Nuclear-Magnetic-Resonance Spectra of Ureas in Aqueous Solution

Abstract

There is considerable confusion in the literature concerning the nuclear-magnetic-resonance spectra of ureas in aqueous solution. Redpath and Smith report that the spectrum of N,N'-dimethylurea exhibits a broad signal for the protons on nitrogen and an unsymmetrical doublet for the methyl groups. This doublet collapses to a singlet in acid solutions and is attributed to spin–spin coupling of the methyl protons with the [Equation] moiety, rather than to any type of hindered rotation. Under similar conditions, Birchall and Gillespie report no observable coupling between the CH3− and =N–H groups of N,N'-dimethylurea, even in the absence of acid. This lack of spin–spin coupling is attributed to rapid proton exchange between the urea nitrogens and the solvent. They report no observable signal due to the =N–H moiety. However, the =N–H signal was found by them in the spectrum of urea itself under similar conditions.