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

摘要

The selective and stereospecific formation of OH radicals from water of crystallization by ionizing radiation has been studied using an irradiated single crystal of dipotassoiumfumaratemiddot; 2H2O. The ESR results show that the major OH radicals produced with high selectivity are formed from a scission of theOsngbnd;Hbond having the shortest hydrogen bond distance among the fourOsngbnd;Hbonds 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 dipotassiumfumaratemiddot; 2D2Ogives 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 H2Oplus;by ionizing radiation. The minor OH radical, which differs inganisotropy but has nearly the same orientation, is assigned to the unrelaxed form of the major OH radical, since the minor one having a largerganisotropy and consequently interacting with surroundings more weakly was found to be less stable and to convert into the major species. At 77deg;K these OH radicals undergo small amplitude oscillation around the coordination bond between the radical oxygen atom and Kplus;.