Martin U. Schmidt (Goethe Universität, Frankfurt am Main, Germany)

The crystal structure of the poorly crystalline, organic red pigment 2-monomethyl-quinacridone (C21H14N2O2) was solved from X-ray powder diffraction data. The structure solution led to a crystal structure in P-1 with Z = 1, with a molecule on the inversion centre. Correspondingly, the molecule, which itself has no inversion symmetry, must be orientionally disordered on two orientations, with a disorder of CH3 versus H. The disorder and the local structure were investigated using various ordered structural models in P1 and P-1, Z = 1, 2, and 4. All models were analysed by three approaches: Rietveld refinement, fit to the pair-distribution function (PDF), and lattice-energy minimisation.

All Rietveld refinements with TOPAS V4 [1] converged with acceptable R-values. All fits to the PDF using TOPAS V6 [2] were quite reasonable. The lattice-energies of the optimised structures using the DREIDING [3] force field were within a range of 6 kJ mol-1. In all methods there were small, but significant differences between the various structural models. In conclusion, all methods favour a statistical orientational disorder with a preferred antiparallel orientation of molecules in neighbouring chains. [4]

[1] Coelho A. A., TOPAS-Academic 4.1, Coelho Software, 2007, Brisbane.

[2] Coelho A. A. J. Appl. Cryst. 2018, 51, 210-218.

[3] Mayo S. L., Olafson B. D., Goddard III W. A., J. Phys. Chem., 1990, 94, 8897.

[4] Schlesinger, C., Hammer, S. M., Schmidt, M. U., submitted.