ESR Study of OH Radicals Originating from Water of Crystallization of Carboxylic Acids. II. Role of Hydrogen Bonding in Selective Formation of OH Radicals

The selective and stereospecific formation of OH radicals from water of crystallization by ionizing radiation has been studied using an irradiated single crystal of dipotassoium $\mathrm{fumarate}\text{· 2}{\mathrm{H}}_2\mathrm{O}$ . The ESR results show that the major OH radicals produced with high selectivity are formed from a scission of the $\mathrm{O}–\mathrm{H}$ bond having the shortest hydrogen bond distance among the four $\mathrm{O}–\mathrm{H}$ bonds in the two inequivalent water molecules in an unit cell. In addition, the evidence has been obtained for the existence of a proton transferred from the water molecule to the neighboring carboxyl oxygen atom when the OH radical is formed. The crystal of dipotassium $\mathrm{fumarate}\text{· 2}{\mathrm{D}}_2\mathrm{O}$ gives OD radicals together with a large amount of OH radicals from H₂O contained as impurity, indicating the marked isotope effect on the formation of OH radicals. These results lead to the conclusion that proton tunneling through hydrogen bond plays an important role when radicals are formed from H₂O⁺ by ionizing radiation. The minor OH radical, which differs in g anisotropy but has nearly the same orientation, is assigned to the unrelaxed form of the major OH radical, since the minor one having a larger g anisotropy and consequently interacting with surroundings more weakly was found to be less stable and to convert into the major species. At 77°K these OH radicals undergo small amplitude oscillation around the coordination bond between the radical oxygen atom and K⁺.