The kinetics of the reaction of chlorine atoms with isoprene (1) has been studied using the discharge-flow mass-spectrometric method: Cl + C 5 H 8 (+M) f C 5 H 8 Cl (+M) (1a); Cl + C 5 H 8 f C 5 H 7 + HCl (1b). As a result of direct and relative measurements, the overall rate constant k 1 ) (6.7 ( 2.0) × 10 -11 exp{(485 ( 85)/T} cm 3 molecule -1 s -1 was obtained at a pressure of 1 Torr of helium over the temperature range 233-320 K. k 1 was found to be pressure-independent (within 10%) at T ) 298 K in the pressure range 0.25-3 Torr. Both HCl and C 5 H 8 Cl adduct were detected as products of reaction 1. The measurements of the HCl yield as a function of temperature led to the branching fraction for the H-atom abstraction channel (1b): k 1b /k 1 ) (1.22 ( 0.4) exp{-(595 ( 90)/T} (P ) 1 Torr, T ) 233-320 K); k 1b /k 1 ) 0.169 ( 0.022 at 298 K. The results are compared with those reported in a recent study 4 carried out at 298 K. In addition, the rate coefficient has been measured for the reaction Cl + Br 2 f BrCl + Br (2), which was used as the reference in the relative study of reaction 1. Combining the present results obtained in the range 233-320 K with those available from a previous study in the range 298-401 K, 10 the following Arrhenius expression is recommended: k 2 ) (2.3 ( 0.4) × 10 -10 exp{(135 ( 60)/T} cm 3 molecule -1 s -1 .

Rate constants for the reactions of O H radicals and chlorine atoms with halogenated aldehydes of the type CX3CHO (X = H, C1, or F) have been determined at 298 f 2 K. The O H radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreased overall reaction enthalpy.

CH 3 SO 2 radical decomposition and the mechanism of SO 2 and CH 3 formation in the reaction of CH 3 SO with NO 2 were experimentally investigated in the pressure range 1-612 Torr of He using laser pulsed photolysis/ laser-induced fluorescence and discharge flow mass spectrometry/laser-induced fluorescence techniques. The upper limit for the thermal decomposition rate of CH 3 SO 2 in the investigated pressure range has been found to be 100 s -1 at 300 K. High-pressure limit, k ∞ ) 2.08 × 10 3 s -1 , and low-pressure limit, k 0 ) 2.72 × 10 -17 cm 3 molecule -1 s -1 , of the rate coefficient of the CH 3 SO 2 decomposition have been derived from ab initio and RRKM calculations. The CH 3 SO 2 decomposition rate at 760 Torr and 300 K has been estimated to be about 200 s -1 from falloff calculations. Reaction of CH 3 SO with NO 2 has been found to form CH 3 and SO 2 with a yield varying from (0.33 ( 0.05) at 13 Torr to (0.18 ( 0.03) at 612 Torr of He. The rate constant for the reaction of CH 3 SO with NO 2 has been found to be (1.5 ( 0.4) × 10 -11 cm 3 molecule -1 s -1 at 300 K, independent of pressure. On the basis of ab initio and RRKM calculations, the experimental results have been interpreted by assuming the reaction of CH 3 SO with NO 2 to form chemically activated CH 3 SO 2 * radical followed by its thermal stabilization or prompt decomposition to CH 3 and SO 2 . The implication of the obtained results for the atmospheric oxidation of dimethyl sulfide is discussed.