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, R1NHC(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 CO bond and the principal component corresponding to the largest shielding, δ33, is perpendicular to the NC(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.

Isotropic and anisotropic NMR chemical shifts in liquid water: a sequential QM/MM study

“…18 The authors verified that the continuum method fails even at reproducing the sign of the isotropic chemical shift and that the discrete approximation is more reliable. They also verified that the rigid cluster approximation, in the case of (H 2 O) 5 provided results close to that obtained from the average over the structures from simulations. However the best agreement between the experimental and theoretical values was given by the Malkin sampling method, 13 that represented the chemical shift as a distribution of values around an average obtained from quantum calculations and not as an absolute value obtained by a single calculation.…”

“…[2][3][4][5]13,14 We used it in this work to describe the isotropic and anisotropic chemical shifts in liquid water. As a first approach, we have employed the hybrid functional B3LYP 36,37 with the purpose of studying the convergence with the number of water molecules used (cluster size).…”

“…The advances in computational resources and the improvement in the theoretical methods provide important refinements to estimate the chemical shielding, such as inclusion of the electron correlation effects and the possibility of studying systems that are more complex than isolated molecules or small clusters. Presently the determination of the chemical shifts for nuclei in proteins, solids and liquids through current methodologies [1][2][3][4][5] are very convincing.…”

Cooperative effects in two-dimensional ring-like networks of three-center hydrogen bonding interactions

“…Masunov and Dannenberg 43 investigated one-dimensional hydrogen-bonding aggregates, chains and ribbons, of urea and thiourea; these authors found the cooperative interactions for the urea and thiourea chains to be similar, whereas the cooperative interactions for both ribbons were found to be negligible. Dong et al 44 reported the first experimental determination of the carbonyl 17 O electric-field-gradient tensor and chemical-shift tensor; the strong hydrogen-bonding effects on these quantities were studied by systematically modeling the H-bond network in crystalline urea with several molecular clusters including bifurcated and other multicenter H bonds. The fundamental role of bifurcated H bonds in proteins, DNA crystal structures, and other biologically relevant systems has been amply demonstrated.…”

Cooperative effects in one-dimensional chains of three-center hydrogen bonding interactions

“…Masunov and Dannenberg 29 investigated one-dimensional hydrogenbonding aggregates, chains and ribbons, of urea and thiourea; these authors found the cooperative interactions for the urea and thiourea chains to be similar, whereas the cooperative interactions for both ribbons were found to be negligible. Wu et al 30 reported the first experimental determination of the carbonyl 17 O electric-field-gradient ͑EFG͒ tensor and chemical-shift tensor; the strong hydrogen-bonding effects on these quantities were studied by systematically modeling the H-bond network in crystalline urea with several molecular clusters including bifurcated and other multicenter a͒ Author to whom correspondence should be addressed. Electronic mail: rparra1@depaul.edu H bonds.…”