A proton NMR investigation of rotation about the C(O)-N bonds of urea

The rates and activation energies have been determined by proton NMR for H_2^(15)N(CO)^(14)NH_2 1 and hydrogen exchange equilibrated mixtures of 1 and H_2^(15)N(CO)^(15)NH_2 2 with D_2NC(O)ND_2 3. Synthesis of 1 was achieved by the classic Wöhler procedure from ^(15)NH_4 C=^(14)N=O with no detectable formation of H_2^(15)N(CO)^(15)NH_2. In the slow-rotation regime, the chemical-shift difference between the protons cis and trans to the carbonyl oxygen decreases rather rapidly with increasing temperature (0.0063 ppm/º). Coupling parameters were extracted from the rather complex six-spin system afforded by H_2^(15)NC(O)^(1)5NH_2. The activation parameters for the rotation in dimethylformamide-dimethyl sulfoxide solution were ΔG^* (245 K) = 11.2 ± 0.3 kcal/mol and E_a = 13.5 ± 0.25 kcal/mol. Significant changes in the sensitivity of the variations of the proton shifts with temperature were found with substitution of deuterium for hydrogen and addition of water to the solvent. The overall results are in accord with the resonance interpretation of the barrier to rotation about the C(O)-N bonds of amides and ureas