Revealing the Configuration and Crystal Packing of Organic Compounds by Solid-State NMR Spectroscopy: Methoxycarbonylurea, a Case Study

• Published in: Chemistry : a European journal 2004-10, Vol.10 (19), p.4808-4816
• Main Authors: Macholl, Sven, Börner, Frank, Buntkowsky, Gerd
• Format: Article
• Language: English
• Subjects: configuration determination; hydrogen bonds; molecular packing; NMR crystallography; REDOR
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200400191

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 Ψ1, Ψ2, and Ψ3. Ab initio calculations showed that these angles are either 0° or 180° (Z or E). From the REDOR experiments, the E configuration was found for Ψ1 and Ψ2 and the Z configuration for Ψ3. 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 13C15N2 MCU with different degrees of isotope dilution and on a cocrystallized 1:1 mixture of 13C(urea) MCU and 15N(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. Dipolar solid‐state NMR spectroscopy is a method for determining the structure of, for example, poorly crystallizing or amorphous organic compounds. Our strategy was based on precise REDOR measurements of selected CN distances in methoxycarbonylurea (see picture). Isotope dilution experiments and numerical simulations of ISn spin systems have been employed to distinguish between intra‐ and intermolecular CN distances, thereby facilitating the molecular modeling of the configuration and packing of molecules: a step towards “solid state NMR crystallography”.