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 vibrationa l predissociation spectroscopy in conjunction with ab initio calculations b ased on density functional theory (DFT). The observed spectra in the freque ncy range 2650-3850 cm(1) show some broad absorption bands attributed to th e free and hydrogen bonded OH stretches of OH (H2O)(4,5) at an estimated cl uster 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 tetrame r 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 mo lecules can act either as a single-donor-single-acceptor, as a double-proto n-donor, or as a double-donor-single-acceptor in both the first and the sec ond solvation shell of the OH ion core. Interconversion among the isomers a ppears 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 correspondin g 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 resul t of sampling configurations with a large distribution of O-solvent-O-ion-O -solvent angles and O-solvent. . .H-O-solvent distances between water molec ules in the first and/or second hydration shells, together with the existen ce of more than one isomer in the supersonic expanson and rapid isomeric in terconversion among them.