Observation of broad EPR spectra of transient free radicals by FT-EPR

Application of electron spin echo Fourier transform EPR (ESE-FT-EPR) to photo-induced chemical reactions is presented. Main purpose of this study is to observe broad EPR spectra of free radicals having very shortT ₂ ^* by means of the ESE-FT-EPR technique. Details of the experimental procedures are described. In ESE experiments design of the resonator is important to obtain sufficient spectral bandwidth because of use of multiple pulses which decrease the bandwidth. We designed and constructed Loop-Gap-Resonantors (LGR) for light irradiation experiments and their specifications were examined. The phase cycling method is essential to obtain pure ESE signals and proper time resolution by eliminating unwanted FID signals which result from imperfect pulse angles. We applied this technique to observe the photo-induced electron transfer reaction between tetraphenylporphinato zinc(II) (ZnTPP) and duroquinone (DQ) in an ethanol solution, and successfully observed the time resolved EPR spectra of the both Zn(TPP) cation and DQ anion radicals by ESE-FT-EPR of the Hahn echo. The half-height full-width of envelope of EPR spectrum of Zn(TPP)⁺, which is never observed in ordinary FT-EPR, is about 16 MHz. Specificity of spectra and the time resolution are compared among the ESE-FT-, FT- and cw-Time-Resolved-EPR (cw-TREPR) techniques.     

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 NH2(X 2 B 1) radicals by CARS spectroscopy

CARS spectra of NH₃ and the free NH₂-radical in the NH stretch region at 3334 and 3220 cm^–1 have been obtained, respectively. The NH₂ radicals were generated by laser photolysis of 0.5 mbar NH₃ at 193 nm with an ArF-exciplex laser and detected by time resolved CARS spectroscopy.     

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.     

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.     

2-mm band EPR spectroscopy of semiquinone radicals in rigid media

2-mm band EPR spectroscopy was used to compare the spectra of paramagnetic particles produced under adsorption of p-quinones (p-benzoquinone, chloranil, fluoranil) on an activated surface of HY zeolites, p-quinone cation and anion radicals, and the complexes of p-quinones with AlCl₃ in a frozen nonpolar solvent. The adsorption is observed to give rise to the paramagnetic complexes of quinones and zeolite surface Lewis sites.     

Investigation of free radicals induced by light ions in CR-39 using ESR spectroscopy

In previous investigations the formation of radicals induced by γ-irradiation in CR-39 samples has been studied by electron spin resonance (ESR) spectroscopy. On the other hand, all attempts fail to detect radicals in samples irradiated by particles (p, and neutrons). The present work succeeded in detection of radicals produced by charged particle radiation in CR-39 (TASTRAK). The detectors were irradiated with protons of 7 MeV and -particles of 3 MeV with fluences in the order of 10¹¹–10¹² particles/cm². The observed ESR signal is composed of at least four different resonance lines for each of which we determined the amplitudes, g-factors, lifetimes and the G-value. The amplitude of the total ESR signal decreases rapidly with a short and a longer lifetime. About 5 days after irradiation the ESR intensity decreased to an undetectable value. The lowest detectable dose (LDD) is of the order of 1 kGy.     

Environmental stress of higher plants monitored by the formation of monodehydroascorbate radicals with continuous-wave EPR

By electron paramagnetic resonance (EPR) inspection of leaves of spinach, maize, peas, wheat and conifer needles as well at room temperature with X-band spectrometer in theg = 2 region the photosignals I and II were observed. Plants exposed to unfavorable living conditions either in a green house or in the open country show in addition the typical hyperfine structure of monode-hydroascorbate (MDA). Careful inspection of these signals, for example, by studying the rise and decay kinetics, the exchange of the surrounding atmosphere or the impact of different chemical or physical stress reveals the mechanism responsible for the MDA production. Under all conditions applied the first reaction of green plants in stress situation was an enhanced concentration of MDA radicals which was built up by the reaction of reactive oxygen species either directly or by the action of enzymes. EPR inspection of the leaves of green plants can be directly performed without any time-consuming sample preparation. Therefore this method can be used for an early diagnosis of environmental stress.     

EPR probes in sedimentary rocks: The features of Mn2− and free radicals distribution in the Permian formation in Tatarstan

EPR data (i.e. the distribution of Mn²⁺ in carbonate rocks and of free radicals in terrigenous rocks) may be used to characterize sedimentary beds, especially if they are unfossilferous. The study of the distribution of Mn²⁺ that substitutes Mg²⁺ and Ca²⁺ in sedimentary dolomites suggests that the distribution is affected by their genesis.     

EPR Spectra of hexafluoride radicals

The preparation of various hexafluoride radicals is described, and their spectra are compared. The central-atom hyperfine interactions are large, indicative of an ²A _1g ground state in O_h symmetry. This is confirmed by the anisotropic ¹⁹F hyperfine structure. The semioccupied orbital consists of an antibonding combination of the central-atom ns atomic orbital, and the six fluorine 2p atomic orbitals. Changes in the central-atom contribution are correlated with electronegativity changes in the central atom.     

EPR of γ-irradiation-induced free radicals in chicken, duck, and quail egg shells

Chicken, duck, and quail egg shells were γ-irradiated and investigated by electron paramagnetic resonance (EPR) at ambient temperature and at 113 K. The EPR-active species after γ-irradiation were attributed to the ĊO₃⁻, ĊO₃³⁻, and ĊO₂⁻ radicals at ambient temperature and at 113 K. Furthermore, when the samples were stored in an oven at 423 K for about 1 h new EPR signals appeared at ambient temperature in addition to the above ones and these signals were attributed to the (CH₃)₂Ċ-R radical. These signals appear also at 113 K, but they are broadened likely due to the freezing of some of their motions. Any significant difference between the egg shells of poultry and those of village-grown chickens could not be observed in EPR spectra. The duck and quail egg shells exhibit the same signals and therefore indicate the inducement of the same radicals. The spectroscopic splitting and the hyperfine interaction values were found to be consistent with the literature data.     

Polysilyl radicals: EPR study of the formation and decomposition of star polysilanes

Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me₃SiMe₂Si)₃Si (1).1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me₃Si)₃Si radical. Thus, (Me₃SiMe₂Si)₃Si^⋅ has a half-life time of ca. 6 min at 20°C, while (Me₃Si)₃Si^⋅ can be observed only at −25°C. Density-functional quantum-mechanical calculations show that1 and (Me₃Si)₃Si^⋅ have the same thermodynamic stability. The high kinetic stability of1 is attributed to its backfold “umbrella”-type conformation where the β-silyl groups point “inwards” towards the radical center. This conformation protects the radical center of1 from dimerization and other reactions. The EPR spectrum of1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that1 is less pyramidal than (Me₃Si)₃Si^⋅ but is more pyramidal than (i-Pr₃Si)₃Si^⋅, with an estimated SiSiSi bond angle around the radical center of 118∘. Photolysis and thermolysis of [(Me₃SiMe₂Si)₃Si]₂ also involves the intermediacy of1. Photolysis of [(Me₃SiMe₂Si)₃Si]₂ leads to (Me₃SiMe₂Si)₄Si, while thermolysis produced the less strained isomer of 1, (Me₃SiMe₂Si)₃SiSi-Me₂Si(Me₃SiMe₂Si)₂SiMe₃. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury.     

Characterization of free radicals from vitamin K1 and menadione by 2 mm-band EPR,ENDOR and ESEEM

The anion and cation radicals of vitamin K₁ and its analog menadione were characterized using the magnetic resonance techniques of Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM), and Electron Paramagnetic Resonance (EPR) at X-band and 2 mm-band. Theg-factor anisotropy of the radicals at 2 mm-band allow them to be distinguished from each other in the solid state. Theg-factor matrix of the radical anion of vitamin K₁ is virtually identical with that reported for the reduced A₁ acceptor in green plant photosystem I thus demonstrating that reduced A₁ is the anion radical of vitamin K₁.     

Detection of glass foreign bodies in soft tissues with low-frequency EPR spectroscopy

The detection of glass particles in glass-induced injuries is often difficult. Most commercial glasses contain iron (Fe³⁺) centers which present a characteristic electron paramagnetic resonance (EPR) spectrum nearg = 4.3. The detection of small glass particles in vivo can be achieved with low-frequency EPR spectroscopy (1.2 GHz) operating at low field. The method was sucessfully applied in anesthetized live animals where small pieces of glass were implanted under the skin in their backs. This could be a new clinically relevant application of EPR spectroscopy.     

Multifrequency EPR of four triarylmethyl radicals

Continuous-wave spectra at W-band of four triarylmethyl (trityl) radicals at 100 K in 1∶1 water-glycerol exhibit rhombic electron paramagnetic resonance spectra. The rigid-lattice line widths at W-band are only 3 to 5 times larger than at X-band or S-band, and fluid-solution line widths are much narrower than those for rigid lattice, which indicates that unresolved anisotropic nuclear hyperfine couplings make significant contributions to the rigid-lattice line widths. Spin-flip lines are observed in glassy-solution spectra at X-band and S-band, but not at W-band or 250 MHz. At 100 KT _m is dominated by spin diffusion of solvent protons and is independent of microwave frequency. Between about 130 and 170 K, 1/T _m for trityl-CH₃ is enhanced by rotation of the methyl groups at a rate comparable to inequivalences in the hyperfine interaction. Motional averaging of anisotropic interactions enhances spin echo dephasing between about 200 and 300 K. The temperature dependence of 1/T ₁ is similar for the four radicals and is consistent with assignment of the Raman process and a local mode as the dominant relaxation processes. The similarity inT ₁ values at W-band and X-band supports this assignment.     

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.     

X-band rapid-scan EPR of nitroxyl radicals

X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals (15)N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1-(15)N-oxyl-d(12)) and (15)N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl-(15)N-oxyl-d(16)). Simulations of spectra for (15)N-mHCTPO and (15)N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B(1) extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal bandwidth, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q.     

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

The time-resolved electron paramagnetic resonance (EPR) spectra are studied in the temperature range of 110–300 K for two mixed solutions of porphyrins, ZnTPP and H₂TPP, in toluene and the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO varied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as 1 mM. The EPR spectra of triplet-state porphyrins and free radicals manifest the chemically induced spin polarization. For the relatively short-lived radical-triplet pairs, with the perturbation theory up to the fourth order, the theoretical expressions are obtained for the triplet and radical spin polarization induced by the enhanced intersystem crossing (ISC) due to the interaction of excited singlet-state porphyrins with free radicals and by the triplet quenching by free radicals. The time-dependent EPR spectra of the triplets are simulated taking into account the spin-lattice relaxation. It is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattice relaxation kinetics. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetic scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the rate of the ISC and the lifetime of the excited singlet state were estimated by fitting the kinetic curves for the triplet EPR spectra intensity. For the mixed porphyrin-TEMPO solutions, a possible set of the rate constants of important bimolecular processes were determined. For this set of parameters, it turns out that the spin polarization transfer has a smaller rate constant than the rate constant of the diffusion collisions of the triplet and radical. It appears that the rate constant of the ISC catalyzed by radicals is relatively high in the solutions close to the melting point of the solvent and in the soft-glassy state. In the triplet porphyrins the initial spin polarization induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.