The study of the electron spin exchange efficiency in liquid solutions of radicals at various EPR frequencies

The electron spin exchange efficiency in toluene solutions of stable nitroxide radicals at different EPR frequencies has been studied. The experimental results are in good agreement with the theory. The spin exchange constantsK _e measured at X-band and 2-mm band coincide. TheK _e values measured from the exchange broadening of the EPR lines do not depend on the value of the nitrogen nuclear spinI _N     

时间分辨ESR与电子自旋极化研究

介绍了近年发展起来的时间分辨TRESR技术，以3个实例介绍了用TRESR研究光解自由基的电子自旋极化的状况，指出研究自旋极化可以得到关于分子激发态和自由基反应动力学等丰富的多方面信息，TRESR技术是研究光与分子相互作用的一种重要手段。     

Photoinduced electron transfer and spin dynamics in mixed porphyrin-quinone solutions studied by time-resolved EPR

From time-resolved direct detection cw EPR with pulsed laser excitation, the photoinduced electron transfer and spin dynamics (CIDEP) in mixed zinc-tetraphenylporphyrin (ZnTPP)/benzo-1,4-quinone (BQ) ethanol solutions were determined as functions of temperature and BQ concentration. At lower temperatures the EPR spectra reveal that mixing of the S and T_?1 states in the charge separated radical pair gains in importance relative to the ST₀ mixing. Furthermore, at lower temperatures, the EPR spectra of the spin-correlated radical pairs of ZnTPP⁺ and BQ⁷ could also be observed. From the temperature/viscosity dependence of the electron transfer rates and of the polarization contributions from the triplet and radical pair mechanisms, deviations from a macroscopic diffusion behaviour are inferred at lower temperatures.     

Nuclear spin relaxation in free radicals as revealed in a stimulated electron spin echo experiment

Electron spin echo envelope modulation (ESEEM) in a three-pulse stimulated echo experiment, when the time interval between the first and second pulses τ is varied, is attributed to a spontaneous change of the electron spin Larmor frequency in the time intervalT between the second and third pulses, due to the longitudinal relaxation of nearby nuclei. It is observed for nitroxide radicals in glassy matrices in the temperature range of 130–240 K. Nuclear relaxation is assumed to arise from fluctuation of the proton hyperfine interaction, due to fast rotation of the methyl groups. This contribution to ESEEM and the conventional one that is induced by the simultaneous excitation of allowed and forbidden electron spin transitions were found to be multiplicative. As the latter does not depend on the timeT, both contributions can be easily separated. The rate of nuclear spin relaxation was determined, and correlation time of methyl group rotation was estimated by Redfield theory of spin relaxation. Arrhenius parameters of this motion were estimated on the basis of these data and those at 77 and 90 K, where the previously developed approach was used (L.V. Kulik, I.A. Grigor’ev, E.S. Salnikov, S.A. Dzuba, Yu.D. Tsvetkov, J. Phys. Chem. A 106, 12066–12071, 2003).     

Dipolar broadening of electron spin resonance lines of free radicals: Spin-trapping studies of free radicals in cigarette smoke

Electron spin resonance spectra of spin-trapped free radicals from tobacco smoke have been examined. The cigarettes studied were standard IRI research cigarettes supplied by the University of Kentucky Tobacco and Health Research Institute. Substantial changes in the spectra were observed upon aging of the samples. Aging for several days resulted in at least two remaining long-lived spin-adducts. For the experimental configuration employed the efficiency of the spin-trapping apparatus was determined to be about 47%. Utilizing measurement of the broadening of the magnetic resonance lines as function of the amount of smoke passed through the phenyl-t-butyl nitrone spin-trapping solution, a concentration of about 1 × 10¹⁸ free radicals per cigarette puff was deduced. The method employed in this determination was tested with a number of stable free radicals in several solvents.     

Mott散射和电子极化术

文章回顾了Mott散射过程及其物理图像,介绍了Sherman函数的定义、计算和测量,简述了探测自旋极化电子的仪器——Mott电子极化仪的一般设计,列举了获得自由极化电子的几种方法和原理.     

Mott散射和电子极化术

文章回顾了Mott散射过程及其物理图像，介绍了Sherman函数的定义、计算和测量，简述了探测自旋极化电子的仪器——Mott电子极化仪的一般设计，列举了获得自由极化电子的几种方法和原理．     

Lineshapes of spin exchange broadened EPR spectra

An unexplained residual in the fits of theoretical and experimental spectra of exchange broadened nitroxide spin probes reported by Robinson et al. [J. Magn. Reson. 138 (1999) 199] is shown to be understood by inclusion of theoretically predicted lineshape changes into the fitting routine. These lineshape changes provide an additional estimate of the spin exchange frequency that is independent of linewidth methods.     

On the applicability of the average exchange model for electron spin polarization phenomena

The applicability of the average exchange model as a tool for describing electron spin polarization in micelles is investigated by comparison with analytical and numerical results. The analytical results are used for the interpretation of the various types of dependence, while the numerical results are used for quantitative tests. It is demonstrated that the dephasing effect of the exchange interaction has a significant influence both on the time dependence and on the stationary value of the polarization. Dephasing arising from the exchange interaction is not included in the average exchange model, and therefore the spin dynamics are not described correctly. The average exchange model can only be used to estimate the frequency and the stationary value of the polarization, and only in the case of very slow singlet-triplet mixing, weak exchange interaction, and low reactivity of the radicals.     

Free radicals in irradiated pepper: An electron spin resonance study

With electron spin resonance (ESR) spectroscopy, we revealed various free radicals in commercially available pepper in Japan before and after γ-irradiation. The representative ESR spectrum of the pepper is composed of a sextet centered atg=2.0, a singlet at the sameg-value and a singlet atg=4.0. The first signal is attributable to a signal with hyperfine interactions of the Mn²⁺ ion (hyperfine constant, 7.4 mT). The second signal is due to an organic free radical apparently induced by a sterilization process. The third signal may originate from the Fe³⁺ ion in the nonheme proteins. The progressive saturation behavior at various microwave power levels indicated quite different relaxation behaviors of those radicals. Namely, the peak intensity of the organic free radical component decreases in a monotonic fashion, whereas the Mn²⁺ and Fe³⁺ ESR signals substantially remain constant. This evidences the presence of three independent radicals in the pepper before irradiation. Upon γ-irradiation, a new pair of signals appeared in the pepper. The progressive saturation behavior of the pair peaks after the irradiation showed a quite different behavior as compared with the free radical centered atg=2.0. For the measure of irradiation effects, we propose a universal index for the ESR analysis of irradiated dry foods.     

Dynamic nuclear polarization in electron spin echo

It is shown that under the action of a proper microwave pulse sequence the equilibrium polarization of the electron spin may be transferred dynamically to the longitudinal nuclear magnetization which will oscillate due to the nuclear spin precession around the effective fields relating to differnt electron quantum number manifolds. These oscillations may be measured directly in the radiofrequency band. Analytical formulae are obtained for the case when all the nuclei coupled to an unpaired electron have spins of 1/2.     

EPR characterization of organic free radicals in marine diesel

The intensive unresolved central line associated with organic free radicals in electron paramagnetic resonance (EPR) spectra of petroleum is interpreted as resulting from the superposition of signals from different radical species with very close g values. The mobility of a free radical in crude oil is relatively low due to the high oil viscosity. Aimed at characterizing free radicals in the oil byproduct, the present study is concerned with the investigation of marine diesel (bunker), a low-viscosity oil. Marine diesel was studied by EPR spectroscopy at X-band (9 GHz) in the temperature range from 170 to 400 K. Despite the viscosity at room temperature (2.5·10⁻³ kg/m·s) and the tumbling correlation time for free radicals of about 10⁻⁷ s suggesting a high mobility of free radicals in marine diesel, the EPR spectra at room temperature did not exhibit resolved lines, but only an envelope with a poor signal-to-noise ratio. The same behavior was observed from 170 to 373 K. Above 373 K, the EPR spectrum exhibited resolved lines: a superposition of a septet-quartet, a sextet-quartet and a quintet-quartet group of lines was identified with the following parameters: g = 2.003 ± 0.001, proton hyperfine couplings A = 6.41 ± 0.03 G (septet) and A′ = 1.82 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.21 ± 0.03 G (sextet) and A′ = 1.64 ± 0.02 G (quartet); g = 2.003 ± 0.001, A = 6.16 ± 0.03 G (quintet) and A′ = 1.83 ± 0.02 G (quartet), which were attributed to phenalenyl radicals and their derivatives. Below 373 K, phenalenyl dimerization prevented its detection by EPR.     

Detection of free radicals produced by a pulsed electrohydraulic discharge using electron spin resonance

The detection of free radicals such as hydroxyl radical and hydrogen radical for plasma in solution induced by a pulsed electrohydraulic discharge are successfully performed using electron spin resonance measurement. The plasma reactor is a barrier-type and consists of a stainless needle high-voltage (+35 to 65 kV) electrode partially immersed in the solution and a Pyrex glass solution container around which an aluminum film grounded electrode is wrapped. Streamers are induced in the solution. After adding iron (II) sulfate or radical trapping agent before the plasma application, the spectrum for radicals is clearly detected. A reactive dye solution is dramatically decolorized.     

Time-resolved EPR spectra of a photoexcited phenanthrene-linked copper(II)-free-base porphyrin dimer: An intermediate-coupling case in a triplet-doublet spin system

Time-resolved electron paramagnetic resonance (TREPR) spectra are presented for a phenanthrene-linked copper(II)-free-base porphyrin dimer. In the lowest electronic excited state, the free-base half is in the (π, π*) triplet state and the copper-porphyrin half is in the doublet ground state due to the copper(II) spin. Because of the interaction between the triplet and doublet spins, the excited state of the dimer is described as a coupled triplet-doublet pair state. By choosing the excitation wavelength, this coupled state is produced via either the intersystem crossing within the free base or the energy transfer from the excited state of the copper porphyrin. The observed TREPR spectra show very large spectral widths compared to that of the triplet state in the free-base monomer. In addition, there is a prominent absorptive band in the center of the spectra regardless of the generation pathway. These features can be interpreted as characteristic properties in an intermediate-coupling case, where the degree of the triplet and doublet mixing largely depends on the molecular orientation relative to the magnetic field.     

EPR study of carbonate derived and ozonide radicals in carbonated apatites synthesized from aqueous solutions

In this study, a series of Na⁺ and¹²CO ₃ ^2? containing apatites synthesized by a hydrolysis of octo-calciumphosphate (OCP) and dried at 25°C until constant weight, were examined with EPR after X-irradiation. A variety of different paramagnetic radicals was formed, giving rise to composite and hence complex EPR powder spectra. The spectra were successfully decomposed into their basic components using a multivariate statistical technique. The different components could be identified unambiguously. In this way, it was found that an O^?, an O ₃ ^? , a CO ₃ ^? , two types of CO ₂ ^? and two types of CO ₃ ^3? ions were formed upon X-irradiation. Also resonances from atomic hydrogen were observed. The most striking feature is the presence of the ozonide ion, which is only rarely observed in irradiated hydroxyapatites. A comparison is made between the EPR spectra of apatite samples prepared by hydrolysis of OCP on the one hand, and those of samples prepared by hydrolyzing monetite, and the tooth enamel spectrum on the other hand.     

Chemically induced electron spin polarization in electron-transfer reactions

Chemically induced electron polarization (CIDEP) has been observed for the durosemiquinone radical anion generated in the flash photolysis of solutions of duroquinone in the presence of various amines. The initial polarization has been measured directly by using a fast response time-resolved E.S.R. spectrometer. The magnitude of polarization is shown to depend on amine concentration and identity, and the solvent medium. Conventional nanosecond flash photolysis has been used to measure duroquinone triplet lifetimes under various conditions. The results are discussed in terms of the triplet mechanism and the radical pair mechanism.     

Energy band gap and electron spin polarization in photoemission

A contribution to the electron spin polarization in photoemission is investigated for the transition from occupied states in conduction bands to the states whose energies lie in a band gap. This contribution can be either positive or negative.     

Electrochemical generation of free radicals in an EPR loop-gap resonator

An electrochemical cell for in situ generation of free radicals in a loop-gap resonator at X band is described. The cell is assembled from commercially available components and requires no glass blowing. The design is flexible and accommodates a variety of electrodes and electrode geometries. Remarkably small amounts of material are required. The filling factor is favorable for short-lived radicals. Performance of the cell is demonstrated by generation of the dianion radical of p-nitrobenzoic acid.     

Pulsed EPR study on large dynamic electron polarisation created in the quenching of photo-excited xanthene dyes by nitroxide radicals in aqueous solutions

Dynamic electron polarisation (DEP) produced by the quenching of dye molecules in the triplet excited states by nitroxide radicals was investigated in aqueous solutions by pulsed electron paramagnetic resonance and transient absorption spectroscopy. An analysis of the measured quenching rate constants suggests that quenching is promoted by either exchange or charge-transfer mechanisms for a triplet dye and a doublet radical pair. An unusually large DEP on the radical was found generated in the nitroxide and Eosin Y or Rose Bengal systems in aqueous solutions. Quantitative analysis indicates that the DEP values in aqueous solutions range from ?40 to ?150 in the unit of thermal spin polarisation, which is in contrast to previously reported small DEP values of less than ?10 for organic triplet molecules in benzene solutions [22–27,29,31,32]. From the theoretical analysis of DEP, an origin of this large DEP was attributed to the notably slow diffusion motion of Eosin Y and Rose Bengal in water.     

Critical broadening of EPR line in molecular crystals of free radicals

Using the EPR method, the members of free radicals of the hydrazyl group have been studied within a wide temperature range. In all the samples at temperatures 0.05?3K the existence of a phase transition to an antiferromagnetic state is found out. Dynamics of critical fluctuations near the phase transition have been investigated. Two regimes of critical spin relaxation are discovered.