Infrared spectra of H+(H2O)5-8 clusters:: Evidence for symmetric proton hydration

Protonated water clusters, H+(H2O)(n) (n = 5-8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 +/- 20 K. Recorded absorption bands at the frequency range of 2700-3900 cm(-1) are attributed to the free- and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H+(H2O)(5-8) isomers are close in energy, with the excess proton either localized on a single water molecule, yielding H3O+(H2O)(n-1), or equally shared by two molecules, yielding H5O2+(H2O)(n-2). Systematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H5O2+(H2O)(4) was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm(-1). The existence of five-membered-ring isomers at n = 7 is also evidenced by the distinct bonded-OH stretches at 3500-3600 cm(-1) and by the free-OH stretch of three-coordinated H2O at 3679 cm(-1). Among these H+(H2O)(7) isomers is a newly discovered H5O2+-containing pentagonal ring, which is computed to be lowest in Gibbs free energy at 170 K. Its spectroscopic signature is the splitting of two equivalent bonded OH stretches into a doublet (3555 and 3555 cm(-1)) by vibrational coupling through ring closure. No profound spectral evidence, however, was found for the formation of four-membered rings although it is predicted to be favorable in terms of total interaction energy for H+(H2O)(7) Six-membered ring and three-dimensional cagelike structures are also stable isomers but they are less strongly bound. The preference of five-membered-ring formation at n = 7 appears to be the result of a delicate balance between entropy and enthalpy effects at the presently investigated cluster temperature. The correlation of this investigation with other studies of neutral water clusters and of the hydration of biological macromolecules is emphasized.