Revealing the configuration and crystal packing of organic compounds by solid-state NMR spectroscopy

The molecular configuration and intermolecular arrangement of polycrystalline methoxycarbonylurea (MCU) has been studied by a combination of chemical editing, rotational echo double resonance (REDOR) spectroscopy and ab initio calculations. From the multispin ISn REDOR experiments several dipolar couplings were determined and converted into distance constraints. Intra- and intermolecular dipolar couplings were distinguished by isotope dilution. The configuration of the MCU molecule can be determined from three torsion angles Psi(1), Psi(2), and Psi(3). Ab initio calculations showed that these angles are either 0degrees or 180degrees (Z or E). From the REDOR experiments, the E configuration was found for Psi(1) and Psi(2) and the Z configuration for Psi(3). Thus the configuration of MCU in the solid state was determined to be EEZ. Distance constraints for the intermolecular arrangement of MCU were obtained by performing REDOR experiments on (CN2)-C-13-N-15 MCU with different degrees of isotope dilution and on a cocrystallized 1:1 mixture of C-13(urea) MCU and N-15(amide) MCU. By combining these distance constraints with molecular modeling, three different possible packing motifs for MCU molecules were found. The molecules in these motifs are arranged as linear chains with methoxy groups at the borders of the chains. All the intermolecular hydrogen bond donors and acceptors in the interior of the chain are saturated.