Analysis of powder EPR and ENDOR spectra of the biphenyl radical cation on H-ZSM-5 zeolite, silica gel and in CFCl3 matrix

Abstract

The biphenyl radical cation absorbed on H-ZSM-5 zeolite and silica gel and isolated in frozen CFCl3 has been studied by EPR and ENDOR spectroscopy. The recorded spectra can be explained by anisotropy of proton hyperfine interactions in para and ortho position and of the g-tensor. The experimental results indicate that the cation has a nearly planar structure, with a twist angle of less than 10°. The ENDOR spectra are analysed by computer simulations. A simple theory to calculate ENDOR transition moments to first order in disordered systems is presented. The simulations show that the observed ENDOR line positions correspond to principal hyperfine tensor values of the para and ortho ring protons.