Electron Spin-Lattice Relaxation Processes of Radicals in Irradiated Crystalline Organic Compounds

Abstract

Electron spin-lattice relaxation times (T 1e ) for the major radicals in γ-irradiated polycrystalline samples of glycylglycine, L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol were measured as a function of temperature using pulsed EPR. CW-saturation recovery (CW-SR) were obtained at X-band (9.1 GHz) and S-band (3.0 GHz) between about 10 and 295 K. Inversion recovery, echo-detected saturation recovery (ED-SR), and pulsed electron-electron double resonance (ELDOR) curves were obtained at X-band between 77 and about 295 K. For 2,4,6-tri-tert-butyl-phenoxy radical, which has a single-line EPR spectrum, the recovery times obtained by the three methods were in good agreement and were assigned as T 1e . For the three radicals with resolved hyperfine splitting, spectral diffusion caused the recovery times observed by inversion recovery or ED-SR to be significantly shorter than T 1e obtained by CW-SR or ELDOR. Spectral diffusion processes were observed directly by pulsed ELDOR experiments, and time constants for cross relaxation and nuclear relaxation were obtained by modeling the ELDOR curves. For irradiated L-alanine and for the 4-methyl-2,6-tert-butyl-phenoxy radical at some temperatures, the effects of rapid cross relaxation on CW-SR curves could not be fully mitigated even by long saturating pulses, and T 1e could only be determined by ELDOR. For the radicals in γ-irradiated L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol, methyl group rotation makes significant contributions to T 1e at temperatures where the rate of rotation of a methyl group is comparable to the microwave frequency. Activation energies for methyl rotation were determined by modeling the temperature dependence of T 1e at X-band and S-band. In temperature ranges where methyl rotation did not dominate, T 1e was dominated by Raman, direct, or local mode processes.

Electron spin relaxation of radicals in γ-irradiated malonic acid and methyl malonic acid

“…Little time dependence of signal intensity was observed for the malonic acid radical, but the signals from MMA1 and MMA2 decreased gradually over a period of several years. CW saturation recovery (CW-SR) at X-band [19] and S-band [20] and Xband spin echo, echo-detected saturation recovery (ED-SR) and inversion ELDOR [21] measurements were performed, as previously described [22], on locally designed and constructed instruments. Inversion ELDOR curves for the malonic acid radical were obtained by pumping on the low-field line and observing the high-field line (Fig.…”

“…The inversion recovery and ELDOR curves were subsequently analyzed with programs on the basis of the models in Fig. 2 [22]. In the inversion recovery or SR experiments, transition 3 ~ 4 was both excited and observed.…”

“…The strategy that was used to analyze the temperature dependence of T~~ was described previously [22,30]. The parameters that were used to fit the temperature dependence of T~, for the radicals in 7-irradiated malonic acid and methyl malonic acid are summarized in Table 1.…”

“…The ELDOR reductions for the malonic acid radical are attributed to a short T1, with Tx. < T~~, Txl [22]. NMR studies of malonic acid have shown that the proton spin-lattice relaxation time, T~,, is temperature-dependent and is affected by hydrogen-bonding dynamics [1].…”

“…10. and decreased to 5% at 253 K. The enhancements are attributed to Txl < Tln , T~o [22]. The inversion recovery and inversion ELDOR curves were fitted with the 16-1evel model (Fig.…”