Diffraction atom type effect

X-Ray diffraction data give the atomic coordinates and thereby the conformations of molecules in the crystalline phase. If many structures of a given type are known, which unfortunately is rarely the case for medium rings, it is likely that an excellent picture of the global conformational energy minimum will be obtained as lattice effects should be more or less random. 

The classification of atomic arrangements into types, together with the consideration of the effect of the type of arrangement Oil the diffraction pattern, will be considered in two stages. First of all, unit cells are either simple, with only one pattern-unit in the cell, or compound, with two or more pattern-units in the cell (see Chapter II). Crystals with compound unit cells give patterns from which maiiy reflections are absent, and the recognition of a compound cell is a very simple matter. 

The shrinkage effect1 is treated in more detail elsewhere in the present article. Due to molecular vibrations interatomic distances observed by electron diffraction do not correspond to a set of distances calculated from a rigid geometrical model. Usually the shrinkage effect is routinely included in electron-diffraction least-squares refinement. In order to do so, it has been found appropriate to introduce a third distance type r defined as the distance between mean positions of atoms at a particular temperature. If the harmonic force field is known, iQ may be calculated from ra according to Eq. (12) ... 

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