Prying apart a water molecule with anionic H-bonding:: A comparative spectroscopic study of the X-•H2O (X = OH, O, F, Cl, and Br) binary complexes in the 600-3800 cm-1 region

A detailed picture of the structural distortions suffered by a water molecule in direct contact with small inorganic anions (e.g., X = halide) is emerging from a series of recent vibrational spectroscopy studies of the gas-phase X-center dot H2O binary complexes. The extended spectral coverage (600-3800 cm(-1)) presently available with tabletop laser systems, when combined with versatile argon "messenger" techniques for acquiring action spectra of cold complexes, now provides a comprehensive survey of how the interaction evolves from an ion-solvent configuration into a three-center, two-electron covalent bond as the proton affinity of the anion increases. We focus on the behavior of H2O in the X-center dot H2O (X = Br, Cl, F, O, and OH) complexes, which all adopt asymmetric structures where one hydrogen atom is H-bonded to the ion while the other is free. The positions and intensities of the bands clearly reveal the mechanical consequences of both (zero-point) vibrationally averaged and infrared photoinduced excess charge delocalization mediated by intracluster proton transfer (X-center dot H2O -> HX center dot OH-). The fundamentals of the shared proton stretch become quite intense, for example, and exhibit extreme red-shifts as the intracluster proton-transfer process becomes available, first in the vibrationally excited states (F-center dot H2O) and then finally at the zero-point level (OH-center dot H2O In the latter case, the loss of the water molecule's independent character is confirmed through the disappearance of the similar to 1600 cm(-1) HOH intramolecular bending transition and the dramatic (> 3000 cm(-1)) red-shift of the shared proton stretch. An unexpected manifestation of vibrationally mediated charge transfer is also observed in the low frequency region, where the 2 <- 0 overtones of the out-of-plane frustrated rotation of the water are remarkably intense in the Cl-center dot H2O and Br-center dot H2O spectra. This effect is traced to changes in the charge distribution along the X-center dot O axis as the shared proton is displaced perpendicular to it, reducing the charge transfer character of the H-bonding interaction and giving rise to a large quadratic contribution to the dipole moment component that is parallel to the bond axis. Thus, all of these systems are found to exhibit distinct spectral characteristics that can