Van der Waals energy contour map describing the orthorhombic hydrocarbon chain packing of symmetry Pbnm

Abstract

The packing of long aliphatic chains was classified by static lattice energy calculations. For the orthorhombic space group 62 (Pbnm), the van der Waals energy of subcells with systematically varied lattice constants aSC and bSC was minimized by changing the dihedral angle between the backbone planes of zigzag chain fragments (–CH2–CH2–). The calculated energies relate to infinite aliphatic chains because of the unlimited periodicity of the fragments in the C–C backbone direction. In order to facilitate the application of the energy criterion in the trial and error based indexing process, the resulting set of 383 energy data was plotted as a van der Waals energy contour map projected onto a plane of the reciprocal space, which is defined by the reciprocal spacings s110s. s020. The absolute and local energy minima correspond to different packing modes of orthorhombic subcells. The energy hyper-surface is divided by lines corresponding to special cell geometries into different areas. The lines are localized near saddle points of the energy surface. For the first time, the geometrical and energetic relations between the different packing modes of symmetry Pbnm are clearly demonstrated using van der Waals energy calculations. Based on these calculations, a new nomenclature for chain packing modes is suggested.