Mechanistic Photodissociation of Glycolaldehyde: Insights from Ab Initio and RRKM Calculations

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 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 C₉ and C₈ aldehydes at air-water interfaces. On the basis of the results, we have found that the formation of C₉ aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T₁ C-O bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C₉ aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C₉ aldehydes are generated. In parallel, the pathway to C₈ aldehydes is initiated by T₁ 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 C₈ aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C₈ 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.

Computational photochemistry of the azobenzene scaffold of Sudan I and Orange II dyes: excited-state proton transfer and deactivation via conical intersections

We employed the complete active space self-consistent field (CASSCF) and its multistate second-order perturbation (MS-CASPT2) methods to explore the photochemical mechanism of 2-hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited-state intramolecular proton transfer (ESIPT) along the bright diabatic (1) ππ* state is barrierless and ultrafast. Along this diabatic (1) ππ* relaxation path, the system can jump to the dark (1) nπ* state via the (1) ππ*/(1) nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay (1) ππ* keto and (1) nπ* enol species to the ground state via two energetically accessible S1 /S0 conical intersections. Finally, we encountered an interesting phenomenon in the excited-state hydrogen-bonding strength: it is reinforced in the (1) ππ* state, whereas it is reduced in the (1) nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I-IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet.