Theoretical Study of Hydroxyl Radical Initiated Degradation Mechanism, Kinetics and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere - 白凤杨

The degradation and transformation of iodinated alkanes are crucial in the iodine chemical cycle in the marine boundary layer. In this study, MP2 and CCSD(T) methods were adopted to study the atmospheric transformation mechanism and degradation kinetic properties of CH3I and CH3CH2I mediated by ·OH radical. The results show that there are three reaction mechanisms including H-abstraction, I-substitution and I-abstraction. The H-abstraction channel producing ·CH2I and CH3C·HI radicals are the main degradation pathways of CH3I and CH3CH2I, respectively. By means of the variational transition state theory and small curvature tunnel correction method, the rate constants and branching ratios of each reaction are calculated in the temperature range of 200―600 K. The results show that the tunneling effect contributes more to the reaction at low temperatures. Theoretical reaction rate constants of CH3I and CH3CH2I with ·OH are calculated to be 1.42×10-13 and 4.44×10-13cm3 molecule−1 s−1 at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH3I and CH3CH2I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of ·CH2I and CH3C·HI in the presence of O2, NO and HO2 indicates that HCHO, CH3CHO, and I-atom are the main transformation end-products. This study provides a theoretical basis for insight into the diurnal conversion and environmental implications of iodinated alkanes.