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

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