COMPUTER-SIMULATION OF THE DYNAMICS OF THE PLASTIC PHASE OF SUCCINONITRILE

The complex dynamics of the plastic phase of crystalline succinonitrile CN-CH2-CH2-CN, involving trans-gauche isomerization as well as molecular reorientations along the diagonals of the cubic unit cell, is investigated by means of molecular dynamics simulation. A simple molecular model in which the end CN groups and the CH2 groups are represented by single pseudoatoms is proposed. The model includes, in addition to the external degrees of freedom, three degrees of internal freedom described by two bending and a torsion coordinate. The intermolecular interactions are treated in terms of Lennard-Jones potentials. Force constants are used for the bending coordinates and the intramolecular torsion is described by a two minima potential function with three parameters. The dynamical properties of the crystal are investigated by calculation of the time dependence of single molecule autocorrelation functions. It is found that the most suitable autocorrelation functions are those involving the torsion and the orientation of the central C-C bond and of the vector joining the two CN end atoms. Three distinct regimes are observed for the orientational relaxation of the model system: (a) a very fast relaxation with a time constant tau < 1 ps due to the overdamped librations and torsional vibrations; (b) a fast decay with tau almost-equal-to 3-4 ps due to the spinning motion of the trans conformers around the long inertia axis; (c) a slow diffusive regime with tau almost-equal-to 20-30 ps due to the molecular tumbling motions and to the trans-gauche isomerization.