Structure and dynamics of nonaqueous mixtures of dipolar liquids. I. Infrared and far-infrared spectroscopy

Mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitri le over their whole composition range were studied with infrared and far-in frared (THz) spectroscopy. Experimental spectra of all mixtures were obtain ed below 120 cm(-1), and spectra of methanolic mixtures were also measured from 400 to 1000 cm(-1). The mixtures display ideal absorption spectra belo w 120 cm(-1), contrasting with significant nonidealities in the absorption of aqueous mixtures in the same frequency range. Dramatic changes as a func tion of composition were found in methanolic mixtures at higher frequencies . The high frequency librational band of methanol, centered at 670 cm(-1), shifts to substantially lower frequencies upon dilution, indicating marked changes in the librational motion of the hydroxyl hydrogen of methanol. Thi s band is a sensitive probe of the hydrogen bonding environment experienced by methanol molecules. (C) 2000 American Institute of Physics. [S0021-9606 (00)51332-4].