Molecular Mechanics in Crystalline Media:  The Case of (E)-Stilbenes

Molecular mechanics simulations within the field of the mean (when disorder is present) crystal lattice have been used to settle most of the ambiguities inherent to the interpretation of the diffraction experiments on ( E )-stilbenes. A dynamic process can be inferred by diffraction methods only if it leaves a track in the atomic displacement parameters or gives rise to some disorder or to some anomaly in the bonding parameters. However, since diffraction experiments map (according to Boltzmann's statistics) only the bottom of the local minima of the potential energy surface, many possible sources of misunderstanding are present. Indeed, the coupling of molecular mechanics to Kitaigorodsky's atom−atom pairwise potential approach, offering a detailed description of the solid-state trajectories (and of the involved energy changes) of ( E )-stilbenes, has allowed a rationalization of the observed dynamic molecular disorder, which differently affects different crystal sites, and has led to a quantitative description of the anomalous temperature dependence of the ethylene bonding parameters in these compounds.