Re-Examination of the Effect of Aprotic Solvents upon the Fundamental Vibrational Peak of the Carbonyl Group in 1,1,3,3-Tetramethylurea

Abstract

The 1650 cm−1 IR absorption band for the fundamental vibration of the C=O group in tetramethylurea (TMU) exhibits a substantial solvent-induced shift in polar and nonpolar aprotic solvents at 298 K. Earlier studies have rationalized the max(C=O) shifts in terms of the empirical Gutmann acceptor numbers (ANs) for the solvents or the Kirkwood parameter in dielectric permittivity. With a larger set of test data, other reaction field models are now compared which incorporate alternative continuum functions as elements in the formulation of an SCRF-MO perturbation description for the aprotic solvent effect upon the max(C=O) of TMU. The magnitude of the band shift appears to depend on both the solvent dipolarity and solvent polarizability, with the first being the dominant factor in the more reliable model.