Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization.

Infrared (IR) spectroscopy based on vacuum-ultraviolet one-photon ionization detection was carried out to investigate geometric structures of neutral and cationic clusters of acetic acid: (CH(3)COOH)(2), CH(3)COOH-CH(3)OH, and CH(3)COOH-H(2)O. All the neutral clusters have cyclic-type intermolecular structures, in which acetic acid and solvent molecules act as both hydrogen donors and acceptors, and two hydrogen-bonds are formed. On the other hand, (CH(3)COOH)(2) (+) and (CH(3)COOH-CH(3)OH)(+) form proton-transferred structures, where the acetic acid moiety donates the proton to the counter molecule. (CH(3)COOH-H(2)O)(+) has a non-proton-transferred structure, where CH(3)COOH(+) and H(2)O are hydrogen-bonded. The origin of these structural differences among the cluster cations is discussed with the relative sizes of the proton affinities of the cluster components and the potential energy curves along the proton-transfer coordinate.