Electron spin relaxation of triarylmethyl radicals in fluid solution

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1:1 water:glycerol, and 1:9 water:glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H(2)O solution, T(1) is 17+/-1 micros at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T(1) in 1:1 water:glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros at L-band. For more viscous solvent mixtures, T(2) is much shorter than T(1) and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T(2). In water and 1:1 water:glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1:9 water:glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T(2)-determined contributions to the linewidths at higher frequencies.     

FID detection of EPR and ENDOR spectra at high microwave frequencies

High-frequency pulsed EPR spectroscopy allows FID detection of EPR spectra owing to the short dead time that can be achieved. This FID detection is particularly attractive for EPR and ENDOR spectroscopy of paramagnetic species that exhibit inhomogeneously broadened EPR lines and short dephasing times. Experiments are reported for the metalloprotein azurin at 275 GHz.     

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.     

A pulsed EPR method to determine distances between paramagnetic centers with strong spectral anisotropy and radicals: The dead-time free RIDME sequence

Methods to determine distances between paramagnetic metal centers and radicals are scarce. This is unfortunate because paramagnetic metal centers are frequent in biological systems and so far have not been employed much as distance markers. Successful pulse sequences that directly target the dipolar interactions cannot be applied to paramagnetic metal centers with fast relaxation rates and large g-anisotropy, if no echos can be detected and the excitation bandwidth is not sufficient to cover a sufficiently large part of the spectrum. The RIDME method Kulik et al. (2002) [20] circumvents this problem by making use of the T₁-induced spin-flip of the transition-metal ion. Designed to measure distance between such a fast relaxing metal center and a radical, it suffers from a dead time problem. We show that this is severe because the anisotropy of the metal center broadens the dipolar curves, which therefore, only can be analyzed if the full curve is known. Here, we introduce five-pulse RIDME (5p-RIDME) that is intrinsically dead-time free. Proper functioning of the sequence is demonstrated on a nitroxide biradical. The distance between a low-spin Fe(III) center and a spin label in spin-labeled cytochrome f shows the complete dipolar trace of a transition-metal ion center and a spin label, yielding the distance expected from the structure.     

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications

The construction and performance of a cryogenic 35 GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE₁₀₂ cavity in which the radio frequency (rf) B₂-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B₂-field strength per square-root of the transmitted rf power over the frequency range 2–180 MHz. The distributions of the microwave B₁- and E₁-field, and the rf B₂-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions.     

Application of 150GHz quasi-optical EPR and endor systems to examination of materials for polarized nuclear targets

Some possibilities of mm and submm magnetic resonance spectrometers using a two-mirror open resonator as the sensor is demonstrated. A comparative analysis of sensitivity has been performed for spectrometer, with cavity and open resonators. Absorption spectra of the HMBA Cr^V complex in propylene glycol and ethylene glycol were investigated at 4.2 K.     

Single Crystal Magnetic Resonance Study of a Rh2☎ Dimer Centre in NaCl at Q-Band

Electron nuclear double resonance spectroscopy at 34 GHz is applied to determine the signs of the principal values and the directions of the corresponding principal axes of the zero field splitting tensor for a Rh^2? dimer centre in NaC1. The results are in good agreement with the hypotheses concerning the spin Hamiltonian parameters deduced from the X- and W-band EPR analysis and support the model proposed for this centre consisting of two Rh^2? ions on cation positions and two Na^? vacancies.     

多核芳烃正离子自由基的ENDOR和EPR研究

用EPR和ENDOR方法检测傅氏烷基化反应(Friedel-Crufts reaction)中所生成的多核芳烃正离子自由基:1,4,5,8-四甲基惠、9,10-二氘代-1,4,5,8-四甲基蒽、2,6,9,10-四甲基蒽、2,6-二甲基-9,10-二(对甲苯基)蒽、3,6,11,14-四甲基二苯并(a,c)三亚苯正离子自由基,测定了它们的超精细偶合常数并提出了其生成机理。     

The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures

A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = (1/2) coupled to a nuclear spin I = (1/2) was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T(1e) and T(1n), respectively, and the cross-relaxation time, T(1x). It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) > T(1e) and T(1n), T(1x) > T(1e), where ENDOR signals from the alpha manifold are negative and those of the beta manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) < T(1e) < t(R) and T(1n), T(1x) > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the alpha manifolds are positive, and those of the beta manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental t(mix) and t(R) dependencies of the W-band (1)H ENDOR amplitudes of [Cu(imidazole)(4)]Cl(2) were reproduced and the values of T(1e) and T(1x) > T(1e) were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum.     

EPR and ENDOR study of an oxygen-vacancy-associated Ti center in RbTiOPO4 crystals

The dominant Ti³⁺ trapped electron center in flux-grown RbTiOPO₄ (RTP) crystals has been characterized using electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR). This center is produced during an X-ray irradiation at room temperature when a Ti⁴⁺ ion traps an electron and becomes a Ti³⁺ ion, and is best studied in the 30–40 K range. The EPR spectrum contains a three-line hyperfine pattern from two nearly equivalent neighboring ³¹P nuclei, along with hyperfine lines from the ⁴⁷Ti and ⁴⁹Ti nuclei. The g matrix, determined from the angular dependence of the EPR spectrum, has principal values of 1.819, 1.889, and 1.947. Hyperfine matrices for four ³¹P nuclei are obtained from the angular dependence of the ENDOR spectrum. The proposed model for this defect is a Ti³⁺ ion adjacent to an oxygen vacancy at an OT position. Analogies are made to a similar Ti³⁺ center in KTiOPO₄ (KTP) crystals.     

Formation and stabilization of persistent free radicals

We demonstrate that stable and relatively unreactive “environmentally persistent free radicals (PFRs)” can be readily formed in the post-flame and cool-zone regions of combustion systems and other thermal processes. These resonance-stabilized radicals, including semiquinones, phenoxyls, and cyclopentadienyls, can be formed by the thermal decomposition of molecular precursors including catechols, hydroquinones and phenols. Association with the surfaces of fine particles imparts additional stabilization to these radicals such that they can persist almost indefinitely in the environment. A mechanism of chemisorption and electron transfer from the molecular adsorbate to a redox-active transition metal or other receptor is shown through experiment, and supported by molecular orbital calculations, to result in PFR formation. Both oxygen-centered and carbon-centered PFRs are possible that can significantly affect their environmental and biological reactivity.     

A Multi-Frequency EPR and ENDOR Study of Rh and Ir Complexes in Alkali and Silver Halides

Aliovalent Rh and Ir cations have been frequently used to influence the photographic properties of silver halide emulsions. The doping introduces several types of related defects with distinct trapping and recombination properties. EPR and ENDOR are, in principle, ideally suited for the determination of the microscopic structure of the individual centres but it will be demonstrated that well-chosen, sometimes sophisticated multi-frequency experiments are necessary in order to (partially) reach this goal. Model studies on single crystals of AgCl and NaCl also appeared indispensable for the unravelling of the spectra.

In the review of Rh-centres in NaCl and AgCl special attention is paid to methods that allow to detect cation vacancies near Rh^2? complexes. An alternative explanation for the high temperature behaviour of the [RhCl₆]^4? complexes in AgCl is presented.     

A Davies/Hahn multi-sequence for studies of spin relaxation in pulsed ENDOR

We extend earlier studies of the effects of relaxation on the intensities of pulsed ENDOR signals by introducing a Davies/Hahn (D/H) pulsed ENDOR multi-sequence that corresponds to a series of Davies sequences with the preparation pulse 'turned off'. In this pulse train, the Hahn [pi/2, pi] detection pulse pair of sequence n-1 both generates the echo detected for that sequence and acts as the preparation portion of sequence n, in effect replacing the pi preparation pulse of the Davies sequence. We show both theoretically, through a master-equation approach, and with both (1)H(I=1/2) and (14)N(I=1) ENDOR experiments on the non-heme Fe enzymes, superoxide reductase (SOR) (S=1/2) and AntDO (S=3/2), that under conditions of high electron-spin polarization (high microwave frequency/low temperature) the D/H multi-sequence allows simplification of ENDOR spectra by suppression of nuclear transitions associated with the m(S)=+1/2 (alpha) manifold. As such suppression depends on the sign of A, it allows determination of this sign. The suppression as a function of the time between individual sequences is found to exhibit behaviors that can be classified into three regimes of the ratio of cross-relaxation to spin-lattice relaxation rates: strong cross-relaxation (X-case); comparable rates (XL); negligible cross relaxation (L). Interestingly, the ENDOR behavior of the S=1/2 SOR center indicates it is an L case, while the S=3/2 AntDO is an L case. Overall, the D/H protocol appears to be a robust and general tool for using relaxation effects to manipulate ENDOR spectra.     

Nano-emulsions of fluorinated trityl radicals as sensors for EPR oximetry

This article reports the development and evaluation of two nano-emulsions (F45T-03/HFB and F15T-03/PFOB) containing fluorinated trityl radicals dissolved in perfluorocarbons. Preparation with a high-pressure homogenizer conferred sub-micronic size to both nano-emulsions. In vitro and in vivo EPR spectroscopy showed that the nano-emulsions had much greater oxygen sensitivity than the hydrophilic trityl, CT-03. In vivo experiments in rodents confirmed the ability of the nano-emulsions to follow the changes in oxygen concentration after induced ischemia. Histological evaluation of the tissue injected with the nano-emulsions revealed some acute toxicity for the F45T-03/HFB nano-emulsion but none for the F15T-03/PFOB nano-emulsion. These new formulations should be considered for further EPR oximetry experiments in pathophysiological situations where subtle changes in tissue oxygenation are expected.     

Annealing studies of irradiation effects in vitreous silica

Abstract

This paper describes a series of annealing experiments devoted to the study of the thermal behaviour of B₂ and E' centres produced by heavy-ion irradiations in v-SiO₂. Thermal bleaching of the B₂ component is shown to be complete and irreversible at relatively low temperatures (?500°C). On the other hand, bleaching of E' in the same range is reversible and the corresponding absorption band can be entirely recovered in subsequent electron irradiations, suggesting the existence of a stable precursor defect for this centre. By alternating isochronal annealing cycles with electron re-irradiations, the annealing of such defects was investigated up to 1000°C, taking care to compare the behaviour of the heavy-ion damaged material with that of undamaged material. The results seem to suggest a structural difference between pre-existing E' precursors of normal SiO₂ and those produced in excess by displacing radiation, the latter being characterized by a distinct process of destruction with an activation energy Q ?0.6 eV.     

Observer-selective double electron-electron-spin resonance, a pulse sequence to improve orientation selection

By pulsed double electron-electron resonance (DEER), distances between spin labels in disordered systems up to 8 nm can be measured. In addition, the relative orientation of the interacting radicals can be determined, provided that the bandwidth of the pulses is sufficiently small. On the other hand, the bandwidth has to exceed the dipolar interaction considerably, because otherwise the DEER modulations become distorted and the modulation depth decreases, making distance determination impossible. Therefore, small bandwidths, i.e. long pulses, place a lower limit on the distance that can be determined. Two new pulse sequences, observer-selective DEER (os-DEER) and dead-time free os-DEER, are introduced that make it possible to use long observer pulses with bandwidths that are smaller than the dipolar interaction. The new pulse sequences do not suffer from the distortions caused by the limited bandwidth of the observer pulses, as demonstrated by measurements on a nitroxide biradical. With observer pulses of 140 ns, i.e., significantly longer than the 32 ns used in the conventional DEER sequence, a dipolar interaction of 7.8 MHz has been measured.     

A new combination of protecting groups and links for encoded synthetic libraries suited for consecutive tests on the solid phase and in solution

Summary A strategy for high-throughput evaluation of combinatorial compound libraries is reported, which circumvents the necessity to test complex mixtures. The method is based on a new combination of protecting groups, solid-phase linker and tags. The bulk of the library first undergoes a binding assay with the components grafted on beads. A selection of beads carrying strong ligands is stripped from the labelled target and distributed into microvessels. The ligands are cleaved and rinsed into microeluates. Subsequently, a more detailed characterization with a functional assay in solution determines the best performers, which are identified through the peptidic tag left behind on the corresponding mother bead.Abbreviations Boc tert-butyloxycarbonyl - CCL combinatorial compound libraries - Ddz ,-dimethyl-3,5-dimethoxybenzyloxycarbonyl - DICD N,N-diisopropylcarbodiimide - DIPEA diisopropylethylamine - DMA dimethylacetamide - ESL encoded synthetic libraries - FITC fluoresceinisothiocyanate - GABA -aminobutyric acid - HOBT N-hydroxybenzotriazole - MALDI matrix-assisted laser desorption ionization - Pbf 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl - Pmc 2,2,5,7,8-pentamethylchroman-6-sulfonyl - TFA trifluoroacetic acid - Trt trityl     

CF自由基双光子共振增强多光子电离研究：3pRy dberg态的转动分析

利用Ar/CF4、Ar/CF2Cl2或Ar/CF3COOH混合气体的直流脉冲放电产生CF自由基,观测了260～360nm范围内转动分辨的CF自由基双光子共振增强多光子电离谱。分析表明,该段光 谱对应于CF自由基3pπD2Πr(ν'=2～6,r=3/2,1/2)←←X2Πr(ν"=0,r=3/2,1/2)的共振激发。对观测的振动带进行了转动分析,并获得了3p里德堡态的转动常数和自旋-轨道分裂值。     

The detection and identification of transient free radicals in solution

The various Electron Spin Resonance methods that have been used for studying transient free radicals are described, and their limitations and advantages discussed. Continuous wave and Fourier Transform time-resolved methods are compared and shown to be complementary in studies of radicals produced using flash photolysis. The role of Chemically Induced Dynamic, Electron Polarization (CIDEP) in determining reaction pathways and in linking the photochemistry and photophysics of molecules is described as is the role played by Reaction Yield Detected Magnetic Resonance (RYDMR) and magnetic field effect (MFE or MARY) studies in understanding the reactions of radicals.