EPR Study of Irradiated Zinc Acetate Dihydrate Single Crystals

Publisher Website
Google Scholar
Abstract
The article of record as published may be found at http://dx.doi.org/10.1063/1.1669954X-ray irradiation of zinc acetate single crystals at 77°K with subsequent examination at 133°K reveals initially an intense EPR signal due to methyl radicals. Decay of the methyl radicals is accompanied by a corresponding increase in another species shown to be the • CH2CO2- radical. The proposed mechanism is a simple abstraction of a hydrogen atom by the methyl radical from a neighboring acetate ion. In support of this mechanism, observation of the reaction using Zn (CH2DC02)2· 2H20 indicates an isotope effect. The • CH2C02- radicals are produced in two magnetically distinct sites. Spectra taken over a range of temperatures reveal both the slow and rapid exchange of the hydrogen atoms. The barrier to internal rotation is 5.0± 1.0 kcal/mole, with less than two entropy units of activation. Parameters characterizing the • CH2CO2- radical are determined with a 12-parameter least-squares calculation utilizing spectra taken at 55 different crystal orientations. The radical is planar with an HCH bond angle of 116.9°. The coupling constants are compared with other radicals using molecular-orbital calculations