Intermolecular interactions in polymorphic p-dichlorobenzene crystals: The α, β, and γ phases at 100 °K

Abstract

The structures of the supercooled α and β phases of p‐dichlorobenzene are determined at 100 °K and a detailed comparison made with the γ‐phase structure previously reported at that temperature [Acta. Crystallogr. B 31, 911 (1975)]. The effects of temperature are investigated by a redetermination of the β‐phase structure at room temperature and by measurement of the thermal expansion coefficients for the α‐ and β‐phase crystals. The average atomic thermal displacements are found to be in the expected thermodynamic order. For chlorine we find Ū (β) =0.184 Å, Ū (α) =0.169 Å, and Ū (γ) =0.157 Å at 100 °K. Rigid body analysis of our data gives good agreement between calculated and observed librational lattice frequencies. New limits are placed on the density changes associated with the phase transitions [ΔV (α→β) =0.1±0.4% and ΔV (γ→α) =0.5±0.4%]. The effects of crystal field on the molecular geometry are limited to a slight out‐of‐plane displacement (0.045 Å) of the chlorine atoms in the γ phase. The possibility of such distortions in the high temperature α and β phases is masked by slight positional disorder (∼0.02 Å). The chlorine–chlorine interactions are found to be attractive and anisotropic. This indicates the necessity of including anisotropic terms in semiempirical atom–atom potential functions. All presently available functions, including Coulomb–Coulomb terms, are found to be inadequate for this reason.