Theoretical IR spectra for water clusters (H2O)n (n = 6-22, 28, 30) and identification of spectral contributions from different H-bond conformations in gaseous and liquid water.

The vibrational IR spectra in the O-H stretching region are computed for water clusters containing 6-22, 28, and 30 molecules using quantum-chemical calculations (B3LYP and an augmented basis set). For the cluster with 20 molecules, several different structures were studied. The vibrational spectrum was partitioned into contributions from different molecules according to their coordination properties. The frequency shifts depend on the number of donated/accepted H-bonds primarily of the two molecules participating in the H-bond, but also of the surrounding molecules H-bonding to these molecules. The frequencies of H-bonds between two molecules of the same coordination type are spread over a broad interval. The most downshifted hydrogen-bond vibrations are those donated by a single-donor 3-coordinated molecule where the H-bond is accepted by a single-acceptor molecule. The H-bonded neighbors influence the downshift, and their contribution can be rationalized in the same way as for the central dimer. Single donors/acceptors cause larger downshifts than 4-coordinated molecules, and the least downshift is obtained for double donors/acceptors. This result is at variance with the conception that experimental liquid water spectra may be divided into components for which larger downshifts imply higher H-bond coordination. A mean spectral contribution for each coordination type for the donor molecule was derived and fitted to the experimental liquid water IR spectrum, which enabled an estimation of the distribution of H-bond types and average number of H-bonds (3.0 +/- 0.2) in the liquid.