Symmetry breaking in the solid state of 9,10-anthracene amino acid conjugates as seen by X-ray diffraction and NMR spectroscopy

Abstract

Three novel structures of 9,10-anthracene amino acid conjugates have been determined by combination of single crystal and powder X-ray diffraction measurements, pseudopotential plane wave DFT optimizations and 13C solid state NMR spectroscopy, including GIPAW calculation of the NMR parameters. All three structures show anti-conformation of amino acid side chains attached to the anthracene unit. For compounds Ant–(CO–Phe-OMe)2, (1) and Ant–(CO–Gly-Phe-OMe)2 (2), anti-conformation is in accordance with nearly centrosymmetric crystallographic structures, while for Ant–(CO–Val-OMe)2 (3), anti-conformation is in accordance with approximate 2-fold molecular axis passing through the plane of anthracene unit. Centres of inversion in 1 and 2 are approximate due to the usage of optically pure amino acids. Rotation axis in 3 is approximate due to the displacement of molecules from crystallographic 2-fold axes present in tetragonal P41212 space group, resulting in a structure more accurate described in space group P212121.