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.