Unexpected high yield of acrolein underlies the importance of the hydrogen-abstraction mechanism in photooxidation of allyl methyl sulfide (AMS)

Abstract

This work explores theoretically the gas phase oxidation of allyl methyl sulfide (AMS, H2C CHCH2SCH3) initiated by •OH radicals, focusing on the H-abstraction pathway at the M06-2X-D3/aug-cc-pVTZ and MN15/augcc- pVTZ levels of theory (m06Tz and mn15Tz). The formation of a prereactive complex (PRC) is involved in H-abstraction processes with two potential directions of approach for the OH radical, denoted as “α” and “β”. The PRCs, demonstrate increased reactivity, primarily due to the interaction between the sulfur atoms and the hydroxyl hydrogen. A scheme for the H-abstraction mechanism that supports the experimentally identified products and predicts the formation of some S-containing low volatility products is proposed. The comparison of the potential energy surface (PES) between the double bond addition and H-abstraction paths in the AMS molecule shows that at the m06Tz level of theory, the H-abstraction on C3 and the addition to C1 have nearly the same profile of energy, while at the mn15Tz level, the minimum energy channel is the addition to C1. The theoretical rate coefficient for each reaction channel was calculated, considering the formation of a PRC prior to reaching the transition state of each channel and assuming thermal equilibrium between reactants and the PRC. The rate constants were calculated in a multi-TS/multi-conformer way at the SVECV-f12/m06Tz and SVECV-f12/mn15Tz levels of theory. The SVECV-f12 method is consistent in its predictions in both systems and exhibits only minor deviations from the experimental rate constants. Despite some specific differences due to the DFT method supporting the SVECV-f12 calculations, both methodologies predict a significant Habstraction contribution in the AMS + OH gas phase reaction, which explains the high formation yield for acrolein determined experimentally.