Diffraction by single crystals symmetry

Radial distribution curve of a 10 1 molar mixture of water and trimethylamine, a composition that gives a crystalline hydrate on cooling. The curve shows peaks corresponding to bonded and non-honded intramolecular contacts, such as C-O, O-H, N-C and C- C, and intermolecular contacts, particularly between oxygen atoms of one water molecule and oxygen or nitrogen atoms of adjacent molecules. The solid line represents experimental data and the dotted line shows values calculated from a theoretical model of the structure. Adapted with permission from [22]. Copyright 1971, American Institute of Physics. 

So we are left with the situation that, by and large, we are ignorant of the structures of liquids and solutions. As the majority of chemical reactions occur in solution, and chemical reactivity is highly dependent on structure, this is unfortunate. But at present it is simply not possible to find out as much about hquids by diffraction methods as we routinely do for the sohd and gaseous states. However, the EXAFS method is often apphed to (frozen) solutions and may give some hmited information on the local surrounding of certain atoms. 

A two-dimensional grating gives a much more compheated pattern, from which it is possible to deduce the spacings between the two sets of ruhngs and the angle between them. In the same way, the diffraction pattern 

This is a sizeable topic, so we have divided the material between several sections of this chapter. There are many textbooks on X-ray crystallography. A very basic text is [23], and [24], [25] and [26] are readable accounts. For an excellent paper, which encourages us all to evaluate pubhshed crystal structures critically, see [27]. A very good teaching website of the International Union of Crystallography can be found at http // www.iucr.org/education. 

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