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Experiments

X-ray and neutron diffraction give a detailed picture of crystal packing. It is difficult to find, in all natural sciences, a more undisputable experimental result, than that of well-performed single-crystal diffraction work. The information is however mainly static, although skillful elaborations may provide a tinge of dynamics to the picture.

Crystal dynamics may be probed by infra-red spectroscopy, for the frequencies of lattice vibrations. Hole-burning spectroscopy can address single impurity molecules in a crystalline environment, and so potentially probe the packing environment. NMR spectroscopy can be used to detect molecular motions and large-amplitude rearrangements, ESR spectroscopy to study the fate of organic radicals produced after a chemical reaction in a crystal.

All measurements of mechanical, electrical, optical, or magnetic properties of crystals are in principle relevant to the study of molecular packing. These experiments are seldom performed by chemists, being beyond the border with the realm of solid-state physics. The relationship between these properties and the crystal structure is strict, but not known in a systematic way.

One most important experiment for the science of crystal packing is a humble one, that is performed every day in every chemical laboratory, but whose results are seldom recorded and almost never published: crystallization from solution. This is a small step for any single chemist, but a systematic analysis of the relationship between molecular structure and ease of crystallization from many solvents and in many temperature conditions would be a giant leap for the chemical sciences.

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