Seeking the structure of water from the combination of bending and stretching vibrations in near infrared spectra

Abstract

Near infrared (NIR) spectroscopy has been used to analyze water structures due to the strong absorption of NIR energy by water. The spectral band around 6900 cm−1, corresponding to the first overtone of the OH stretching vibration, is generally studied because the OH in the water molecule with different numbers of hydrogen bonds can be distinguished. In this work, the spectral band around 8600 cm−1, corresponding to the combination of HOH bending and stretching vibration, ν1+ν2+ν3, was studied to extract spectral information about water structures. Continuous wavelet transform was used to enhance the resolution of the spectra. Seven peaks related to the possible molecular structures of water with different numbers of hydrogen bonds were identified based on the spectral changes with temperature. The identification was validated by varying the spectral peaks with molar ratio of H2O–D2O in mixtures and the effect of hydration around the cations on the structure of water. NIR spectroscopy is therefore proven to be a powerful technique for identifying water structures with different hydrogen bonds.