Outline of a crystal structure determination

The stages in a crystal structure analysis by diffraction methods are summarized in Figure 14.1 for a substance with fewer than about 1000 atoms. The principal steps are: (1) First it is necessary to obtain or grow suitable single crystals; this is sometimes a tedious and difficult process. The ideal crystal for X-ray diffraction studies is 0.2–0.3mm in diameter. Somewhat larger specimens are generally needed for neutron diffraction work. Various solvents, and perhaps several different derivatives of the compound under study, may have to be tried before suitable specimens are obtained. (2) Next it is necessary to check the crystal quality. This is usually done by finding out if the crystal diffracts X rays (or neutrons) and how well it does this. (3) If the crystal is considered suitable for investigation, its unitcell dimensions are determined. This can usually be done in 20 minutes, barring complications. The unit-cell dimensions are obtained by measurements of the locations of the diffracted beams (the reciprocal lattice) on the detecting device, these spacings being reciprocally related to the dimensions of the crystal lattice. The space group is deduced from the symmetry of, and the systematic absences in, the diffraction pattern. (4) The density of the crystal may be measured if the crystals are not sensitive to air, moisture, or temperature and can survive the process. Otherwise an estimated value (about 1.3g cm−3 if no heavy atoms are present) can be used. This will give the formula weight of the contents of the unit cell. From this it can be determined if the crystal contains the compound chosen for study, and how much solvent of crystallization is present. (5) At this point it is necessary to decide whether or not to proceed with a complete structure determination. The main question is, of course, whether the unit-cell contents are those expected. One must try to weigh properly the relevant factors, among which are: (i) Quite obviously, the intrinsic interest of the structure. (ii) Whether the diffraction pattern gives evidence of twinning, disorder, or other difficulties that will make the analysis, even if possible, at best of limited value.