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 O-17 electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R1NH-C(O)-NHR2. Analysis of magic-angle spinning (MAS) and stationary O-17 NMR spectra of crystalline [170]urea yields not only the principal components of the carbonyl O-17 EFG and CS tensors, but also their relative orientations. The carbonyl O-17 quadrupole coupling constant (QCC) and the asymmetry parameter (eta) in crystalline urea were found to be 7.24 +/- 0.01 MHz and 0.92, respectively. The principal components of the O-17 CS tensor were determined: delta (11) = 300 +/- 5, delta (22) = 280 +/- 5 and delta (33) = 20 +/- 5 ppm. The direction with the least shielding, delta (11), is perpendicular to the C=O bond and the principal component corresponding to the largest shielding, delta (33), is perpendicular to the N-C(O)-N plane. The observed O-17 CS tensor suggests that, in crystalline urea, the O-17 paramagnetic shielding contributions from the sigma --> pi* and pi --> sigma* mixing are greater than that from the n --> pi* mixing. Quantum chemical calculations revealed very large intermolecular I-I-bonding effects on the O-17 NMR tensors. It is demonstrated that inclusion of a complete intermolecular H-bonding network is necessary in order to obtain reliable O-17 EFG and CS tensors. B3LYP/D95** and B3LYP/6-311++G** calculations with a molecular cluster containing 7 urea molecules yielded O-17 NMR tensors in reasonably good agreement with the experimental data.