Notes: 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. © 1994 American Institute of Physics.

Notes: An UV multipass optical absorption method to increase the sensitivity for radical species detection has been developed for high temperature chemical kinetics experiments in a shock tube. The specific illustration is for OH radicals in the reflected shock wave regime. With a resonance lamp source, 12 optical passes were found to give a sufficient signal-to-noise ratio for a large range of [OH]. Two different calibration procedures using the reaction systems H2/O2 and C2H5I/NO2 were used, and a curve of growth was determined. The measured absorbance (ABS), was found to be dependent on both temperature and [OH]. The results can be expressed in a modified Beer's law form as,(ABS)=9.49×10−12T−0.5281[OH]0.8736.Using this curve of growth, the absorbance data from the above kinetics experiments were converted to concentration profiles. These were fully modeled with previously established mechanisms, giving excellent fits. The multipass method is compared to earlier systems that used both resonance lamp and laser absorption sources, and the increase in sensitivity is found to be substantial due primarily to the increased path length. This increased sensitivity inhibits the effects of any possible secondary chemistry thereby allowing chemically isolated experiments on OH + molecules to be performed at high temperatures. © 1995 American Institute of Physics.

Notes: Rate constants for the thermal decomposition of CH3Cl and the atom–radical reaction O+CH3 have been measured in shock tube experiments. The decomposition of CH3Cl, at three loading pressures, has been carried out between 1663–2059 K with mixtures that varied from 1.25 to 24.9 ppm in Ar. The first-order rate constant k1=1.21×1010 ×exp(−27 838 K/T) s−1 describes the experimental results to within ±29% at the one standard deviation level. The bimolecular rate experiment has then been carried out over the temperature range 1609–2002 K using mixtures of SO2 (49.9 ppm) and CH3Cl (5.14 and 8.2 ppm) in Ar. The technique first involves allowing the thermal decomposition to proceed forming CH3 radicals, and this is then followed by delayed photolysis of SO2 forming the O-atom species. This new method is called the pyrolysis photolysis-shock tube (PyPh-ST) technique. A reaction mechanism had to be used to simulate the measured O-atom profiles for the various experimental conditions, and the bimolecular rate constant was found to be temperature independent with a value of k2=1.4×10−10 cm3 molecule−1 s−1. Reactions (1) and (2) are both theoretically discussed.

Notes: The reactions of Si+ with CH3SiH3, CH3SiD3, C2H6, and CH3CHD2 have been studied in a tandem mass spectrometric apparatus over the kinetic energy range of 1–10 eV laboratory-frame-of reference (LAB). In all systems, the major process is the formation of SiCH+3, as well as SiCH2D+ and SiCHD+2 in the case of the reaction with CH3CHD2. It is shown that in the reaction of Si+ with CH3SiH3 and CH3SiD3, the process is best described as a Walden inversion, while in the reaction with C2H6 and CH3CHD2, the process appears to approximate the spectator stripping model or modified spectator stripping (polarization-reflection model). In the reaction with CH3CHD2, the slight preference of Si+ to strip the CH3 radical rather than the CHD2 radical is shown to be in accord with a cross-sectional energy dependence of approximately E−1.