Dynamic simulation of orientational disorder in organic crystals: Methyl groups, tri fluoro methyl groups and whole mol ecules

Abstract

Large amplitude librations of atomic groups or of entire mol ecules in their crystals are simulated using optimized inter molecular potentials and crystal structures deposited in the Cambridge Structural Database. The analysis proceeds by a simple static model in which reorientations take place in a fixed environment, or by Monte Carlo (MC) simulation of equilibria dotted by rotational defects, or eventually by full Mol ecular Dynamics (MD). The simplest approach provides a valuable qualitative preview, but MC and MD are becoming easily accessible to the general solid-state chemist thanks to the facilities of the newly developed Milano Chemistry Mol ecular Simulation (MiCMoS) platform. Their combined results offer a wealth of information on the behaviour of phen yl-methyl and phen yl-tri fluoro methyl groups, almost invariably affected by rotational flipping, whose nature and consequences are discussed with respect to disorder modelling in the refinement of X-ray structures. Whole-body reorientation takes place in flat mol ecules, benzene being the well-known prototype, but also in a very large mol ecule like coronene. Mol ecular dynamics of rotations in the cyclo hexa-1,4-diene crystal offer a spectacular picture of the energetic profiles with jumping times. The dynamic oscillations described here are seldom considered in the formulation of crystal 'bonds' or of 'synthon' stability.