Structure determination of 6,6′-bis(trifluoromethyl)-thiazine-indigo from laboratory powder data and lattice-energy minimisation

Abstract

Abstract 6,6′-bis(trifluoromethyl)-thiazine-indigo, C18H8F6N2O2S2, is a yellow derivative of the thiazine-indigo pigments. Due to its insolubility single crystals could not be grown; the structure had to be determined from powder diffraction data. The X-ray powder pattern – measured on a laboratory diffractometer in routine manner – could be indexed, although the crystallite size was only about 20 nm. The crystal struc ture was solved by lattice-energy minimisation using the program CRYSCA. Subsequently, the structure was refined by the Rietveld method using restraints. The compound crys tallises in space group P21/c with a = 15.4136(12), b = 7.8384(5), c = 7.7102(6) Å, β = 112.736(6)°, Z = 2, with molecules on inversion centres. The molecules are connected by double hydrogen bonds, resulting in planar chains. These chains form stacks in [001] direction. The density is unusually high (ϱ = 1.7875 g/cm3), this explains the observed insolubility of the compound.