Raman OD-Stretching Overtone Spectra from Liquid D₂O between 22 and 152 °C

Raman OD-stretching (first) overtone spectra, X(Z, X+Z)Y, X(ZZ)Y, and X(ZX)Y, from pure D 2 O between 22 and 98 °C yield an isosbestic frequency near 5130 cm -1 . This value corresponds to the X(ZX)Y fundamental isosbestic observed near 2624 cm -1, but not to the X(ZZ)Y fundamental isosbestic near 2429 cm -1, because the fundamental 2375 cm -1 correlated deuteron OD stretch, ν 1, and its overtone, 2ν 1, are weak in the X(ZX)Y spectra, compared to the three higher-frequency fundamentals, ν 2, ν 3, and ν 4, and their overtones. The 2375 and 2475 cm -1 fundamentals and their overtones probably engage in coupling with a ≈175 cm -1 LA phonon which is supported by H-bonded aggregates or patches, whereas the corresponding data for H 2 O provide clear and unequivocal evidence for this LA coupling. Deconvolution of the X(Z,X+Z)Y overtone spectra, 22−152 °C, plus van't Hoff treatment yields an enthalpy for H bond, O−D···O, rupture of 2.8 ± 0.2 kcal/mol O−D···O, in good agreement with present values of 2.6 ± 0.2 and 2.8 ± 0.2 kcal/mol O−D···O, obtained from the fundamental X(ZZ)Y and X(ZX)Y spectra. Raman overtone measurements are important because the spacing between the four-Gaussian components is large in the overtone OD- and OH-stretching contours compared to the fundamentals.