The reactions of alkyl radicals (R = CHI, C2H5, i-C3H7, and t-C4H9) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. Rate constants were obtained in the following temperature ranges and provided Arrhenius parameters for each reaction (A/(cm3 molecule-' s-I), E,/(kJ mol-')): R = CH3, 299-536 K ((-1.57 i 0.26) X lo-", 1.6 i 0.6); R = C2H5, 297-530 K ((1.70 f 0.55) X -4.2 i 1.2); R = i-C3H7, 298-530 K ((1.58 i 0.38) X -6.4 i 0.9); R = t-C4Hg, 298-530 K ((1.37 i 0.47) X lo-'*, -7.8 i 1.4). R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C&, C3H8, n-C4Hl,,, i-C&Il0), have also been investigated using laser flash photolysis/resonance fluorescence methods. Rate constants were obtained in the following temperature ranges and provided Arrhenius parameters for each reaction (same units): RH = C2&, 473-621 K ((2.35 i 1.12) X 1O-Io, 53.3 i 2.1); RH = C3H8, 476-667 K ((8.78 f 3.00) X lo-", 36.0 i 2.0); RH = n-C4Hlo, 447-625 K ((2.86 i 0.90) X 10-lo, 37.7 i 2.0); RH = i-C4H10, 423-621 K ((1.61 f 0.60) X lO-'O, 28.8 i 1.5). These results, combined with previously obtained kinetic information, were used in second-and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C& alkyl radicals involved. Second-and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol-'). The heats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol-'. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determinations (kJ mol-'): C2Hs, 121.0 i 1.5; i-C3H7, 90.0 i 1.7; sec-C4Hg, 67.5 i 2.2; t-C4Hg, 51.3 i 1.8. Accurate determinations of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol-'): primary C-H in C2H6 (422.8 i 1.5); secondary C-H in C3H8 (412.7 i 1.7) and in n-C4Hlo (41 1.1 i 2.2); tertiary C-H in i-CIHIO (403.5 i 1.8).

The unimolecular decomposition of C2H5 in helium has been investigated near the low-pressure limit (7 = 876-1094 K; P = 0.8-14.3 Torr). Rate constants (*1) have been determined as a function of temperature and pressure in the indicated ranges in time-resolved experiments. The reaction was isolated for quantitative study in a heated tubular reactor coupled to a photoionization mass spectrometer. Weak collision effects (fall-off behavior) were analyzed using a master equation analysis. Values of (A£'>down for the exponential down energyloss probability were obtained for each experiment performed. The microcanonical rate constants, *, (£), needed to solve the master equation were obtained from a transition state model for the reaction which is described. The temperature dependence of these (AE)down determinations was apparent and fits the expression (A£)down = 0.255710(±01) cm-1. It is shown that this expression (derived from experiments conducted between 876 and 1094 K) provides a reasonable representation of observed weak collision effects in helium down to 285 K. Values for (A£)down for C2H5 decomposition in other bath gases were obtained by reexamining published data on the fall-off of the C2H5 unimolecular rate constant in N2, SF6, and C2H6. The experimental results and data simulation were used to obtain a parametrized expression for * (7, ), the low-pressure limit rate constant for C2H5 decomposition in helium (200-1100 K); fci°= 6.63 x lfpT"4•99 exp(-20,130 K/7) cm3 molecule"* 1 s"1. Prior published experiments on both the forward and reverse reactions (C2H5 + (M) <= C2H4 + + (M)) in the fall-off region were reevaluated and used in conjunction with an RRKM model of the transition state to obtain a new recommended expression for the high-pressure limit rate constant for the temperature range 200-1100 K, k\" = 1.11 X io1071037 exp(-18,504/7) s"1. Parametrization of the density and temperature dependence of *1 in helium according to the modified Hinshelwood expression introduced by Gilbert et al. is provided.

The reaction of trimethylsilyl radicals with nitric oxide, Si(CH3)3 + NO Si(CH&NO, has been studied using pulsed excimer-laser photolysis coupled with time-resolved photoionization mass spectrometry over the temperature range 300-812 K. The reaction rate constant was measured as a function of temperature and density (He) in the range 0.82 x loi6 5 [He] I 15.7 x 10l6 molecules ~m -~. Equilibrium constants for the reaction were measured between 685 and 787 K. The standard enthalpy of the reaction Si(CH3)3 + NO Si(CH3)3NO was obtained from the measured equilibrium constants using both second-and third-law methods. The two procedures yielded results in good agreement: m 2 9 8 = -183 f 11 kJ mol-' (second law) and AH0298 = -190.2 f 3.6 kJ mol-' (third law), the latter being more accurate. The high-pressurelimit rate constant of reaction Si(CH3)3 + NO -Si(CH3)3NO, obtained by a short extrapolation of the experimental data using the Troe factorization technique, has a small negative temperature dependence: k,,,, = (3.8 f 0.4) x 10-''(T/298)-(0.6*02). Ab initio calculations with empirical bond additivity corrections have been performed to determine the structure, vibrational frequencies, and energies of the low-lying electronic singlet and triplet states of the Si(CH&NO molecule. The calculated thermodynamic functions of the molecule were used to obtain the standard entropy of the reaction @SO298 = -147.9 J mol-' K-I) that was used in the third-law thermochemical calculations. The theoretical standard enthalpy of the reaction, W 2 9 8 (~a l~) = -190.9 kJ mol-', is in excellent agreement with that determined experimentally using the third-law procedure.The intrinsic Si(CH&-NO bond strength of 187.4 f 4.0 kJ mol-' ( -M o o for the reaction) was determined using the measured enthalpy of the reaction at 298 K (by the third-law method) and the calculated relative enthalpy functions. This study provides the first direct experimental determination of a Si-N bond strength.

The kinetics of the reaction between CH3C0 and HBr has been studied using a heatable tubular reactor coupled to a photoionization mass spectrometer. CH3C0 was produced homogeneously by laser photolysis in the presence and absence of HBr. Radical decays were monitored in time-rcsolved experiments. Rate constants were determined at five temperatures in the range 300-400 K and fitted to the Amhenius expression, 6.4 (*3.6) X lO-" exp(4.45 (fl.50) kJ mol-'/RI) an3 molecule-l s-', This kinetic information was combined with known rate constants and Arrhenius parameters for the reverse reaction to obtain the heat of formation of CH3C0. Both m n d law and third law procedures were used to obtain this thermochemical information from these rate constants. The two determinations of this heat of formation were in close agreement (differing by only 0.4 kJ mol-'). These results, taken together, provide a CH3C0 heat of formation of -10.0 f 1.2 kJ mol-' at 298 K which is 14 kJ mol-' higher than the value in common use. The current results imply a CH3-C0 bond enthalpy of 45.1 ( f 1 . 5 ) kJ mol-' which is 14 kJ mol-' lower than currently believed and a CH3CO-H bond enthalpy of 373.8 (*1.5) kJ mol-' which is higher by this same figure. Former disparities between reported CH3C0 heats of formation associated with the equilibrium systems studied to obtain this thermochemical information are resolved.