Revealing CSA tensors and hydrogen bonding in methoxycarbonyl urea:: A combined 13C, 15N and 13C14N2 dipolar chemical shift NMR and DFT study

Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by C-13 and N-15 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 N-15 nuclei and of two out of the three C-13 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 C-13(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tenser 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 C-13(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the C-13 CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where delta(11) points approximately in direction of N(imide), delta(22) approximately in direction of the C=O bond, and delta(33) is oriented perpendicular to the molecular frame, is adopted in the molecule.