The influence of radiation chemical effects on X-ray structure determinations

Abstract Implicit in the use of x-ray diffraction for structural determination is the assumption that the x-rays do not seriously perturb the crystal. This is not true for many organic crystals. X and y rays induce chemical changes including the formation of free radicals. These usually arise by the loss of hydrogen from a molecule or the addition of hydrogen lost from some other molecule, to a double bond. Such effects are quite general and are apart from such special reactions as photodimerization. Normally this would cause minimal perturbation in the usual structural determination. However, free radicals are very reactive. The initial radicals may decompose or undergo chemical change to form more stable trapped radicals. They may react with each other, commonly by dismutation or by addition to another radical or molecule. If oxygen from the surrounding air can penetrate the crystal, the free radicals will react to generate peroxide radicals initiating a chain reaction in which several hundred molecules undergo oxidation for every free radical present.