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.
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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