Temperature-dependent rate coefficients for the reactions of Cl atoms with methyl methacrylate, methyl acrylate and butyl methacrylate at atmospheric pressure

Abstract

Rate coefficients for the gas-phase reactions of Cl atoms with a series of unsaturated esters CH 2 C(CH 3)C(O)OCH 3 (MMA), CH 2 CHC(O)OCH 3 (MAC) and CH 2 C(CH 3)C(O)O(CH 2) 3CH 3 (BMA) have been measured as a function of temperature by the relative technique in an environmental chamber with in situ FTIR detection of reactants. The rate coefficients obtained at 298 K in one atmosphere of nitrogen or synthetic air using propene, isobutene and 1,3-butadiene as reference hydrocarbons were (in units of 10 −10 cm 3 molecule −1 s −1) as follows: k (Cl+MMA) = 2.82 ± 0.93, k (Cl+MAC) = 2.04 ± 0.54 and k (Cl+BMA) = 3.60 ± 0.87. The kinetic data obtained over the temperature range 287–313 K were used to derive the following Arrhenius expressions (in units of cm 3 molecule −1 s −1): k (Cl+MMA) = (13.9 ± 7.8) × 10 −15 exp[(2904 ± 420)/ T], k (Cl+MAC) = (0.4 ± 0.2) × 10 −15 exp[(3884 ± 879)/ T], k (Cl+BMA) = (0.98 ± 0.42) × 10 −15 exp[(3779 ± 850)/ T]. All the rate coefficients display a slight negative temperature dependence which points to the importance of the reversibility of the addition mechanism for these reactions. This work constitutes the first kinetic and temperature dependence study of the reactions cited above. An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and unsaturated and oxygenated volatile organic compounds (VOCs) at 298 K has shown that they can be related by the expression: log k OH = 1.09 log k Cl − 0.10. In addition, a correlation between the reactivity of unsaturated VOCs toward OH radicals and Cl atoms and the HOMO of the unsaturated VOC is presented. Tropospheric implications of the results are also discussed.