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

Abstract

We report the first experimental determination of the carbonyl 17O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R1NHC(O)NHR2. Analysis of magic-angle spinning (MAS) and stationary 17O NMR spectra of crystalline [17O]urea yields not only the principal components of the carbonyl 17O EFG and CS tensors, but also their relative orientations. The carbonyl 17O 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 17O CS tensor were determined: δ11 = 300 ± 5, δ22 = 280 ± 5 and δ33 = 20 ± 5 ppm. The direction with the least shielding, δ11, is perpendicular to the CO bond and the principal component corresponding to the largest shielding, δ33, is perpendicular to the NC(O)N plane. The observed 17O CS tensor suggests that, in crystalline urea, the 17O 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 17O NMR tensors. It is demonstrated that inclusion of a complete intermolecular H-bonding network is necessary in order to obtain reliable 17O EFG and CS tensors. B3LYP/D95** and B3LYP/6-311++G** calculations with a molecular cluster containing 7 urea molecules yielded 17O NMR tensors in reasonably good agreement with the experimental data.