Dynamic simulation of orientational disorder in organic crystals: methyl groups, tri­fluoro­methyl groups and whole mol­ecules

Large amplitude librations of atomic groups or of entire mol­ecules in their crystals are simulated using optimized inter­molecular 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 Mol­ecular 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 Mol­ecular Simulation (MiCMoS) platform. Their combined results offer a wealth of information on the behaviour of phen­yl–methyl and phen­yl–tri­fluoro­methyl 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 mol­ecules, benzene being the well-known prototype, but also in a very large mol­ecule like coronene. Mol­ecular dynamics of rotations in the cyclo­hexa-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.