Far-infrared optical constants of liquid acetonitrile at 238 to 343 K as measured with a synchrotron radiation source

Far-infrared absorption spectra of liquid acetonitrile at temperatures ranging from 238 to 343 K were measured with a source of synchrotron orbital radiation. On the basis of an optical theory, a theoretical expression for transmittance of a cell filled with liquid was obtained as a function of the optical constants of the liquid. Assuming an analytical form with seven parameters for the absorption coefficients, the refractive indices were analytically calculated by the Kramers-Kronig relation. The parameters of the absorption coefficients were adjusted by least-squares fitting of the theoretical transmittance to the observed transmittance. The resulting optical constants at 298 K were in good agreement with the previously reported results by a laser ATR method. The effect of temperature on the band shape of the absorption is remarkable and will provide an important clue to molecular reorientational dynamics. On the other hand, the molecular dipole moments obtained from the integrated intensities are nearly independent of temperature, being 4·5 ± 0·1 D in the 238 to 343 K range.