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

Mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their whole composition range were studied with infrared and far-infrared ~THz! spectroscopy. Experimental spectra of all mixtures were obtained below 120 cm, and spectra of methanolic mixtures were also measured from 400 to 1000 cm. The mixtures display ideal absorption spectra below 120 cm, contrasting with significant nonidealities in the absorption of aqueous mixtures in the same frequency range. Dramatic changes as a function of composition were found in methanolic mixtures at higher frequencies. The high frequency librational band of methanol, centered at 670 cm, shifts to substantially lower frequencies upon dilution, indicating marked changes in the librational motion of the hydroxyl hydrogen of methanol. This band is a sensitive probe of the hydrogen bonding environment experienced by methanol molecules. © 2000 American Institute of Physics. @S0021-9606~00!51332-4#