Near-Infrared Spectroscopic Study of Interaction between Methyl Group and Water in Water—Methanol Mixtures

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 CH₃ asymmetric stretching modes of methanol near 5950 and 5900 cm⁻¹ show a downward shift by about 30 cm⁻¹ with the increase in the concentration of methanol. This is unambiguous evidence for the direct interaction of the CH₃ 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 CH₃ group of methanol and water. This intermolecular interaction is analogous to the intramolecular “(O) CH … O” interaction in compounds having both a CH₃ 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.