Crystallography Without a Lattice

Molecular structure determination in crystals depends on the presence of the crystal lattice. In liquids, there is no underlying lattice, so phase relationships between scattered X-rays or neutrons are not preserved. Hence, only pair-distance distributions can be obtained from simple diffraction experiments. Advances in the past 30 years in radiation sources, instrumentation, and computing have enabled us to go beyond this apparent limitation. We can now obtain detailed structural information on relatively complex liquids and thus see clearly for the first time how molecules actually interact in solution, and how the solvent is perturbed by the presence of the solute. Using as an example recent work on aqueous solutions of tertiary butanol, the ways in which we can obtain high quality structural information in the absence of a lattice are summarized. Not only can we see how intermolecular interactions are influenced by changes in temperature and concentration, we can also begin to see where the structural source of the entropic driving force for the hydrophobic interaction may be found.