Structure elucidation of cyameluric acid by combining solid-state NMR spectroscopy, molecular modeling and direct-space methods L. Seyfarth a , J. Sehnert a , N.E.A. El-Gamel b,c , W. Milius a , E. Kroke b , J. Breu a , J. Senker a, * a Anorganische Chemie I, Universita ¨ t Bayreuth, Universita ¨ tsstrasse 30, 95440 Bayreuth, Germany b Institut fu ¨ r Anorganische Chemie, TU Bergakademie Freiberg, Leipziger Strasse 29, 09596 Freiberg, Germany c Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt Received 6 December 2007; accepted 6 February 2008 Available online 16 February 2008 Abstract We present the structure solution for solvent-free cyameluric acid which has first been synthesized more than 150 years ago. By den- sely intertwining the complementary methods solid-state NMR spectroscopy, molecular modeling as well as direct-space methods for the analysis of the X-ray powder diffraction data we succeeded where conventional powder diffraction analysis alone failed. In a first step, the correct tautomer was identified by combining solid-state NMR and ab initio calculations. The quantum-chemically optimized molecule was further employed in direct-space methods for the crystal structure solution. After a subsequent Rietveld refine- ment the positions of the hydrogen atoms in the crystal were determined by comparing experimental and calculated NMR chemical shift parameters. In this context, the sensitivity of the anisotropy d aniso and the asymmetry parameter g of the carbonyl 13 C towards the hydro- gen bonding environment proved to be very valuable. Cyameluric acid crystallizes in space group P2 1 2 1 2 1 (Z = 4, a = 6.4701(5) A ˚ , b = 9.9340(6) A ˚ , c = 12.0985(7) A ˚ ) with one complete molecule in the asymmetric unit. It consists of the symmetric trioxo tautomer which is arranged in a three-dimensional hydrogen bond network where all three NH groups interact with carbonyl groups. Ó 2008 Elsevier B.V. All rights reserved. Keywords: s-Heptazine derivatives; Molecular modeling; Chemical shift anisotropy; X-ray powder diffraction; Hydrogen bonding 1. Introduction Nowadays the design of new materials with tailored properties is a central topic in materials research. A key step in this target-oriented approach is the understanding of relationships between the chemical structure and its spe- cific physical properties. For the investigation of such structure–property relationships it is essential to determine the structures of a broad variety of material classes. For single crystals the structure solution can be per- formed routinely with X-ray diffraction techniques. A chal- lenge then pose only the localization of light atoms because of their low scattering forces and the differentiation of atoms with adjacent atomic numbers due to their similar scattering forces. Compounds with good crystallization properties have therefore thoroughly been characterized over the last 50 years. Materials for which only microcrys- talline powder can be obtained pose a much greater chal- lenge. The structure solution from powder diffraction data is distinctly more demanding as in this case the three-dimensional information about the crystal structure is reduced to one-dimensional data. In addition, severe overlap of the diffraction reflexes further complicates the analysis. In most cases, a routine treatment is hence not possible and individual strategies have to be developed depending on the structural details of the material [1–4]. Mostly, the deficit of information is overcome using complementary information from alternative methods like molecular modeling or various spectroscopies (e.g. liquid and solid-state NMR, IR) [5–9]. For molecular crystals 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.02.005 * Corresponding author. Tel.: +49 921552532; fax: +49 921 55 2788. E-mail address: juergen.senker@uni-bayreuth.de (J. Senker). www.elsevier.com/locate/molstruc Available online at www.sciencedirect.com Journal of Molecular Structure 889 (2008) 217–228