An NMR Investigation of the Effect of Hydrogen Bonding on the Rates of Rotation about the C−N Bonds in Urea and Thiourea

The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/dimethyl sulfoxide solutions using ^1H and ^(15)N NMR. The chemical-shift behavior of the urea protons is consistent with a urea-acetate hydrogen-bonded complex involving both carboxylate oxygens and the urea hydrogens trans to the carbonyl oxygen with K_(assoc) = 120 ± 10. Line shape analysis of the temperature-dependent ^1H NMR spectra show that ΔG^⧧ for rotation about the C−N bond of urea changes only slightly from 11.0 ± 0.1 to 11.2 ± 0.1 kcal/mol on 1:1 molar addition of tetrabutylammonium acetate to a dilute solution of urea. A parallel investigation of the interaction of thiourea with tetrabutylammonium acetate gave a binding constant, K_(assoc) = 90 ± 10. The ΔG^⧧ for rotation about the C−N bond of thiourea was found to increase from 13.5 ± 0.1 to 14.0 ± 0.1 kcal/mol on 1:1 addition of tetrabutylammonium acetate to a dilute solution of thiourea in dimethylformamide/dimethyl sulfoxide. Measurements were also made of the self-association of several ureas and of ΔG⧧ for rotation about both C(O)−N bonds of 1,1-dimethylurea