Temperature dependence of the Raman OH-stretching overtone from liquid water

The first Raman overtone of the OH-stretching vibration from highly purified liquid water has been examined quantitatively in the temperature range of 20–95 °C, and at frequencies from about 5500–8000 cm−1. The overtone Raman spectra show contour shapes, depolarization ratio dispersion, and an isosbestic frequency (6960±25 cm−1, uncorrected; 6900±25 cm−1, density and refractive index corrected) which, apart from the effects of anharmonicity, tend to mimic the fundamental, including its major four-component substructure. The two components at about 7030 and 6850 cm−1 were interpreted, respectively, in terms of nonhydrogen-bonded OH, and triply hydrogen-bonded OH groups, which are common to the three-bonded H2O species, whereas two components at about 6665 and 6160 cm−1 were interpreted as intermolecular coupling components of the fully hydrogen-bonded, i.e., four-bonded H2O species. A ΔH° value of −2.5 Kcal/mol hydrogen bond was obtained from the temperature dependence of ratios of the combined component intensities of these two pairs, in agreement with results from previous Raman fundamental measurements. The feature near 6160 cm−1 was found to be better resolved in the spectrum than its fundamental analog, indicating that anharmonicity is advantageous in elucidating contour substructure.