Theoretical Investigation of the Atmospheric Cl Oxidation Chemistry of 4-Methyl-3-penten-2-one

The reaction mechanisms and kinetics of 4-methyl-3-penten-2-one (MPO) with atomic Cl have been studied using second-order Moller-Plesset perturbation theory (MP2) and canonical variational transition-state (CVT) theory with smallcurvature tunneling (SCT) correction methods. Eight different reaction paths are considered: two possible Cl addition reactions and six possible hydrogen abstraction reactions. The results of the theoretical investigation indicate that Cl addition dominates the initial reactions of MPO with Cl atoms, and the most energetically favorable pathway is the Cl addition to the double bond carbon adjacent to the carbonyl in the molecule. Among all of the six Habstraction paths, the H abstraction from the two methyl groups linked to the double bond is predicted to occur readily according to the calculated data. In addition, the 1,5-H shift isomerization process, involving the Cl addition mechanism, was proposed and found to be feasible. The major possible products that were detected in experiment and derived from our theoretical study have been identified. The rate constants and branching ratios of the main reaction paths have been obtained over the temperature range of 180-380 K. The total rate constants are dependent on temperature and the Arrhenius equation is fitted as k(total) (2.99 X 10(-13))exp(1795.60/T). The atmospheric lifetime is also estimated and shows a positive temperature dependence.