Abstract
Near-infrared spectra were measured for water-methanol mixtures with a methanol content of 0-100 wt % at 25 °C. The second derivative of the NIR spectra clearly reveals that the first overtones of the CH<sub>3</sub> asymmetric stretching modes of methanol near 5950 and 5900 cm<sup>-1</sup> show a downward shift by about 30 cm<sup>-1</sup> with the increase in the concentration of methanol. This is unambiguous evidence for the direct interaction of the CH<sub>3</sub> group of methanol with the OH group of water. Similar downward shifts were also observed for water-ethanol and water-1-propanol mixtures, although the shifts are much smaller for the water-ethanol and water-1-propanol mixtures. Based upon the results of NIR spectra, we propose a model for the interaction between the C-H bond of the CH<sub>3</sub> group of methanol and water. This intermolecular interaction is analogous to the intramolecular "(O) CH · · · O" interaction in compounds having both a CH<sub>3</sub> group binding to an oxygen atom and an OH group. This paper demonstrates novel potential of NIR spectroscopy in investigating interactions containing a C-H bond.
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