ESR Study of the Effects of Alkali Metal Substitution on the Radical Species Produced and the Stereospecific Formation of Radicals in Irradiated Carboxylic Acids

A remarkable difference has been found in the radical species produced when the alkali metal was changed from potassium to sodium in irradiated single crystals of acid salts of maleic acids. The room temperature stable species in an irradiated single crystal of potassium hydrogen maleate was previously found to be the symmetrical delocalized $\pi$ radical formed by removal of the acidic proton. In contrast to this result, the stable species found in sodium hydrogen maleate was the radical formed by the addition of hydrogen to the double bond. Furthermore, it has been found that the addition takes place selectively to one of the two vinylene carbon atoms, as is unexpected from the symmetrical structure of the hydrogen maleate ion. A similar difference was also observed in the unstable species at 77°K. The potassium crystal gave the symmetrical $\sigma$ radical formed by removal of the acidic proton while the sodium crystal gave the unsymmetrical $\sigma$ radical. Here again the unpaired electron is selectively located in one of the two COO groups. These results indicate that the formation of radicals in crystalline solids is strongly affected by the surrounding crystalline field, since the sodium crystal is known to have a different crystal structure with hydrated water.