The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.

Directly measured low-pressure thermal HCO dissociation rate constants and isotope effects are presented for the first time. The temperature range of the measurements is 637-832 K. A theoretical model developed in the preceding paper is found to be highly consistent with these results and with all available H + CO thermal addition rate constant measurements. The calculations are used to extend the measured dissociation rate constant to combustion temperatures. The calculated low-pressure dissociation rate constant k¡ in various buffer gases is accurately represented from 300 to 3000 K by At,(Ar) = 3.09 X 10-7 7•°/* , A:,(He) = 3.80 X 10"7r1e"171/-Rr, A:,(N2) = 3.07 X lO^rV170/*7; and A:,(H2) = 5.79 X lO^r'e"17,0/*7", where kx is in cm3 4/(molecule s) and R is in kcal/(mol deg). The calculations suggest that {AE)lot, the average energy transfer between metastable HCO* and the buffer gas, varies between -40 and -50 cm"1 for buffer gases N2, H2, He, and Ar.

The gas-phase kinetics of the reactions of HCO with four molecules (02, N02, Cl2, and Br2) have been studied as a function of temperature in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants for each reaction were determined at a minimum of five temperatures to obtain Arrhenius parameters (k = A exp(-EJRT)). The results obtained are as follows (the numbers in the brackets are log A/(cm3 molecule-1 s-1), £a/(kJ mol-1), and the temperature ranges covered): HCO + 02 {-10.9 (±0.3), 1.7 (±1.5), 295-713 K); HCO + N02 {-10.6 (±0.3), -1.8 (±2.0), 294-713 K); HCO + Cl2 {-11.2 (±0.3), 0.3 (±2.0), 296-582 K¡; HCO + Br2 {-10.8 (±0.3), -3.7 (±2.0), 296-669 K). The reactivity of HCO was found to be between that of CH3 and C2H5 in the reactions of these radicals with Cl2 and Br2, which is consistent with proposed correlations of reactivity in exothermic reactions based on free-radical ionization potentials.

The complexes formed between cis- and trans-HONO isomers and ammonia have been observed and characterized in argon matrices. Five perturbed HONO vibrations and one perturbed NH3 deformation vibration were identified for the H3N···HONO-trans complex, and one perturbed HONO vibration and perturbed NH3 deformation vibration were identified for the H3N···HONO-cis complex. The OH stretching vibration in the H3N···HONO-trans complex is ca. 800 cm-1 red-shifted and NOH bending vibration is ca. 190 cm-1 blue-shifted with respect to the trans-HONO monomer, indicating formation of a very strong molecular hydrogen bond. Theoretical studies of the structure and spectral characteristics of the H3N···HONO-trans and H3N··· HONO-cis complexes were carried out on the electron correlation level and G-311+G(2df,2pd) basis set. The calculated binding energy at the MP2 level is −40.13 and −36.39 kJ mol-1 for the H3N···HONO-trans and H3N···HONO-cis complexes, respectively. The calculated spectra reproduce very well the frequencies and the intensities of the measured spectra.