Reaction mechanisms and kinetics of the O2 addition pathways to the main thiophene-OH adduct: a theoretical study

Density functional theory, along with the omega B97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH](center dot) (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O-2 addition at the C-5 position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure.