On the stacking disorder of DL-norleucine

Abstract

DL-Norleucine (2-aminohexanoic acid, C6H13NO2) forms a double-layer structure in all known phases (α, β, γ). The crystal structure of the β-phase was redetermined at 173 K. Diffraction patterns of the α- and β-phases frequently show diffuse streaks parallel to c*, which indicates a stacking disorder of the layers. A symmetry analysis was carried out to derive possible stacking sequences. Lattice-energy minimizations by force fields and by dispersion-corrected density functional theory (DFT-D) were performed on a set of ordered model structures with Z = 4, 8 and 16 with different stacking sequences. The calculated energies depend not only on the arrangement of neighbouring double layers, but also of next-neighbouring double layers. Stacking probabilities were calculated from the DFT-D energies. According to the calculated stacking probabilities large models containing 100 double layers were constructed. Their simulated diffraction patterns show sharp reflections for h + k = 2n and diffuse streaks parallel to c* through all reflections with h + k = 2n + 1. Experimental single-crystal X-ray diffraction revealed that at 173 K norleucine exists in the β-phase with stacking disorder. After reheating to room temperature, the investigated crystal showed a diffraction pattern with strong diffuse scattering parallel to c* through all reflections with h + k = 2n + 1, which is in good agreement with the simulated disordered structure.