Mechanism of Hydrogen Atom Transfer on Water Surfaces from Nonanoic Acid to Molecular Photosensitizers

Photosensitized reactions at water plays an important role in atmospheric chemistry, but their microscopic mechanism are largely unknown. In this work, we explore computationally the photosensitized hydrogen-atom transfer reactions from nonanoic acid (NA) to the photo sensitizers 4-benzoylbenzoic acid (4BBA) and 4-imidazolecarboxaldehyde (4IC) in large water cluster. Excited-state ab initio molecular dynamics (AIMD) simulations as the principal method used. The following main results are obtained, (i) The process occur preferentially at the air-water interface. Agreements with the experiments on the photo-excitation spectra is good, much better than for 4BBA-NA and 4IC-NA complexes computed in gas phase, (ii) Excited-state dynamics simulation shows ultrafsat hydrogen-atom transfer of the carboxylic O-H hydrogen of NA to the carbonyl oxygen of the photosensitizers for the lowest excited singlet state, (iii) No hydrogen-atom transfer is observed for the corresponding triplet trajectories. It is suggested that excited-state AIMD simulation for photosensitizer-reagent complex in large water cluster can be suitable for unraveling photosensitized reaction mechanism for other photosensitizers.