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

Abstract

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 photoproducts 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 and C aldehydes at air-water interfaces. On the basis of the results, we have found that the formation of C aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T C-O bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C aldehydes are generated. In parallel, the pathway to C aldehydes is initiated by T C-C bond fission of NA, 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 aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C 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.

Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids

“…Each of the fatty acids and fatty alcohols investigated has weak absorbance in their respective UV–vis absorption spectra at ∼270 nm (Figures S10 and S11), which is slightly decreased in intensity following distillation (Figure S12). For nonanoic acid, this peak has previously been assigned to its triplet state. , It is intriguing that the fatty alcohols also have a peak in this same wavelength region given that their electronic structures are quite different. Interestingly, upon photolysis of 20 mM hexanoic acid, while no new photoproducts are detected by NMR or ESI – MS, the 270 nm peak in the UV–vis absorption spectra is preferentially depleted compared to the acid peak at ∼204 nm (Figure S13).…”

“…For nonanoic acid, this peak has previously been assigned to its triplet state. 67 , 68 It is intriguing that the fatty alcohols also have a peak in this same wavelength region given that their electronic structures are quite different. Interestingly, upon photolysis of 20 mM hexanoic acid, while no new photoproducts are detected by NMR or ESI – MS, the 270 nm peak in the UV–vis absorption spectra is preferentially depleted compared to the acid peak at ∼204 nm ( Figure S13 ).…”

Probing the OH Oxidation of Pinonic Acid at the Air–Water Interface Using Field-Induced Droplet Ionization Mass Spectrometry (FIDI-MS)

“…3 In addition, organic molecules at the air−water interface may have preferred orientation as a result of hydrogen bonding involving substituent groups; 4 thus, the reaction mechanisms at the air−water interface can differ from those of gas-and aqueous-phase reactions. 5−11 Studies of the air−water interfacial oxidation of organic compounds have been focused on the effects of substitutions, 12 unsaturation, 13,14 and chain length 15 on reaction kinetics and mechanisms. Pinonic acid (PA) is a major oxidation product of α-pinene, one of the most abundant biogenic volatile organic compounds emitted to the atmosphere (global emissions estimated at 66.1 Tg/y).…”

“…Moreover, heterogeneous reaction at the air–water interface can be facilitated if the air–water interface lowers the reaction energy barrier . In addition, organic molecules at the air–water interface may have preferred orientation as a result of hydrogen bonding involving substituent groups; thus, the reaction mechanisms at the air–water interface can differ from those of gas- and aqueous-phase reactions. − Studies of the air–water interfacial oxidation of organic compounds have been focused on the effects of substitutions, unsaturation, , and chain length on reaction kinetics and mechanisms.…”

Production of Peroxy Radicals from the Photochemical Reaction of Fatty Acids at the Air–Water Interface

“…As shown by reaction A in Figure , H abstraction from the saturated aldehyde can be done by an excited nonanoic acid molecule [NA]* as well. [NA]* does not compete with OH radicals since, according to a quantum chemical investigation into the photochemical reaction of nonanoic acid at an air–water interface, dehydrogenation of the saturated aldehyde by an OH radical is easier than that by an excited nonanoic acid molecule [NA]* . Therefore, the formation of the unsaturated aldehyde (C 9 H 16 O) is due to the reaction of OH radicals with the saturated aldehyde (C 9 H 18 O).…”