A Combined Experimental and Theoretical ¹⁷O NMR Study of Crystalline Urea:  An Example of Large Hydrogen-bonding Effects

We report the first experimental determination of the carbonyl 17 O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R 1 NH C(O) NHR 2 . Analysis of magic-angle spinning (MAS) and stationary 17 O NMR spectra of crystalline [ 17 O]urea yields not only the principal components of the carbonyl 17 O EFG and CS tensors, but also their relative orientations. The carbonyl 17 O quadrupole coupling constant (QCC) and the asymmetry parameter (η) in crystalline urea were found to be 7.24 ± 0.01 MHz and 0.92, respectively. The principal components of the 17 O CS tensor were determined: δ 11 = 300 ± 5, δ 22 = 280 ± 5 and δ 33 = 20 ± 5 ppm. The direction with the least shielding, δ 11, is perpendicular to the C O bond and the principal component corresponding to the largest shielding, δ 33, is perpendicular to the N C(O) N plane. The observed 17 O CS tensor suggests that, in crystalline urea, the 17 O paramagnetic shielding contributions from the σ → π* and π → σ* mixing are greater than that from the n → π* mixing. Quantum chemical calculations revealed very large intermolecular H-bonding effects on the 17 O NMR tensors. It is demonstrated that inclusion of a complete intermolecular H-bonding network is necessary in order to obtain reliable 17 O EFG and CS tensors. B3LYP/D95** and B3LYP/6-311++G** calculations with a molecular cluster containing 7 urea molecules yielded 17 O NMR tensors in reasonably good agreement with the experimental data.