What we can learn about fast chemical processes from slow diffraction experiments.

The potential energy surface is an important determinant of a chemical reaction. Three ways of deducing non-trivial properties of such surfaces from the results of crystal structure analyses are discussed and illustrated with examples. (1) The mapping approach brings together structures of the same molecular fragment from different environments to outline reaction coordinates and vibrations. (2) Correlations between molecular structures and activation energies for a given reaction type reveal general and quantitative relations between seemingly independent entities such as ground state structure, force constants, reaction path length, activation energy and catalysis. (3) The evolution of atomic mean square amplitudes (displacement parameters) with temperature uncovers frequencies and atomic displacement patterns of large-amplitude vibrations in molecular crystals. Examples include the vibrations of molecular zeolite building blocks, the crankshaft motion of stilbenes, the dynamic coupling between pyramidal deformation of the amide NH2 group and hydrogen bonding, the bowl inversion of corannulenes and nucleophilic addition/elimination reactions.