Raman Spectral Studies of HDO in H2O

Argon‐ion‐laser and mercury‐excited photoelectric Raman spectra have been obtained from a 6.2M solution of D₂O in H₂O in the temperature range of 16°‐97°C and at frequencies from 2000‐3000 cm⁻¹. Examination of the OD stretching contours observed near 2500‐2600 cm⁻¹ revealed marked asymmetry, and a high‐frequency shoulder was apparent in all cases. In addition, an isosbestic point was found at 2570 ± 5 cm⁻¹. The OD stretching contours were analyzed by means of a special‐purpose analog computer, and two Gaussian components near 2510‐2540 and 2630‐2660 cm⁻¹ were found to provide adequate fits of the contours in the temperature range involved, although some indication of a third weak Gaussian Raman component was found, and three Gaussian components were employed to fit the infrared‐absorbance contours also obtained in this work. The integrated Raman intensities of the two principal Gaussian components were observed to have opposite temperature dependences, and the component intensities yielded a value of 2.5 ± 0.6 kacl/mole for the van't Hoff $\mathrm{\Delta H°}$ associated with the disruption of one O–D···O unit in good agreement with the O–H···O values of 2.₈ kcal/mole obtained from previous Raman studies of water and 2.4 kcal/mole obtained by Worley and Klotz from near‐infrared studies of HDO in D₂O. The presence of asymmetry in the form of a pronounced high‐frequency shoulder, and of an isosbestic point, constitute the principal experimental observations indicating that the OD stretching contours observed in the Raman spectra are composed of two or more components. The opposite temperature dependences of the integrated Raman intensities of the principal Gaussian components considered to comprise the OD stretching band, the resulting $\mathrm{\Delta H°}$ value, and agreements between $\mathrm{\Delta H°}$ values from various methods provide additional support for the two‐state model of water structure. No evidence was obtained in support of the continuum model proposed by some workers.