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 ambiguiti es inherent to the interpretation of the diffraction experiments on (E)-sti lbenes. 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 diff raction experiments map (according to Boltzmann's statistics) only the bott om of the local minima of the potential energy surface, many possible sourc es of misunderstanding are present. Indeed, the coupling of molecular mecha nics to Kitaigorodsky's atom-atom pairwise potential approach, offering a d etailed description of the solid-state trajectories (and of the involved en ergy changes) of(E)-stilbenes, has allowed a rationalization of the observe d dynamic molecular disorder, which differently affects different crystal s ites, and has led to a quantitative description of the anomalous temperatur e dependence of the ethylene bonding parameters in these compounds.