Revealing CSA Tensors and Hydrogen Bonding in Methoxycarbonyl Urea: A combined ¹³C, ¹⁵N and ¹³C¹⁴N₂ Dipolar Chemical Shift NMR and DFT Study

Abstract

Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by 13C and 15N 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 15N nuclei and of two out of the three 13C 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 13C(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 13C(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the 13C CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where δ11 points approximately in direction of N(imide), δ22 approximately in direction of the C=O bond, and δ33 is oriented perpendicular to the molecular frame, is adopted in the molecule.

Polymorphism of N,N′′‐Diacetylbiuret Studied by Solid‐State 13C and 15N NMR Spectroscopy, DFT Calculations, and X‐ray Diffraction

“…All experiments performed with the 14 T machine employed TPPM decoupling [19] and ramped amplitude cross-polarization (RAMP-CP). [20] 13 C NMR chemical shifts are referenced [16] to tetramethylsilane (TMS) using external secondary shift standards: adamantane (d = 29.50 and 38.56 ppm for the two adamantane signals [21] ) or glycine (d = 43.3 and 176.48 ppm for the two glycine signals [22] ). 15 N NMR chemical shifts are referenced to solid NH 4 Cl employing either an external sample of solid NH 4 Cl or glycine using the expression d(NH 4 Cl,solid)Àd(glycine,solid) = 6.4 ppm.…”

“…The calculated chemical shielding values s were converted to chemical shifts d by using the 13 C absolute chemical shielding of TMS at 188.1 ppm [29] and the 15 N absolute chemical shielding of ammonia at 267.3 ppm calculated in the same way as described above. [16] X-ray diffraction: Reflections were collected with a CCD Bruker-AXS Smart diffractometer with an w scan at 173(2) (a-DAB) and 223 (2) The crystal and refinement data were produced using the SHELXS-97 [30] and SHELXL-97 [31] programs (see Table 4). CCDC-620843 and CCDC-620844 contain the supplementary crystallographic data for this paper.…”

“…This analytical approach was recently applied by us to methoxycarbonylurea (MCU), another promising long-term fertilizer. [16,17] In that case, singlecrystal X-ray crystallography could not be applied because the single crystals of MCU obtained exhibit lattice disorder. Instead we resorted to standard [16] and more elaborate [17] techniques of solid-state NMR spectroscopy in combination with an isotope-labeling strategy and DFT calculations for structure elucidation ("NMR crystallography").…”

“…[16,17] In that case, singlecrystal X-ray crystallography could not be applied because the single crystals of MCU obtained exhibit lattice disorder. Instead we resorted to standard [16] and more elaborate [17] techniques of solid-state NMR spectroscopy in combination with an isotope-labeling strategy and DFT calculations for structure elucidation ("NMR crystallography"). In this study on DAB, the results obtained by using these techniques were rather ambiguous compared with MCU owing to the inherent molecular symmetry of the primary structure of DAB.…”

“…Only the asymmetry parameters are considerably larger for DAB, with h = 0.4 compared with 0.11 and 0.17, respectively. The CSA tensors for 15 NA C H T U N G T R E N N U N G (imide) in methoxycarbonylurea [16] and phthalimide [47] show pronounced differences to these three tensors. There is no simple explanation for this behavior.…”