Raman Intensities of HDO and Structure in Liquid Water

Abstract

Photoelectric Raman spectra of HDO in 5-mole % isotopically substituted water show single, broad, slightly asymmetric bands in both of the regions containing the uncoupled fundamental stretching motions. An empirical correlation between O–H or O—D stretching frequency and O···O hydrogen-bonded distance has been used to map the frequency coordinate of the spectra onto an intermolecular distance scale, and the Raman bands are replotted on this abscissa. The resulting intermolecular distribution functions for liquid water agree with x-ray results and support the suggestion that the breadth of the HDO uncoupled stretching bands reflects the variation of intermolecular distance. Moreover, it is noted that the corresponding infrared and Raman bands are almost superposable, and that this indicates little hydrogen-bond breakage in liquid water. The distribution of intermolecular distances represented by the spectra is compared to that incorporated in some of the structural models for liquid water. It appears that a successful model should incorporate the homogeneity, isotropy, coherence, and disorder of the liquid in its local structure.