Kinetics of CH2OO Criegee intermediate reactions with SO2, NO2, NO, H2O and CH3CHO and CH2I radical reactions with NO2 are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH2I2-O2-N2 gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO2, direct detection of CH2OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH2OO + SO2 and CH2OO + NO2 are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10(-11) cm(3) s(-1) (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10(-12) cm(3) s(-1) (measured between 25 and 300 Torr), respectively. The rate coefficient for CH2OO + CH3CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10(-13) cm(3) s(-1) and 9 × 10(-17) cm(3) s(-1) are placed on the rate coefficients for CH2OO + NO and CH2OO + H2O, respectively. The upper limit for the rate coefficient for CH2OO + H2O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH2OO chemistry.

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).