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Notes: Rate constants for the reaction of O(3P) atoms with deuterium, O+D2→OD+D, have been measured over the temperature range 825–2487 K. The experimental method that has been used is the flash photolysis–shock tube (FPST) technique. This technique utilizes atomic resonance absorption spectroscopy (ARAS) to monitor O-atom depletion in the presence of a large excess of reactant, D2. The measurement is made in the stagnant reflected shock wave region. Thus, shock heating simply serves to prepare the gas density and temperature for a flash photolytically induced absorption photometric experiment. The results that have been obtained between 825 and 2487 K can be represented by the Arrhenius expression: k=(3.22±0.25)×10−10 exp(−7293±98 K/T) cm3 molecule−1 s−1. The average deviation of the present data from this equation is ±17%. An alternative three parameter expression that represents the data to within ±16% is k=1.95×10−15 T1.45 exp(−5250 K/T) cm3 molecule−1 s−1. When the recent results of Zhu, Arepalli, and Gordon (the preceding paper) are considered, a three parameter expression can be determined for the temperature range, 343–2487 K. This combined result is k=2.43×10−16 T1.70 exp (−4911 K/T) cm3 molecule−1 s−1. The average deviation of the data from this equation is ±16%, whereas the data of Gordon and co-workers agree to within ±5%. The combined result is compared to earlier experimental results and, also, to theoretical calculations by Bowman, Wagner, Walch, and Dunning; Garrett and Truhlar; and Joseph, Truhlar, and Garrett. The present result is used along with recent data for O+H2 to specify the experimental isotope effect, kH2/kD2, over the experimental temperature range. Lastly, the experimental rate constant ratio is compared to the theoretical predictions.

Notes: Equilibrium constants K1 for the reaction H+NH3(arrow-right-and-left)NH2+H2 were measured over the temperature range 900 to 1600 K using a flash photolysis–shock tube apparatus. The experimental values of K1 ranged from 1.0 at 900 K to 2.2 at 1620 K with an estimated experimental error of about ±10%. The value obtained from the third law analysis for the enthalpy of formation of the amidogen radical, ΔH0f298 (NH2), is 45.3 kcal/mol (ΔH0f0 =46.0). The corresponding value for the bond dissociation energy, D0(NH2–H), is 107 kcal/mol. These values are in good agreement with the data tabulated in the revised JANAF tables (1982), with those derived from new measurements of the photoionization threshold for the amidogen radical, and with those from independent measurements of the rate constants of the forward and back reactions. The Arrhenius rate expression derived for reaction (−1), NH2+H2→NH3+H, is k−1(T)=5.38×10−11 exp(−6492/T) cm3 molecule−1 s−1 for the temperature range 900–1620 K. The estimated error in k−1(T) is about ±25%.

Notes: Thermal rate constants measured by the flash photolysis-shock tube (FP-ST) technique are reported for the reaction, H+D2→HD+D, over the temperature range, 724–2061 K. H-atom concentration has been monitored by atomic resonance absorption spectroscopy (aras). The results can be represented by the Arrhenius expression: k1=(3.95±0.35)×10−10 exp(−5919±95 K/T) cm3 molecule−1 s−1, to within ±25% over the temperature range. These results are then combined with lower temperature direct determinations, and a three parameter expression is derived which expresses the rate behavior between 256–2061 K: k1=1.69×10−17T1.10 exp(−3527 K/T) cm3 molecule−1 s−1. The experimental results are then compared to theoretical calculations that utilize ab initio potential energy surfaces that are presumably the most exact that have ever been determined. Thus, the theoretical to experimental comparison constitutes a stringent test of the ab initio surfaces and the dynamical calculations in which they are used. The conclusion from this comparison is that transition state theory supplies a high quality prediction for the rate behavior, being within ±30% of the experimental data over the entire temperature range.

Notes: The rate constant for the reaction of hydroxyl radical with acetaldehyde OH+CH3CHO → products has been measured from 244–528 K with the discharge flow-resonance fluorescence technique. The temperature dependence, expressed in Arrhenius form, is k1(T)=(5.52±0.80)×10−12 exp(610±103/RT) cm3 molecule−1 s−1, where R is in cal mol−1 and the errors are at the two standard deviation level. This result is compared to earlier flash photolysis-resonance fluorescence work. Mechanistic considerations are additionally discussed, and it is concluded that acetyl radicals are the most probable products of the reaction. Lastly, theoretical calculations are presented which delineate theoretical issues to be addressed in ab initio potential energy calculations.

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: Rate constants for the reactions (1) H+O2→OH+O and (2) D+O2→OD+O have been measured over the temperature ranges 1103–2055 K and 1085–2278 K, respectively. The experimental method that has been used is the laser-photolysis–shock-tube technique. This technique utilizes atomic resonance absorption spectrophotometry (ARAS) to monitor H- or D-atom depletion in the presence of a large excess of reactant, O2. The results can be well represented by the Arrhenius expressions k1(T)=(1.15±0.16)×10−10 exp(−6917±193 K/T) cm3 molecule−1 s−1, and k2(T)=(1.09±0.20)×10−10 exp(−6937±247 K/T) cm3 molecule−1 s−1. Over the experimental temperature range, the present results show that the isotope effect is unity within experimental uncertainty. The Arrhenius equations, k−1(T)=(8.75±1.24) ×10−12 exp(1121±193 K/T) cm3 molecule−1 s−1 and k−2 (T)=(9.73±1.79)×10−12 exp(526±247 K/T) cm3 molecule−1 s−1, for the rate constants of the reverse reactions were calculated from the experimentally measured forward rate constants and expressions for the equilibrium constants that have been derived from the JANAF thermochemical database. The theoretical implications of the present results are also discussed.