Frequency dependence of electron spin–lattice relaxation for semiquinones in alcohol solutions

Abstract

The spin-lattice relaxation rates at 293 K for three anionic semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, and 2,3,5,6-tetramethoxy-1,4-benzosemiquinone) were studied at up to 8 frequencies between 250 MHz and 34 GHz in ethanol or methanol solution containing high concentrations of OH-. The relaxation rates are about a factor of 2 faster at lower frequencies than at 9 or 34 GHz. However, in perdeuterated alcohols the relaxation rates exhibit little frequency dependence, which demonstrates that the dominant frequency-dependent contribution to relaxation is modulation of dipolar interactions with solvent nuclei. The relaxation rates were modeled as the sum of two frequency-independent contributions (spin rotation and a local mode) and two frequency-dependent contributions (modulation of dipolar interaction with solvent nuclei and a much smaller contribution from modulation of g anisotropy). The correlation time for modulation of the interaction with solvent nuclei is longer than the tumbling correlation time of the semiquinone and is consistent with hydrogen bonding of the alcohol to the oxygen atoms of the semiquinones.

Multifrequency Pulsed EPR and the Characterization of Molecular Dynamics

“…To distinguish between these types of processes it is very helpful to measure T 1 as a function of Zeeman frequency and of tumbling correlation time. Processes that have been characterized by studying the frequency dependence of T 1 include molecular tumbling, intramolecular dynamics, and solvent motions (Biller et al, 2013; Elajaili, Biller, Eaton, & Eaton, 2014; Hyde et al, 2004; Owenius, Eaton, & Eaton, 2005; Owenius, Terry, Williams, Eaton, & Eaton, 2004). In this chapter the mechanisms that contribute to electron spin relaxation of radicals in fluid solution are discussed as the basis for designing experiments to use the frequency dependence of relaxation to elucidate motions.…”

Rapid-scan coherence signals in X-band EPR spectra of semiquinones with small hyperfine splittings

“…B 1 was set at a low value (50 mG) to avoid saturation. The sample relaxation times were T 1 = 8 µs and T 2 = 3 µs [6]. The m x - and m y -components of the computed rapid-scan signal were combined into a complex function for deconvolution to obtain the slow-scan spectrum.…”

“…As reported previously [6, 7] high-yield generation of long-lived semiquinones involves attention to details of mixing solutions initially in air, followed promptly by purging with N 2 . After mixing the reagents listed above, the vapor space in the tube was purged vigorously with N 2 , after which N 2 was gently bubbled into the solution through thin Teflon tubing immersed in the sample.…”

“…Semiquinone radicals were generated as in [6]. DTBSQ was prepared by mixing equal volumes of air-saturated solutions of 2,5-di-t-butyl hydroquinone (5 mM) and KOH (5 mM) in ethanol.…”

Rapid-Scan EPR of Nitroxide Spin Labels and Semiquinones

“…The underlying spin physics that causes the oscillations can be mathematically modeled, so the corresponding slow-scan absorption spectrum can be calculated by deconvolution. An example of a rapid-scan signal and deconvolution is shown in Figure 1 for 2,3,4,5-tetramethoxybenzosemiquinone (TBMSQ) in methanol (Elajaili, Biller, Eaton, & Eaton, 2014). In this experiment a sinusoidal scan was generated with a scan frequency of 41.6 kHz and scan width of 10 G. The signal was recorded as the magnetic field increased and decreased through resonance (Fig.…”