Structure of succinonitrile in its plastic phase

The structure of succinonitrile in its orientationally disordered phase was reexamined through extended X-ray diffraction measurements. It was solved by using both the analytic procedures of symmetry-adapted functions and a Frenkel model assuming discrete orientations. A possible translation-rotation coupling was included in this latter case via an offset vector e. The study confirms that the nitrogen atoms are localised along the fourfold axis of the cubic cell but evidences a strong offset of the centre of mass for gauche conformations. It is shown that this can be explained by steric hindrance between some configurations of neighbouring molecules.