Infrared spectra and isomeric structures of hydroxide ion-water clusters OH- (H2O)1-5:: a comparison with H3O+ (H2O)1-5

Size-selected hydroxide ion water tetramers and pentamers [OH-(H2O)(4,5)], produced by a supersonic expansion, have been investigated using vibrational predissociation spectroscopy in conjunction with ab initio calculations based on density functional theory (DFT). The observed spectra in the frequency range 2650-3850 cm(1) show some broad absorption bands attributed to the free and hydrogen bonded OH stretches of OH (H2O)(4,5) at an estimated cluster temperature of 170 K. DFT calculations performed at the B3LYP/6-31+G* level reveal five and eight possible low lying isomeric forms for OH-(H2O)(4) and OH-(H2O)(5), respectively. The global minimum isomer of the tetramer is tri-solvated cyclic, which is energetically more stable than the tetra-solvated wheel-shaped form with an OH ion at its centre. Compact cage-like lowest energy structures are found for the pentamer, in which the water molecules can act either as a single-donor-single-acceptor, as a double-proton-donor, or as a double-donor-single-acceptor in both the first and the second solvation shell of the OH ion core. Interconversion among the isomers appears to be rapid as manifested in the observed spectra dominated by broad and congested absorptions. To understand the nature of spectral broadening and congestion, systemic comparisons of the results are made against those of the corresponding protonated cations, H+(H2O)(n+1) and the corresponding halide anions, X-(H2O)(n), X = F, Cl, Br, and I. It is suggested that the spectral complexities observed for OH-(H2O)(4,5) are predominantly a result of sampling configurations with a large distribution of O-solvent-O-ion-O-solvent angles and O-solvent. . .H-O-solvent distances between water molecules in the first and/or second hydration shells, together with the existence of more than one isomer in the supersonic expanson and rapid isomeric interconversion among them.