Quantum Chemical Investigation on Photochemical Reactions of Nonanoic Acids at Air-Water Interface

Photoinduced chemical reactions of organic compounds at the marine boundary layer have recently attracted significant experimental attention because this kind of photoreactions has been proposed to have substantial impact on local new particle formation and their photo products could be a source of secondary organic aerosols. In this work, we have employed first-principles density functional theory method combined with cluster models to systematically explore photochemical reaction pathways of nonanoic acids (NAs) to form volatile saturated and unsaturated C-9 and C-8 aldehydes at air water interfaces. On the basis of the results, we have found that the formation of C-9 aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T-1 C-O bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C-9 aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C-9 aldehydes are generated. In parallel, the pathway to C-8 aldehydes is initiated by T-1 C-C bond fission of NAs which generates octyl and carboxyl radicals; then, an octanol is formed through recombination reaction of octyl with hydroxyl radical. In the following, two dehydrogenation reactions result into an enol intermediate from which saturated C-8 aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C-8 aldehydes. In these reactions, water and NA molecules are found to play important roles. They significantly reduce relevant reaction barriers. Our work has also explored oxygenation reactions of NA with molecular oxygen and radical radical dimerization reactions.