Pulse-shift measurement: a direct observation of radical pair dynamics in a micelle

The hydrogen abstraction reactions of benzophenone in a micellar solution in the absence and presence of 1,4-cyclohexadiene are compared with time-resolved electron spin resonance (ESR) and optically detected (OD) ESR. “Pulse shift” measurement by OD-ESR, which observes the effect of a resonant microwave pulse at different delay times after laser excitation, reveals that the lifetime of the radical pair becomes much shorter in the presence of 1,4-cyclohexadiene. This explains the change of chemically induced dynamic electron polarization from spin-correlated radical pair polarization in the absence of 1,4-cyclohexadiene to conventional, E^*/A polarization in its presence. The rate constants determined by this technique indicate that the escaping rate of cyclohexadienyl radical is ten times larger than those of alkyl and benzophenone ketyl radicals.     

Magnetic field dependence of spin dynamics of radical pairs studied by time-resolved RYDMR

The photochemical reaction of 2-methyl-1,4-naphthoquinone in sodium dodecyl-sulphate micellar solution was investigated with an optical detection ESR apparatus working at 17.44 GHz (Ku-band). The Ku-band RYDMR spectra are obtained from the transient optical absorption and the stationary absorption of the reaction product, and the shift of the spectral peak compared with the spectra at 331 mT is reproduced well by the difference in g of the component radicals. The microwave pulse length dependence shows the quantum beat originated from the conversion between triplet ±1 states and the mixed state of singlet and triplet 0 states by the microwave field. The decay rate of the radical pair in triplet ±1 at 622 mT was determined to be 7.1 ± 1.1 × 10⁵ s^?1 by changing the irradiation time of a short (20 ns) microwave pulse with reference to the laser excitation. This value is smaller than that at 331 mT, as expected by the relaxation mechanism.     

亚细胞器诱导的2,4,6-三硝基甲苯阴离子自由基的ESR研究

本文在无氧条件下利用ESR分别观察了肝脏和晶状体的微粒体及线粒体酶在NADPH存在下还原三硝基甲苯(TNT)的过程,检测到参数为g=2.0048±0.0005,A_对位^N=1.215mT,A_对位^N=0.800±0.010mT,A^H=0.122±0.0206mT的自由基信号,并通过电子计算机对ESR谱的模拟证明该自由基信号为TNT硝基阴离子自由基,根据其超精细分裂常数认为,其不配对电子的电子云密度主要分布在对位硝基上。     

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.     

Radical pair dynamics observed with pulse-mode product-yield-detected ESR

Three pulse sequences have been employed for laser and microwave-pulse irradiation in the pulse mode product-yield-detected ESR technique to detect the dynamical feature of transient radical pair. In this method, the decrease in spin adduct yield as functions of delay times for irradiation/termination of the microwave pulse after the laser pulse are simulated with the Runge-Kutta method for a detailed reaction model to obtain the rate constants of cage product formation, escape of the radical pair, as well as the hydrogen abstraction reaction. As the representative reaction systems, photoreduction of anthraquinone, anthraquinone-2-sulfonate, and benzophenone in SDS micellar solution were employed. Effects of cyclohexene, added as a hydrogen donor, NaCl and Ethanol, added to change the microviscosity of micelle, on these reaction rate constants have been tested.     

Modeling magnetic field effects in multispin systems

A model is proposed for calculating magnetic field effects formed in a radical triad composed of a biradical and a paramagnetic particle. To describe the influence of the “third” spin on the spin evolution in a biradical, the electron spin exchange interaction of the added spin with one of the paramagnetic centers of the biradical has been considered. Calculating the field dependence of the recombination probability of the biradical-oxygen complex revealed both an increase in recombination probability earlier attributed to spin catalysis and the influence of the values and signs of the exchange interaction in the complex on the shape of the magnetic-field effect dependence. Calculation results are in agreement with the experimental data on the photolysis of 7,7’-dimethyl-silanorbornadiene in aerated and deaerated solution.     

Low field RYDMR: effects of orthogonal static and oscillating magnetic fields on radical recombination reactions

Reactions involving spin correlated radical pairs as intermediates are known to be sensitive to applied static and/or oscillating magnetic fields. In the reaction yield detected magnetic resonance (RYDMR) technique, an electromagnetic field in resonance with the electron Zeeman splitting produced by a strong static field is used to perturb the singlet ? triplet interconversion of the radical pair and so to affect the yield of geminate recombination. New experiments are described in which weak radiofrequency fields (? 300μT) in the frequency range 1–80 MHz are applied to radical ion pairs derived from pyrene and 1,3-dicyanobenzene, in the presence of a weak (? 3.0 mT) static magnetic field. Such experiments test the viability of RYDMR in low fields, provide insight into the crossover region between the zero-field and high field cases, and may give information on the distribution of radical pair lifetimes.     

Coupled (localized+delocalized) electron spins dynamics in polymer phase of RbC60 fulleride

The transverse response (transverse dynamic susceptibility) of coupled localized (s) and delocalized (e) electron spins of a metal paramagnet as well as the longitudinal dynamic response of such a system (to be registered by a longitudinal coil) to the modulation of microwave power saturating electron spin resonance (ESR) are calculated. The ESR spectrum is analytically decomposed into two Lorentzians with normal resonance frequencies and decay rates of the coupled localized +delocalized electron spin system. In the case of relaxationally coupled s- and e-spins the longitudinal response is decomposed into two “relaxational” Lorentzians squared with amplitudes containing ESR lineshapes with above-mentioned shifted frequencies and linewidths as well as enhancement-suppression coefficients of magnetization evolution. These results are essential for the interpretation of experiments on longitudinal response in metal paramagnets, the latter being the source of important information on longitudinal electron relaxation; in particular, for extraction of information from longitudinal response experiments in the polymer phase of RbC₆₀ fullende, where the obtained results describe the observed form of the ESR spectrum and the main features of the longitudinal response.     

L波段三维ESR成像系统的研制(Ⅳ)--系统组成与各部分性能指标

报道了自行研制的L波段三维电子自旋共振成像（3D-ESRI）系统的整机结构及各部分性能指标. 该系统主要由L波段ESR谱仪、三维梯度磁场装置、数据处理及图像重建软件组成. 系统的微波频率为1.05 GHz;最大微波功率500 mW. 采用3-环2-缝再进入式谐振腔,无载Q值>1 000;最大测量体积为φ 20 mm, 高30 mm柱状水溶液样品. 接收系统采用100 kHz锁相放大电路,最大增益可达1×10⁶;时间常数0.02 ms～1 s;磁场调制幅度>0.5 mT. 最大梯度磁场2 mT/cm;三维梯度线性度均优于5 ％;稳定度可达10^-5;主磁场可在1.6～96 mT范围内任意点选择扫场起始点;在0.2～16 mT范围选择磁场扫描宽度. 数据系统为12位A/D实现数据采集,三路8位D/A控制梯度磁场. 采用滤波反投影法实现图像重建, 成像功能包括：二维、三维自旋浓度成像;等浓度线2D图像显示;3D立体和断层图像显示等. 对水溶液和固体模型样品进行ESR成像的结果表明：本系统可以开展较大体积生物样品的ESRI研究.     

The theory of nuclear orientation via electron spin locking (NOVEL)

Nuclear orientation via electron spin locking (NOVEL) is a technique to orient nuclear spins embedded in a solid. Like other methods of dynamic nuclear polarization (DNP) it employs a small amount of unpaired electron spins and uses a microwave field to transfer the polarization of these unpaired electron spins to the nuclear spins. Traditional DNP uses CW microwave fields, but NOVEL uses pulsed electron spin resonance (ESR) techniques: a 90 degree pulse–90 degree phase shift–locking pulse sequence is applied and during the locking pulse the polarization transfer is assured by satisfying the Hartmann–Hahn condition. The transfer is coherent and similar to coherence transfer between nuclear spins. However, NOVEL requires an extension of the existing theory to many, inequivalent nuclear spins and to arbitrary, i.e. high electron and nuclear spin polarization. In this paper both extensions are presented. The theory is applied to the system naphthalene doped with pentacene, where the proton spins are polarized using the photo-excited triplet states of the pentacene molecules and found to show excellent agreement with the experimentally observed evolution of the polarization transfer during the locking pulse.     

Observation and reaction control of a singlet born radical pair formed photochemically in an SDS micelle by pulsed microwave irradiation

The photochemical reaction of tetraphenylhydrazine in an SDS micellar solution is studied using a transient absorption detected magnetic resonance (ADMR) method. This system is photo-dissociated via the singlet excited state and forms a transient radical pair. Strong microwave irradiation of this system under an × band EPR magnetic field provides an ADMR spectrum of the singlet born radical pair as the optical absorbance change of the diphenylaminyl radical. The employment of a short duration microwave pulse that efficiently flips the electron spin quantum (Δm _s = 1) controls the reaction of the radical pair, and the reaction rate constant of this system is determined. Furthermore, changing the microwave duration shows the quantum beat of this system induced by the electromagnetic field. This beat signal carries both a single beat that corresponds to the Rabi frequency and a double-frequency beat that is due to simultaneous two-spin controlling.     

Radical cations of n-alkanes in irradiated solutions as studied by time-resolved magnetic field effects

The time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs has been measured to study n-alkane radical cations in irradiated solutions at room temperature. The magnetic field effect was recorded as a ratio of fluorescence decay curves in the 0.1 T and zero magnetic fields for solutions of C₈, C₉, C₁₀, C₁₂, and C₁₆ n-alkanes in n-hexane with addition of 3 × 10^?5 M p-terphenyl-d ₁₄. A distinct maximum at 10–30 ns followed by a slowly decaying plateau was observed for all the solutions. Simulation shows that the maximum corresponds to an unresolved ESR spectrum with the peak-to-peak line-width ranging from about 1.6mT to 0.5mT for C₈ to C₁₆ radical cations. The unresolved structure is believed to result from the hyperfine couplings with many protons of the radical cation, the increase in the number of interacting protons compared with low temperature matrices being caused by the methyl group rotation and conformational motion of the carbon chain. With increase in concentration of dissolved n-alkane, the maximum in the curves first moves to longer times and finally disappears; this was attributed to the narrowing of ESR spectrum contour due to degenerate electron exchange.     

Multiplet-to-net CIDEP conversion by MW-pulses with adiabatically ramped amplitude

ABSTRACT

A method is proposed to manipulate electron spin order of spin-correlated radical pairs. As radical pairs are often born in a well-defined spin state, e.g. in the singlet state, they acquire Chemically Induced Dynamic Electron Polarisation (CIDEP). In the case of singlet-state preparation CIDEP is of the multiplet (or anti-phase) type resulting in reduction of EPR (electron paramagnetic resonance) signals due to overlap of absorptive and emissive lines in the spectrum. Here we propose to convert the singlet spin order into net magnetisation of the radical pair by applying a microwave field, with its amplitude slowly (adiabatically) reduced to zero. We demonstrate that by properly choosing the microwave frequency one can completely convert the singlet order into net polarisation of the radical pair with significant enhancement of the signal as compared to multiplet CIDEP. Calculations show that the technique is operative for both weakly coupled and strongly coupled spin pairs. Potential applications of the method are discussed.     

Feasibility of radiation sterilization and dosimetric features of ampicillin: an electron spin resonance study

In the present work, it was aimed to identify radical species produced by gamma irradiation (3–34?kGy) of solid ampicillin, to determine its spectroscopic, dosimetric, stability and kinetic behavior, and to investigate feasibility of the radiation sterilization feature of ampicillin by using electron spin resonance (ESR) spectroscopy. ESR experiments were performed at low and high temperatures (130–400?K) to examine the characteristic properties of the radical intermediates that are produced in ampicillin by gamma radiation treatment. Unirradiated ampicillin presented no ESR signal but irradiated samples exhibited ESR spectra with four resonance peaks spread over a magnetic field range of 8?mT. The spectral parameters of the central resonance line of the spectrum are g?=?2.0044 and ΔH_pp?=?0.08?mT. An exponential growth function of the applied dose was found to describe best the experimental dose–response data and it was found that ampicillin did not exhibit the feature of a good dosimetric material as its ESR intensity was relatively weak even for the samples irradiated at high level of doses. G_mean value of gamma-irradiated ampicillin was found to be 4.6?±?0.9?×?10^?9?mol/J, which is very small compared to irradiated alanine solid sample. However, the discrimination of irradiated ampicillin from unirradiated one was possible even ～3 months after storage at normal conditions. The simulation calculations indicated that gamma irradiation created two different radical species in solid ampicillin. Decay activation energy of the radical species which is mostly responsible from central intense resonance line is calculated to be 55.6?±?3.2?kJ/mol by using the signal intensity decay data derived from annealing studies. It was concluded that ampicillin could be sterilized by gamma radiation and ESR spectroscopy can be used as a potential technique to monitor its radiosterilization process.     

Investigation of radiosterilization feasibility of sulfamethoxazole by ESR spectroscopy

In the present study, the spectroscopic features of the radiolytic intermediates that were produced in gamma-irradiated (5, 10, 25 and 50?kGy) sulfamethoxazole (SMX) have been investigated by electron spin resonance (ESR) spectroscopy and the radiation sterilization feasibility of SMX by ionizing radiation was examined. Gamma-irradiated SMX exhibited a complex ESR spectrum consisting of 13 resonance lines where spectral parameters for the central resonance line were found to be g?=?2.0062 and ΔH_pp?=?0.6?mT. The radiation yield of SMX was calculated to be relatively low (G?=?0.1) by ESR spectroscopy and no meaningful difference was observed in the comparison of unirradiated and 50?kGy gamma irradiated SMX by the Fourier transform infrared (FT-IR) technique, confirming that SMX is a radioresistive material. Although SMX could not be accepted to be a good dosimetric material, the identification of irradiated SMX from the unirradiated sample was possible even for the low absorbed radiation doses and for a relatively long time (three months) after the irradiation process. Decay activation energy of the radical species, which is mostly responsible for the central intense resonance line, is calculated to be 45.15?kJ/mol by using the signal intensity decay data derived from annealing studies. Four radical species with different spectroscopic properties were accepted to be responsible for the ESR spectra of gamma-irradiated SMX, by simulation calculations. It is concluded that SMX and SMX-containing drugs can be sterilized by gamma radiation and ESR spectroscopy is an appropriate technique for the characterization of these induced radical intermediates during the gamma irradiation process of SMX. Toxicology tests should also be done for its safe usage.     

Nonselective excitation of pulsed ELDOR using multi-frequency microwaves

The use of a polychromatic microwave pulse to expand the pumping bandwidth in pulsed electron-electron double resonance (PELDOR) was investigated. The pumping pulse was applied in resonance with the broad (～100 mT) electron paramagnetic resonance (EPR) signal of the manganese cluster of photosystem II in the S2 state. The observation pulses were in resonance with the narrow EPR signal of the tyrosine radical, YD·. It was found that in the case of the polychromatic pumping pulse containing five harmonics with the microwave frequencies between 8.5 and 10.5 GHz the PELDOR effect corresponding to the dipole interaction between the Mn cluster and YD· was about 2.9 times larger than that achieved with a monochromatic pulse. In addition to the dipolar modulation, the nuclear modulation effects were observed. The effects could be suppressed by averaging the PELDOR trace over the time interval between the observation microwave pulses. The polychromatic excitation technique described will be useful for improving the PELDOR sensitivity in the measurements of long distances in biological samples, where the pair consists of a radical with a narrow EPR spectrum and slow phase relaxation, and a metal center that has a broad EPR spectrum and a short phase relaxation time.     

ESR response of gamma-irradiated sulfamethazine

In the present work, characteristic features of the radiolytical intermediates produced in gamma-irradiated solid sulfamethazine (SMH) were investigated by electron spin resonance (ESR) spectroscopy. The heights of the resonance peaks, measured with respect to the spectrum baseline, were used to monitor microwave saturation, temperature and time-dependent kinetic features of the radical species contributing to the formation of recorded experimental ESR spectra. Three species having different spectroscopic and kinetic features were observed to be produced in gamma-irradiated SMH. SO₂, which is the most sensitive group of radiation in the SMH molecule, was found to be at the origin of radiation-produced ionic radical species. Based on the experimental results derived from the present study, the applicability of ESR spectroscopy to radiosterilization of SMH was discussed. In the dose range of interest (0.5–10 kGy), the radiation yield of solid SMH was calculated to be very low (G=0.45) compared with those obtained for sulfonamide aqueous solutions (G=3.5–5.1). Based on these findings, it was concluded that SMH and SMH-containing drugs could be safely sterilized by gamma radiation and that ESR spectroscopy could be successfully used as a potential technique for monitoring their radiosterilization.     

Effects of Gd<Superscript>3+</Superscript> doping on structural and dielectric properties of PZT (Zr:Ti = 52:48) piezoceramics

The purpose of this research is to study the effect of doping Gd into Pb(Zr_0.52Ti_0.48)O₃ ceramics prepared by solid state reaction. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole % Gd at which the unreacted oxides start to appear. The electron spin resonance (ESR) spectra of PGZT's indicate that Gd³⁺ can enter both A site of the perovskite structure instead of only A site as widely believed. The ESR peaks resonance shift towards low fields as the concentration is higher, which is due to the change in crystal field experienced by Gd³⁺ ions. At x =0.001, 0.005 and 0.01 dopings, two sets of powder ESR signals arising from Gd³⁺ (4f⁷, spin 7/2) ions at A site. The first set shows some fine structure having strong absorption peaks centered at 76.26 mT (g = 8.550). The second is a seven-peak spectrum centered at 206.01 mT (g = 3.165), which belongs to the Gd³⁺ ions at B sites. Furthermore, the overlapped ESR strong absorption peaks from 309.17 mT to 314.49 mT (g = 2.2818-2.1087) belong to Gd³⁺ of unreacted Gd₂O₃. The local environments of Gd³⁺ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameter, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A.     

Abnormal electron spin echo and multiple-quantum coherence in a spin-correlated radical pair system

The phenomena of the abnormal “out-of-phase” electron spin echo in a photo-induced spin-correlated radical pair system are examined theoretically. It is shown that such abnormal phenomena are a consequence of initial non-Boltzmann distribution and zero-quantum coherence produced by laser excitation. The analysis of echo amplitude versus the pulse-angle of the microwave pulse reveals two sources for the formation of the echo. The method of excitation and detection of multiple-quantum coherence using a phase-cycled 2-pulse sequence is also discussed. Such a technique is complementary to the ESE method.     

Large magnetic field effect on back electron transfer from uncharged radical to its cationic partner in anionic micelle

The magnetic field effect (MFE) on the radical pair (RP) generated by photoexcitation of the acetyl derivative of phenyl pyrylium ion (APP⁺) in the presence of biphenyl, an electron donor, has been investigated. The escape yield at 3.5 T is more than ten times the zero-field value. The MFE reaches near-saturation twice, once at fields of the order of 10mT and again at about 3.5 T. The low-field variation of the MFE conforms to the pattern expected for the isotropic HFC mechanism, and the high-field variation to that expected for the relaxation mechanism. In this particular system two types of radical pair are generated, one by electron transfer from the donor to the acceptor and another by H-abstraction from the micelle. The MFEs on the two types of ³RP have been compared.