Cryogenic investigation of molecular hydrogen in fullerenes by means of nmr , ir and

S (LECTURES) Abstract: CEUM 2008, Zagreb, September 29, 2008 L 1 CEUM 2008, Zagreb, September 29, 2008 L 1 NMR POWDER CRYSTALLOGRAPHY Lyndon EMSLEY 1 Université de Lyon, CNRS / ENS Lyon / UCB Lyon1, Centre de RMN à Très Hauts Champs, 5 rue de la Doua, 69100 Villeurbanne, France We will discuss various aspects of progress towards powder crystallography by NMR. Using high-resolution proton spectroscopy of powders, we present the refinement of the three-dimensional structure of organic compounds, at natural isotopic abundance, obtained by an approach that in the first step combines molecular modeling (MM) in the Xplor-NIH program with experimental proton spin-diffusion data (PSD) obtained from high-resolution solid-state NMR of protons. This approach enables us to refine the molecular structure of b-Asp-Ala at natural abundance and in powder form to obtain a group of structures with an average rmsd of 0.1Å, and which deviates from the known structure by only ~0.6Å. Additionally, the conformation of thymol in its crystalline arrangement is investigated following the same MM-PSD optimization scheme. Thymol is a monoterpene phenol found in oil of thyme, with strong antiseptic properties. Due to the ease of obtaining large crystals, it was among the first systems studied by crystallographers, even before the advent of X-Ray methods. We then show how the PSD-MM structures can be used as a starting point for further refinement based on plane wave DFT geometry optimization and chemical shift calculations. This procedure results in structures that are identical to the known X-ray structure to within <0.2Å, and its validity is confirmed by comparing the DFT calculated chemical shifts for H and C with the experimental shifts. We observe a substantial improvement in the agreement between the calculations and experiments after DFT structure optimization. We will also invoke aspects of small molecule crystallography involving paramagnetic metal centers as reference points for determination of structures in organometallic coordination complexes.