Infrared spectra of H(2)16O, H(2)18O and D(2)O in the liquid phase by single-pass attenuated total internal reflection spectroscopy

Mid-infrared attenuated total internal reflection (ATR) spectra of H(2)16O, H(2)18O and D(2)16O in the liquid state were obtained and normal coordinate analysis was performed based on the potential energy surface obtained from density functional theory (DFT) calculations. Fits of the spectra to multiple Gaussians showed a consistent fit of three bands for the bending region and five bands for the stretching region for three isotopomers, H(2)16O, H(2)18O and D(2)16O. The results are consistent with previous work and build on earlier studies by the inclusion of three isotopomers and mixtures using the advantage of single-pass ATR to obtain high quality spectra of the water stretching bands. DFT calculation of the vibrational spectrum of liquid water was conducted on seven model systems, two systems with periodic boundary conditions (PBC) consisting of four and nine H(2)16O molecules, and five water clusters consisting of 4, 9, 19, 27 and 32 H(2)16O molecules. The PBC and cluster models were used to obtain a representation of bulk water for comparison with experiment. The nine-water PBC model was found to give a good fit to the experimental line shapes. A difference is observed in the broadening of the water bending and stretching vibrations indicative of a difference in the rate of pure dephasing. The nine-water PBC calculation was also used to calculate the wavenumber shifts observed in the water isotopomers.