Paramagnetic Relaxation of Alkyl Radicals in Solid Alkanes As Studied by Electron Spin Echo Method: Possibility of High-Resolution Solid-State ESR

Article abstract of DOI:10.1021/j100317a014

Electron spin echo detected ESR spectra of alkyl radicals in γ-irradiated solid alkanes (n-hexane, n-octane, n-decane, n-dodecane, polyethylene, and cyclohexane) have been measured at 77 K as a function of external magnetic fields and transverse or longitudinal relaxation rate in order to investigate the possibility of the selective detection and the resolution enhancement of the ESR spectra of radical species in solids by the electron spin echo method.Among the two types of alkyl radicals stably trapped at 77 K in the solids, the longitudinal and transverse relaxation rates of CH3CH^.CH2R radicals are much faster than those of RCH2CH^.CH2R' radicals, so that the RCH2CH^.CH2R' radicals are selectively detected by the electron spin echo method.The relaxation rates of the RCH2CH^.CH2R' radicals decrease with increasing length of the C-C chain and are lowest for cyclohexyl radicals, which indicates that relaxation is mainly caused by the irregular twisting motion of the C-C^. bond.The ESR spectra of the magnetically saturated RCH2CH^.CH2R' radicals become sharper with increasing time during saturation recovery.This effect is interpreted as due to the spectral diffusion of the RCH2CH^.CH2R' radicals caused by the slow time fluctuation of hyperfine fields.It is concluded that the resolution of a solid-state ESR spectrum is improved by measuring the saturated component of the spectrum when the line width of the ESR spectrum is comparable to the width of the spectral broadening caused by the time fluctuation of the hyperfine fields.