The carbonyl stretching frequencies for a series of ketones occur at a lower frequency in dimethyl sulfoxide solution than in carbon tetrachloride solution, and occur at intermediate frequencies depending upon the mole percent ratio (CH<sub>3</sub>)<sub>2</sub>SO/CCl<sub>4</sub>. Ketones of form (CH<sub>3</sub>)<sub>2</sub>C=O, Φ(C=O)CH<sub>3</sub>, and (Φ)<sub>2</sub>C=O exhibit carbonyl stretching frequencies which are affected in the same manner by solvent effects. In general, inductive and resonance effects play an important role in determining the unperturbed carbonyl stretching frequencies of ketones.
R. A. Nyquist, V. Chrzan, and J. Houck, "Chemical and Physical Effects Upon Carbonyl Stretching Frequencies: Ketones," Appl. Spectrosc. 43, 981-983 (1989)