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

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⋯OC 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⋯OC(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.