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Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by  13 C and  15 N dipolar chemical shift NMR spectroscopy and  ab initio  calculations. Employing a combination of dipolar chemical shift NMR, selective isotope labeling and  ab initio  gas phase calculations, possible molecular structures and chemical shielding tensors of all  15 N nuclei and of two out of the three  13 C nuclei were revealed. Four possible stable configurations of the molecule with different energies were found in the calculations. The CSA tensors were calculated for these configurations. While the calculated  13 C(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tensor orientation, there are pronounced differences between calculated and experimental tensor eigenvalues. These differences are a clear indication of the presence of intermolecular hydrogen bonds in the experimental sample, which are neglected in the gas phase calculations. Four different possible orientations of the experimental  13 C(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the  13 C CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where δ 11  points approximately in direction of N(imide), δ 22  approximately in direction of the C=O bond, and δ 33  is oriented perpendicular to the molecular frame, is adopted in the molecule.

Revealing CSA Tensors and Hydrogen Bonding in Methoxycarbonyl Urea: A combined 13 C, 15 N and 13 C 14 N 2 Dipolar Chemical Shift NMR and DFT Study