Solvent effects in infra-red spectra: C=O, C-H, and C-C vibrations

Abstract

Frequency displacements and intensities are reported for the C=O stretching fundamental and first overtone in acetone, acetaldehyde, and diisopropyl ketone, for the chloroform C-H stretching fundamental, and for the acetonitrile C-C stretching fundamental, all in a variety of non-polar and polar solvents. The solvent displacement of C-C is very small (_?1 K), for C-H and C=O it is to the red and of the order of 10-20 K, with C=O overtones being displaced about twice as much as the fundamentals. The Kirkwood relation between the frequency displacement and solvent dielectric constant is inadequate if the static D is used. The C=O results can be. interpreted in terms of two superimposed effects : (i) the electronic polarization of the solvent causes a frequency shift related to the solvent refractive index, and (ii) in polar solvents there is an additional shift due to solvent dipole orientation. Effect (ii) causes an added contribution to the intensity. The C-H results do not fit easily into this interpretation.