Diffraction methods

Diffraction is an interference phenomenon taking place when waves are scattered by objects in distinct positions. Electron diffraction relies on the wave-like properties of electrons and can be employed in several ways. In inorganic chemistry one application is the determination of bond angles and lengths of molecules in the gas phase. Scope of it is limited as only volatile substances might be studied and a full interpretation is only feasible for molecules consisting of rather few atoms. Of much greater usual use is X-ray diffraction that is by far the most significant structural method in chemistry. It relies on the fact that X-ray wavelengths are comparable to the spacing among atoms in crystals. So Interference takes place between radiation scattered by different atoms and scattered X-rays emerge only at specific angles from a crystal is depending on the wavelength of radiation and the inter-atomic spacings. Two different methods may be used. X-ray powder diffraction (XRPD) is performed on finely divided powdered (so polycrystalline) samples. It allows the dimensions of the crystal unit cell to be determined. As such types of dimensions are feature of specific crystalline substances, XRPD is an important fingerprinting method for solids, and may be employed to follow the course of solid-state reactions. More comprehensive structural information depends on finding the positions of atoms in the unit cell that can be done from the intensities of the distinct diffraction lines.

Even though simple structures can be determined from powder diffraction, several structural information comes from single-crystal X-ray diffraction. The good quality crystal of around 0.3 mm dimension is needed and additional information over that from XRPD comes from its particular orientation in the X-ray beam. Detailed calculations are needed to match the structure against the observed diffraction intensities but modern computational techniques combined with automated data collection permit this to be done routinely in many laboratories. The structure 1 can be established with reasonable certainly on the basis of its NMR, MS and IR spectra for the compound X discussed above. None of these methods however gives any information about the bond angles and lengths. On this compound the Single crystal X-ray diffraction confirms its structure and gives this additional information. All C-C bond lengths in the benzene ring are equivalent to 140 pm and the C=O lengths equivalent to 114 pm; both lengths are very little longer than ones in the free ligands (139 and 113 pm, correspondingly).

Even though X-ray diffraction is generally reliable, some problems can arise. Crystals with disorder-sometimes not suspected-can give misleading products. Problems can be due to the fact the X-rays are scattered by electrons so that the scattering power of an atom is proportional to its atomic number. It can be difficult to locate light atoms like hydrogen in the neighborhood of heavy elements. It may be also be not possible from X-ray measurements alone to differentiate between elements of almost similar atomic number. In the principle some of these problems can be conquer by using neutron diffraction but that is a much more costly method not routinely available. Generally it is significant that X-ray structures should be backed up by other information, particularly a good elemental analysis.