Knowledge Bank

Isotropic Raman spectra of $H2O,D2O$ and HOD bonded to dimethyl sulfoxide have been obtained over a range of concentrations and temperatures. At low water concentrations we find that the hydrogens of a water molecule are bound to the base molecule with weak but symmetric hydrogen bonds. Also present are water molecules whose hydrogens are connected by unsymmetric hydrogen bonds and evidence is presented that indicates that the Raman band commonly attributed to a ``free OH stretch” is not associated with a completely free OH stretch but with a very weak hydrogen bond. Fermi resonance between the OH stretch and the first overtone of the bending mode is shown to be important. The Fermi-resonance operator W agrees well with the $36cm-1$ and $25cm-1$ values obtained by Overend (1) et. al. for gaseous $H2O$ and $D2O$. We have used our model to interpret the isotropic Raman spectrum of water/DMSO mixtures from 5 mole % to 100 mole % water. At low temperatures the model indicates that pure water contains one species having a distribution of symmetric dihydrogen bonded molecules. At high temperatures the symmetric species are largely converted to the asymmetric type.