Hindered Intermolecular Rotation in the Solid State: Thermal and Dielectric Phenomena in Long-Chain Compounds

Abstract

It has been assumed that the potential energy barrier hindering rotation of chain molecules about their long axes in the solid state has one deep minimum and Ω other minima with a potential energyV above that of the deeper well. Molecular models suggest Ω should be between 8 and 18. Application of the Bragg‐Williams cooperative approximation to this system leads to first‐order thermal and dielectric transitions. Dielectric and thermal data on n‐C22H45Br, n‐C30H61Br, and n‐C28H58 show that the transitions are first‐order. The static dielectric constant of the polar compounds calculated from Onsager's equation on the basis Ω=12 agree closely with experiment. The model suggests the existence of two Debye‐type loss regions below the transition temperature. Loss data confirm this and further indicate Ω is close to 12. There is no evidence of resonance absorption. Ω may also be found from the entropy of transition at constant volume, R lnΩ. Thermal data on all three compounds are consistent with a value of Ω of 12. The theory indicates that the transition temperatures of a homologous series should arise almost linearly with increasing chain length if the chains do not twist appreciably. Data from the literature confirm this.