Abstract
A vibrational study of two crystalline forms of the N,N′‐diphenyl‐1,4‐phenylenediamine molecule is reported. This compound shows polymorphism in its solid state, i.e. either an orthorhombic or a triclinic structure is obtained depending on the conditions of crystallization. Three conformational isomers are evidenced in these structures, showing significant differences between their geometric parameters, especially their torsional angles. These observations are also supported by the vibrational features characteristic of the conformers in the crystals. Several changes are observed in the infrared absorption (ir) and Raman scattering data. This work concerns mainly the polarized Raman measurements on these two crystalline forms. A detailed analysis and a good knowledge of phenyl compounds allow a very precise assignment of the vibrational modes and also reveals the structure–properties (especially molecular conformations–vibrational properties) relationships in these systems. In particular, the effect of the crystalline arrangement and of the local symmetry between neighbouring molecules is discussed. Copyright © 2001 John Wiley & Sons, Ltd.