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

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 CH₃ I and CH₃ CH₂ I 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 ⋅CH₂ I and CH₃ C ⋅ HI radicals are the main degradation pathways of CH₃ I and CH₃ CH₂ I, 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 CH₃ I and CH₃ CH₂ I with ⋅OH are calculated to be 1.42×10^-13 and 4.44×10^-13  cm³  molecule^-1  s^-1 at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH₃ I and CH₃ CH₂ I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of ⋅CH₂ I and CH₃ C ⋅ HI in the presence of O₂ , NO and HO₂ indicates that HCHO, CH₃ CHO, 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.