Behaviors of an excess proton in solute-containing water clusters: A case study of H+(CH3OH)(H2O)(1-6)

Behaviors of an excess proton in solute-containing water clusters were inve stigated using infrared spectroscopy and ab initio calculations. This inves tigation characterized the structures of protonated methanol-water clusters , H+(CH3OH)(H2O)(n) with n=2-6, according to their nonhydrogen-bonded and h ydrogen-bonded OH stretches in the frequency range of 2700-3900 cm(-1). Ab initio calculations indicated that the excess proton in these clusters can be either localized at a site closer to methanol, forming a methyloxonium i on core (CH3OH2+), or at a site closer to water, forming a hydronium ion co re (H3O+). Infrared spectroscopic measurements verified the calculations an d provided compelling evidence for the coexistence of two distinct structur al isomers, CH3OH2+(H2O)(3) and H3O+(CH3OH)(H2O)(2), in a supersonic expans ion. The spectral signatures of them (either CH3OH2+ or H3O+ centered) are the free-OH stretching absorption band at 3706 cm(-1) of a single-acceptor- single- donor H2O, and the band at 3673 cm(-1) of a single-acceptor CH3OH. At n=4-6, the clusters adopt structures similar to their pure water analogs with five-membered rings starting to form at n=5. The position of the exce ss proton in them varies sensitively with the number of solvent water molec ules as well as the geometry of the clusters. To further elucidate the beha viors of the excess proton in these clusters, we analyze in detail the pote ntial energy surface along the proton transfer coordinate for two specific isomers of n=2 and 4: MW2II and MW4I. It is found that the proton can be ne arly equally shared by methanol and the water dimer subunit in the form of CH3OH-H+-(H2O)(2), as substantiated by hydrogen bond cooperativity and zero -point vibrational effects. (C) 2000 American Institute of Physics. [S0021- 9606(99)01248-9].