Revealing the Configuration and Crystal Packing of Organic Compounds by Solid-State NMR Spectroscopy: Methoxycarbonylurea, a Case Study Sven Macholl, [a, b] Frank Börner, [c] and Gerd Buntkowsky* [a] Introduction In a recent study supported by BASFAG, [1] methoxycarbo- nylurea (MCU) and other related urea compounds were in- vestigated as potential long-term nitrogen fertilizers with in- termediate solubility in water. Understanding the solubility properties of MCU could help in the more efficient screen- ing of the fertilizer properties of other urea derivatives. For this, detailed knowledge of the molecular configuration and intra- and intermolecular hydrogen-bonding properties of the urea compounds is necessary. Single-crystal X-ray crys- tallography could not be applied to the system because the single crystals exhibit lattice disorder. Recently we reported astaticdipolarsolid-stateNMRstudyofMCU [2] . For the study reported herein, we chose a more general strategy. The molecular configuration of MCU in the solid state was studied by a combination of selective 13 C and/or 15 N-isotope labeling schemes (chemical editing), dipolar solid-state NMR spectroscopy [3] and ab initio calculations. The experimental NMR data were used to obtain a network of interatomic distance constraints. These constraints were converted into torsion angles, which yielded the molecular configuration.Additionally,intermoleculardistancesprovide hints about the intermolecular arrangement of MCU in the crystal (NMR crystallography). [4–7] Our strategy was based on the following basic idea: that the nitrogen and all the carbon atoms, except for the me- thoxy group, of the amide, urea and ester functions of the MCU molecule (see Figures1–3) are expected to be mainly sp 2 hybridized. In this case, the C N bonds show some double-bond character and a planar arrangement is formed (apart from the H(methoxy) atoms). This assumption is sup- ported by results of our previous study [2] and by ab initio calculations reported in this study (see below). By using bondlengthsandbondanglesderivedfromageometryopti- mization by a molecular modeling program, the secondary structure of the MCU backbone is characterized by three torsion angles Y 1 , Y 2 and Y 3 (see Figure2). Owing to the [a] Dr.S.Macholl,Dr.G.Buntkowsky InstitutfürChemie,FreieUniversitätBerlin Takustrasse3,14195Berlin(Germany) Fax:(+ 49)30-838-55310 E-mail:bunt@chemie.fu-berlin.de [b] Dr.S.Macholl Presentaddress:InstitutfürBiometrieundMedizinischeInformatik Abteilung Experimentelle Bildverarbeitung Otto-von-Guericke-UniversitätMagdeburg LeipzigerStrasse44,39120Magdeburg(Germany) [c] Dr.F.Börner Fraunhofer-InstitutfürAngewandtePolymerforschung Postfach126,14476Golm(Germany) Abstract: The molecular configuration and intermolecular arrangement of polycrystalline methoxycarbonylurea (MCU)hasbeenstudiedbyacombina- tion of chemical editing, rotational echo double resonance (REDOR) spectroscopy and ab initio calculations. From the multispin IS n REDOR ex- periments several dipolar couplings were determined and converted into distance constraints. Intra- and inter- molecular dipolar couplings were dis- tinguished by isotope dilution. The configuration of the MCU molecule can be determined from three torsion angles Y 1 , Y 2 , and Y 3 . Ab initio calcu- lations showed that these angles are either 08 or 1808 (Z or E). From the REDOR experiments, the E configura- tion was found for Y 1 and Y 2 and the Z configuration for Y 3 . Thus the con- figuration of MCU in the solid state was determined to be EEZ. Distance constraints for the intermolecular ar- rangement of MCU were obtained by performing REDOR experiments on 13 C 15 N 2 MCU with different degrees of isotope dilution and on a cocrystallized 1:1 mixture of 13 C(urea) MCU and 15 N(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 hy- drogen bond donors and acceptors in the interior of the chain are saturated. Keywords: configuration determi- nation · hydrogen bonds · molecular packing · NMR crystallography · REDOR #2004Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim DOI: 10.1002/chem.200400191 Chem. Eur. J. 2004, 10,4808–4816 4808 FULL PAPER