Barrierless proton and hydrogen atom migrations in photoionized benzaldehyde clusters result in benzyl alcohol formation: an ion–molecule perspective

Abstract

Reported here is a study on intermolecular proton and H atom transfers in photoionized small clusters of benzaldehyde in a supersonic jet expansion. The clusters were photoionized by the fourth harmonic wavelength (266 nm) of a Q-switched picosecond (30 ps) Nd:YAG laser, and the resulting photoproducts were probed by means of time-of-flight mass spectrometry. The recorded mass spectrum reveals the formation of protonated benzaldehyde (PhCHO)H⁺, a protonated benzaldehyde dimer [(PhCHO)₂H]⁺, and a benzyl alcohol cation (PhCH₂OH)˙⁺ as the signatures of intracluster H⁺/H transfer reactions. Electronic structure theory calculation predicts that, while in the neutral ground state, the dimer is stabilized by weak C–H⋯O hydrogen bonded interactions, the optimized geometry of the photoionized dimer cation corresponds to a proton transferred configuration (PhCHOH⋯PhCO)˙⁺, wherein the H atom from the aldehydic C–H group of the ionized counterpart of the dimer is completely shifted to the carbonyl oxygen of the other molecular moiety. Potential energy scans along the C–H⋯OC coordinate of the dimer cation also show that the intracluster proton transfer in [(PhCHO)₂]˙⁺ is a barrierless process that results in (PhCHO)H⁺ formation. The calculation also reveals that the trimeric ion of benzaldehyde can also be converted similarly to an intracluster proton transferred configuration, which can be converted to a proton bound benzaldehyde dimeric species, [PhCHO⋯H⋯OC(H)Ph]⁺, and/or transformed further to a benzyl alcohol radical cation involving a H atom transfer process. The possibility for the occurrence of H transfer in the neutral S₂ state of the clusters is also discussed. These findings establish a reaction pathway for photo-induced aldehyde conversion to alcohol in small gas-phase clusters of aromatic aldehydes that might have relevance to atmospheric and interstellar chemistry.