Tropospheric oxidation of cyclic unsaturated ethers in the day-time: Comparison of the reactions with Cl, OH and O₃ based on the determination of their rate coefficients at 298K

The rate coefficients of the reactions of three unsaturated cyclic ethers, 2,5-dihydrofuran (2,5-DHF), 2,3-dihydrofuran (2,3-DHF), and 3,4-dihydropyran (3,4-DHP), with the major day-time tropospheric oxidants - OH (k_OH) and O₃(kO3), and that of 3,4-DHP with Cl atom (k_Cl) are determined usingrelativeratemethod, at 298K. The values of k_OH are (6.45±1.69), (11.95±2.79) and (11.38±2.64) × 10^-11cm³molecule^-1s^-1 for 2,5-DHF, 2,3-DHF and 3,4-DHP, respectively and kO3 are (1.65±0.31), (443.20±79.0) and (31.36±5.80)×10^-17cm³molecule^-1s^-1 for 2,5-DHF, 2,3-DHF and 3,4-DHP, respectively. The value of k_Cl for 3,4-DHP is (6.15±0.84)×10^-10cm³molecule^-1s^-1_. The dominant pathway of tropospheric degradation of 2,5-DHF is the reaction with OH, whereas that of 2,3-DHF is the reaction with O₃. Reaction with both OH and O₃ are equally important in the case of 3,4-DHP. Under the conditions of marine boundary layer, the reaction with Cl atom is important in the tropospheric degradation of 2,5-DHF and 3,4-DHP, but insignificant in the case of 2,3-DHF (only 1.8%). The estimated tropospheric lifetimes in the day-time are 2.0, 0.08 and 0.62hfor 2,5-DHF, 2,3-DHF and 3,4-DHP respectively. The presence of O atom, unsaturation, conjugation etc. influences the rate coefficients in a complex manner. However, there is a good correlation of k_OH with HOMO energy, which further improves by treating the molecules separately, either as ethers and hydrocarbons, or as cyclic and acyclic molecules. The values of kO3 of these ethers fit very well with the straight line regression derived for simple alkenes with their HOMO energy, except 2,3-DHF, which has a high strain enthalpy. The values of k_Cl also show a similar trend of increase with HOMO energy, but the correlation is not as good as that of k_OH.