On the calculations of the vibrational Raman spectra of small water clusters

The static and frequency-dependent Raman intensities for small water clusters ((H2O)n, n = 2-8) were calculated by means of the DFT(B3LYP)/d-aug-cc-pVDZ method. The obtained intensities were analyzed in order to find a correlation between the topology of a cluster and its calculated Raman spectrum. A change from 170 for water dimer to 677 Å4 u−1 for hexamer-ring cluster reveals a tendency of a practically linear increase in the Raman intensity of the lowest-frequency symmetric OH stretching mode with a number of water molecules in cyclic clusters (including dimer). The stretching vibrations of the three-coordinated single donor-double acceptor (DAA) molecules are marked by higher Raman intensities than the double donor-single acceptor (DDA) molecules. The simple average Raman intensity over the stretching modes of the DAA molecules yields 110 Å4 u−1 while the corresponding value for the DDA molecules is 70 Å4 u−1. The Raman intensities calculated at 351.1 nm are larger by 10-40% than the static ones. Previously suggested in literature origin of the shoulder on the blue side of the Raman spectrum of liquid water as arising from the water molecules whose protons are not engaged to hydrogen bond can be confirmed by our calculations.