Intense depolarized Rayleigh scattering in Raman spectra of acids caused by large proton polarizabilities of hydrogen bonds

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Abstract
The depolarized Rayleigh scattering of liquid H2O and D2O and of aqueous solutions of HCl and DCl was studied as a function of concentration and temperature. This scattering is caused by time dependent changes of the anisotropy of the molecular polarizability. The scattering below 30 cm−1 is determined by a Lorentzian component. As shown in Ref. 1 with pure water, the scattering of this component is caused by the change of the molecular anisotropy of the polarizability due to breaking and formation of hydrogen bonds, whereby the mean lifetime of the hydrogen bonds determines the half-width. This mechanism is confirmed by our results obtained with pure liquid H2O and D2O. With the acid solutions, the intensity of the scattering of this Lorentzian increases strongly with increasing acid concentration. This intensity increase is caused by the large anisotropic proton polarizability of the hydrogen bond in the H5O2+ groupings. Thus, this intense scattering is in addition to the intense continua in the IR spectra, a second access to study proton polarizabilities of hydrogen bonds. The time dependence of these proton polarizabilities is caused by changes of the local electrical fields due to breaking and formation of hydrogen bonds in the neighborhood of the H5O2+ groupings and therefore determined by the mean lifetime of these bonds. The half-width of this Lorentzian becomes smaller with increasing acid concentration, indicating a larger mean lifetime of the hydrogen bonds in the neighborhood of the H5O2+ groupings. From the fact that the scattering intensity in the DCl solutions is less than in the HCl solutions, it is concluded that the easily polarizable hydrogen bonds are already considerably polarized by their polar environments.