An NMR investigation of the existence of halide and carboxylate co-solute effects on the rotational barrier about the C—N bonds in urea and thiourea

Rotational barriers of the NH_2 group in thiourea and benzylurea in solution become slightly larger in the presence of anionic hydrogen-bond acceptors. The interactions between the NH bond and different anions were investigated in acetonitrile solution using ^1H{^(14)N} variable-temperature NMR. Line-shape analyses of the spectra show a weakly anion-dependent increase in ΔG^‡ of rotation, with benzoate having the strongest effect and iodide the weakest. Hydrogen-bonding acidities of benzylurea and thiourea were also measured on the Abraham solute hydrogen-bond acidity scale by proton NMR