (200≤T≤2950 K) and kD2th=9.20×10−16 T1.459 exp(−2274 K/T) cm3 molecule−1 s−1 (255≤T ≤3050 K). The KIE predictions are also compared to experiment. The ‘‘derived’’ PES is compared to a new ab initio calculation, and the differences are discussed. Suggestions are noted for reconciling the discrepancies in terms of better dynamics models.

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Rate constants for the reaction H + NO 2 f OH + NO have been measured over the temperature range 1100-2000 K in reflected shock wave experiments using two different methods of analysis. In both methods, the source of H-atoms is from ethyl radical decomposition in which the radicals are formed essentially instantaneously from the thermal decomposition of C 2 H 5 I. The first method uses atomic resonance absorption spectrometry (ARAS) to follow the temporal behavior of H-atoms. Experiments were performed under such low [C 2 H 5 I] 0 that the title reaction could be chemically isolated, and the decay of H-atoms was strictly firstorder. The results from these experiments can be summarized as k ) (1.4 ( 0.3) × 10 -10 cm 3 molecule -1 s -1 for 1100 e T e 1650 K. The second method utilizes a multipass optical system for observing the product radical, OH. A resonance lamp was used as the absorption source. Because this is the first OH-radical kinetics investigation from this laboratory, extensive calibration was required. This procedure resulted in a modified Beer's law description of the curve-of-growth, which could subsequently be used to convert absorption data to OH-radical profiles. Rate constants by this method required chemical simulation, and the final result can be summarized as k ) (1.8 ( 0.2) × 10 -10 cm 3 molecule -1 s -1 for 1250 e T e 2000 K. Because the results from the two methods statistically overlap, they can be combined giving k ) (1.64 ( 0.30) × 10 -10 cm 3 molecule -1 s -1 for 1100 e T e 2000 K. The present results are compared to earlier work at lower temperatures, and the combined database yields the temperature dependence over the large range, 195-2000 K. The combined results can be summarized as k ) (1.47 ( 0.26) × 10 -10 cm 3 molecule -1 s -1 for 195 e T e 2000 K. The reaction is subsequently considered theoretically using ab initio electronic structure calculations combined with modern dynamical theory to rationalize the thermal rate behavior. † Part of the special issue "Donald Setser Festschrift".

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Previously measured Cl dissociation rate constants for CCl 4 and CFCl 3 were analyzed with three different kinetics modeling calculations. The three models differ in detail but primarily are distinguished by the manner in which the high-pressure limiting rate constant is determined: model 1 involves a calibration to transport properties of the dissociated fragments, model 2 uses a Gorin model with a hindrance parameter, and model 3 requires variational transition state theory on an ab initio reaction path where all low-frequency motion off the path is presumed to be a free rotation. All three models have two adjustable parameters: the dissociation energy E 0 and the average energy transferred to the buffer gas 〈∆E〉 down . All three models are found to give comparable fits to the experiment and produce quite similar values for the adjustable parameters. For CCl 4 , the values are E 0 ) (68.2 ( 1.2) kcal mol -1 and 〈∆E〉 down ) (750 ( 125) cm -1 . For CFCl 3 , the values are E 0 ) (76.5 ( 0.5) kcal mol -1 with 〈∆E〉 down ) (800 ( 215) cm -1 . These values are compared to those obtained in similar studies for CF 2 Cl 2 and CF 3 Cl. The results indicate a substantial and consistent decrease in the C-Cl bond energy with each additional chlorine substitution in the chlorofluoromethanes. Isodesmic electronic structure calculations at the MP2 level confirm this effect but find it to be a little smaller than the experimental results indicate. Extended electronic structure calculations provide heats of formation for all nine CH x F y Cl z methyl radicals.

The unimolecular decomposition CF3C1 + Kr -CF3 + C1 + Kr has been studied using two different techniques, atomic resonance absorption spectrometry (ARAS) and laser schlieren (LS) density gradients, in two laboratories. As in our previous joint investigation of C C 4 dissociation, the ARAS and LS techniques give completely consistent results over the overlapping temperature range, 1800-2200 K. The title reaction is found to be fairly close to the low-pressure limit. The ARAS measurements between 1521 and 2173 K give k2nd = 1.73 x IO-' exp(-33837K/T) cm3 molecule-' s-l ( f 2 7 % at lo). This is in good agreement with the earlier ARAS measurements of b g e r and Wagner. k2nd = 1.15 x exp(-28330K/T) cm3 molecule-' s-' ( f 2 0 % at l o ) is obtained from the LS results between 1800 and 3000 K. The good agreement between methods verifies both the C-C1 fission path for the CFX1 dissociation and the curve-of-growth used in the C1 atom ARAS analysis. RRKM analysis of these rate data indicated a larger than usual magnitude for hEdown, in agreement with our previous findings on CC4, but here it was necessary that it increase with T.The best fit was with = 270 cm-'. The RRKM fit to the data is given by the three-parameter expression k2nd = 2.95 x 1024T8.50 exp(-48133K/7') cm3 molecule-' s-l for a pressure of 300 Torr. This final expression agrees with both the ARAS and LS results to within f 2 5 % and can be used in modeling applications between 1500 and 3000 K. IntroductionC1 atom ARAS,8 and we can also compare with these results.