Ionic radicals on silica surfaces — an EPR, ENDOR and ESE study of benzene radical cations adsorbed on HY and silica gel

Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo, (ESE) spectroscopy have been used to characterise radical cations of benzene (benzene-d6 and benzene-d1), generated by ionising radiation, and stabilised on silica gel and HY molecular sieve surfaces. The electronic structure and dynamic features of the benzene cation were found to be different from those in the case of stabilisation in a Freon matrix (CFCl3). The monomeric cation was found to undergo pseudorotation at 3.5 K with hyperfine structure (h.f.s.) constants due to six averaged hydrogen nuclei. Simulations of the time-domain deuterium electron spin echo envelope modulation (ESEM) (C6D+6 cation) gave the following h.f.s. constants: the perpendicular component of the axially symmetric tensor T⊥ = −0.8 MHz, and the isotropic component a = 2.17 MHz, consistent with ENDOR results at 105 K. It was concluded that the monodeuterated benzene cation ‘slowed down’ the rotation at 3.5 K, although not enough to allow an analysis in the rigid limit. ENDOR spectra of the protonated and deuterated monomeric and dimeric benzene radical cations on silica gel and HY molecular sieve surfaces are presented. On HY molecular sieve the ENDOR results revealed two types of dimer, one stabilised at low temperature (below 110 K) characterised by h.f.s. constants of 4.8 and 8.9 MHz, and the other stabilised at higher temperatures with an h.f.s. constant of 6.5 MHz. Only the latter type of dimer could be detected on silica gel. In addition to hyperfine splittings from ring protons, a smaller hyperfine splitting attributed to hydroxyl protons situated on the surface was determined from the ENDOR results.