Study of molecular motions in liquids by electron spin-lattice relaxation measurements, I: Semiquinone ions in hydrogen bonding solvents

Rengan, S. K.; Khakhar, M. P.; Prabhananda, B. S.; Venkataraman, B.

doi:10.1007/bf02847118

Cited by 34 publications

(28 citation statements)

References 19 publications

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“…1969;Freed 1972Freed , 1976Goldman et aI., 1973;Schneider and Freed, 1989). Direct measurements of Tl by saturation recovery by Rengen (1971Rengen ( , 1974aRengen ( , 1974b and by Hyde and coworkers Hyde 1974a,b, 1976;Hyde 1974Hyde , 1998, and then of T2 by Freed and coworkers (Gorchester et aI., 1990) and by Schwartz et aI. (1979;Stillman and Schwartz, 1979) solidified understanding of the rates and mechanisms of relaxation of small organic radicals in fluid solution (Freed 1972(Freed , 1976(Freed , 1998Schneider and Freed, 1989).…”

Section: Experimental Datamentioning

confidence: 93%

Relaxation Times of Organic Radicals and Transition Metal Ions

Eaton

2002

Distance Measurements in Biological Systems by EPR

152

165

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Abstract:Review of electron spin relaxation times for organic radicals and transition metal ions in magnetically dilute samples. Emphasis is placed on studies that have been performed as a function of temperature and that provide insight into the relaxation processes. INTRODUCTION Scope of this ChapterMost of the methods for determining distances between spins (ch.l) are either experimentally feasible or theoretically applicable for a limited range of electron spin relaxation times. For example, analysis of the dipolar contribution to the CW line shape to determine interspin distance requires that the relaxation rate for the interacting partner is slow enough that the dipolar splitting is not collapsed by electron spin relaxation. In addition, analysis of the line shapes assumes that spectra are obtained under nonsaturating conditions and are free from passage effects. Application of methods based on changes in relaxation times requires that the relaxation rate of the interacting partner fall within certain limits. The longest distance that can be obtained by pulsed techniques is limited, in principle, by the relaxation-determined width of an individual spin packet. Thus, knowledge of electron spin relaxation times is foundational to the methodologies presented in this book.

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Section: Experimental Datamentioning

confidence: 93%

Relaxation Times of Organic Radicals and Transition Metal Ions

Eaton

2002

Distance Measurements in Biological Systems by EPR

152

165

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“…It was established by a series of power saturation experiments that there is no significant difference in T, for radicals 1 and 2. Since the absolute value of T, is not available for radicals 1 and 2, we used the reported T, values for the semiquinone radical anion of t BBQ (19) corresponding to the viscosity of the solution containing phenol. The estimation' using the semiquinone anion T, values has been found to be satisfactory (7).…”

Section: Cidepmentioning

confidence: 99%

An electron spin resonance and CIDEP study of the photoreduction of tert-butyl-p-benzoquinones

Foster¹,

Elliot²,

Adeleke³

et al. 1978

Can. J. Chem.

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The photoreduction of 2-tert-butyl-p-benzoquinone and 2,5- and 2,6-di-tert-butyl-p-benzoquinone were studied by esr spectroscopy. In the photolysis of 2,6-di-tert-butyl-p-benzoquinone, only one semiquinone radical was observed and its decay kinetics were monitored by esr. The reduction of the triplet 2-tert-butyl-p-benzoqinone leads to the formation of isomeric semiquinone radicals 1 and 2 in the primary photochemical processes, and this is then followed by a chemical equilibrium between the two isomeric radicals. The relative rates of formation of the two radicals cannot be monitored by conventional esr methods but the CIDEP results show that the formation of radical 2 is about five to seven times faster than that of radical 1. A secondary reaction of the phenoxy radical with a product benzohydrofuran 4 leads to the observation of a polarized radical 3 which derives all its polarization from the primary phenoxy radical.

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“…The activation energy for the thermally activated process (1100 K) for tempol in the solid host is in good agreement with the activation energy for rotation of the nitroxyl ring methyl groups (1100 K) obtained by ENDOR for tempone in a solid matrix 25 so the thermally activated process was tentatively assigned as methyl group rotation. 24 Saturation recovery studies of T 1e for semiquinones as a function of solvent, temperature and viscosity [26][27][28][29] are consistent with where A and B are adjustable parameters, η is solvent viscosity and ∆E is an activation energy of the order of 1 kcal/mol (500 K).…”

Section: Introductionmentioning

confidence: 97%

Frequency Dependence of Electron Spin Relaxation of Nitroxyl Radicals in Fluid Solution

et al. 2004

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Electron spin lattice relaxation rates (1/T 1e ) of nitroxyl radicals for tumbling correlation times between about 0.1 and 10 ns in water:glycerol or water:sorbitol mixtures at room temperature were measured by saturation recovery at X-band (9.2 GHz), S-band (3.1 GHz), and L-band (1.9 GHz) for natural abundance 2,2,6,6-tetramethylpiperidinyl-1-oxy (tempol), tempol-d 17 , and 15 N-tempol-d 17 and for a spin-labeled derivative of ethylenediaminetetraacetic acid. Tumbling correlation times were calculated from the continuous wave EPR line shapes at X-band. The dependence of T 1e on tumbling correlation time was modeled with contributions from modulation of nitrogen nuclear hyperfine and g anisotropy, from spin rotation and from one or more thermally activated processes. At these microwave frequencies, modulation of nitrogen hyperfine anisotropy makes a substantially larger contribution than modulation of g anisotropy. Spin rotation makes a very small contribution to T 1e at the tumbling correlation times examined in these studies. Replacement of 14 N by 15 N decreases the relaxation rates as predicted for samples in which modulation of nuclear hyperfine anisotropy contributes to relaxation. Deuteration of the solvent does not affect T 1e , which indicates that the electronnuclear dipolar interaction with solvent nuclei does not make a significant contribution to T 1e of nitroxyl radicals in this motional regime. The contributions to relaxation from one or more thermally activated processes that occur at rates comparable to the microwave frequency dominate the relaxation at tumbling correlation times longer than about 2 ns at X-band and S-band. The contributions from the thermally activated process-(es) and from modulation of nitrogen nuclear hyperfine anisotropy increase as the microwave frequency is decreased, and the contribution from hyperfine anisotropy dominates at L-band.

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Study of molecular motions in liquids by electron spin-lattice relaxation measurements, I: Semiquinone ions in hydrogen bonding solvents

Cited by 34 publications

References 19 publications

Relaxation Times of Organic Radicals and Transition Metal Ions

Relaxation Times of Organic Radicals and Transition Metal Ions

An electron spin resonance and CIDEP study of the photoreduction of tert-butyl-p-benzoquinones

Frequency Dependence of Electron Spin Relaxation of Nitroxyl Radicals in Fluid Solution

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