An electron spin resonance study of proton transfer equilibria involving the semiquinone radicals derived from hydroquinone and catechol

The electron spin resonance spectra of the semiquinone radicals derived from hydroquinone and catechol have been studied in aqueous solutions of varying ph in a rapid flow system. The spectra were found to change drastically with ph; these effects are attributed to protonation of the semiquinones at rates which are dependent on the ph. Models for the systems are proposed, and the effects of exchange are described by means of the modified Bloch equations. Estimates are made of the rate constants and equilibrium constants for protonation of the semiquinone radicals and limits are set for the rate of hydroxyl proton exchange between the radical species and solvent in the low ph region.     

EPR studies of15N- and2H-substituted pH-sensitive spin probes of imidazoline and imidazolidine types

The isotopically substituted analogs of pH-sensitive imidazoline and imidazolidine radicals have been synthesized and investigated with electron paramagnetic resonance (EPR) spectroscopy. The introduction of²H and¹⁵N into the structure of the radical is a useful approach to enhance the information obtained from spin-labeling experiments. The spectra of the radicals have been analyzed with 9.8 (X-band) and 130 GHz (D-band) EPR spectroscopy. The substitution of¹H for²H leads to significant narrowing of Gaussian line width, while the substitution of¹⁴N for¹⁵N in the nitroxyl fragment decreases both the number of spectral lines and Lorentzian line width. These effects result in a significant increase in the peak intensities up to 5–7 times for X-band EPR spectra of one of the imidazoline radicals (R4). The increase in spectral resolution allowed us to reveal the hyperfine interaction splitting with the attached proton (0.36 G) in the protonated form of the radical R4. The influence of proton exchange of the radicals with phosphate and acetate buffers on their EPR spectra has been studied in H₂O and D₂O. The corresponding rate constants of the proton exchange have been calculated from fitting of the simulated EPR spectra line shapes to experimental spectra. The data obtained demonstrated the advantages of the isotopically substituted spin pH probes in spectral resolution and sensitivity which can be an important factor particularly for applications in vivo where the fundamental sensitivity is much lower. The sensitivity of EPR spectra of these spin probes to the buffer capacity could be of practical importance taking into account the biological relevance of monitoring this parameter in some pathological states.     

Results from the CMS collaboration on the search of heavy dilepton and diphoton resonances

A survey of the results on the search of new heavy resonances decaying into muon, electron, or photon pairs in the CMS experiment at proton beams of the LHC in the run of 2015 for the total center of mass energy √s = 13 TeV is presented. The limits for heavy resonance production cross sections are set. Excess of diphoton events in the region of mass spectra near 750 GeV with a local statistical significance of 3.4 σ and a global statistical significance of 1.6 σ is observed.     

An Electron Spin Resonance Study of Gamma-Irradiated 5,6-Dihydropyrimidines

Electron spin resonance (ESR) spectra are observed from gamma-irradiated, poly-crystalline samples of 5,6-dihydrouracil, 5,6-dihydro-5-methyluracil, and 5,6-dihydro-6-methyluracll. The spectra indicate that the radiation-induced free radicals are formed by the loss of hydrogen atoms from the parent compounds. The crystal structure of the dihydropyrimidines is apparently such that there is a proton near the unpaired electron which can simultaneously flip with the flip of the unpaired electron resulting in the formation of satellite lines relative to the main spectral lines. Annealing the irradiated samples at 100°C produces a decrease in the radical content of all compounds except in dihydro-5-methyluracll where the radical content increases. These differences are discussed in terms of the close proximity of the radicals formed during irradiation of dihydro-5-methyluracil. The     

Electron paramagnetic resonance study of gamma-irradiated potassium hydroquinone monosulfonate single crystal

Gamma-irradiated potassium hydroquinone monosulfonate (PHM) single crystals were investigated between 125 and 300?K using the electron paramagnetic resonance (EPR) technique. Between 125 and 300?K no changes in the spectra have taken place. The spectra were found to be temperature independent. The investigation of EPR spectra of irradiated single crystals of PHM showed the presence of two PHM anion radicals. The g values of the radiation damage centers observed in the PHM single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained.     

吩噻嗪-1,4萘醌-乙二醇体系光化学过程的时间分辨ESR研究

用时间分辨ＥＳＲ方法研究了吩噻嗪（ＰＴＨ）－１，４－萘醌（ＮＱ）－乙二醇（ＲＨ）体系中的光化学过程．只观察到萘醌的中性半醌自由基ＮＱＨ．＊的全发射极化的ＣＩＤＥＰ（化学诱导动态极化）信号，并未观察到极化的萘醌负离子ＮＱ－·＊的ＣＩＤＥＰ信号，表明ＲＨ与三重态ＮＱ间的质子转移反应远比ＰＴＨ与ＮＱ间的电子转移反应为快．ＮＱＨ．＊的ＣＩＤＥＰ信号随ｐＨ值的变化表明体系中同时有极化自由基ＮＱＨ．＊与ＮＱＨ．＋＊２存在，并且其间有质子交换．     

The Mo−OH proton of the low-pH form of sulfite oxidase: Comparison of the hyperfine interactions obtained from pulsed ENDOR, CW-EPR and ESEEM measurements

Pulsed electron nuclear double resonance (ENDOR) spectra have been obtained for the exchangeable Mo-OH proton of the low-pH form of native chicken liver sulfite oxidase (SO) and recombinant human SO for the first time. The spectra of the two enzymes are very similar, indicating a similar binding geometry of the hydroxyl ligand to the Mo center. The isotropic hyperfine interaction (hfi) constant for the proton of the OH ligand in both enzymes is about 26 MHz. The anisotropic components of the hfi obtained from the pulsed ENDOR spectra are about 1.6–1.8 times larger than those obtained by continuous-wave electron paramagnetic resonance and electron spin echo envelope modulation. These hfi differences are explained by a rotational disorder of the Mo-OH group. A similar rotational disorder of the coordinated exchangeable ligand has been found previously for the high-pH and phosphate-inhibited forms of SO.     

The electron spin resonance spectra of the toluene,p-xylene and m-xylene anions

The electron spin resonance (e.s.r.) spectra of the anions of toluene, p-xylene and m-xylene have been studied and the proton hyperfine coupling constants determined. These are in excellent agreement with the predictions of Hückel molecular orbital theory.     

Electron resonance spectra of some radicals derived from acetylenes and nitriles

The electron resonance spectra of a series of radicals derived by hydrogen abstraction from substrates containing carbon-carbon or carbon-nitrogen triple bonds have been studied in fluid solutions. The radicals ·CH₂CN, ·CH₂C ≡ CH and ·CH₂C ≡ C·CH₃ which have a linear heavy-atom framework show interesting temperature dependent line-width variations. These are attributed to spin-rotational interactions arising from anisotropic molecular reorientation. It is suggested that the motion about the long axis of these radicals is inertial in character.

The equilibrium geometries and hyperfine coupling constants of the propargyl and cyanomethyl radicals have been estimated from INDO-LCAO-SCF calculations.     

吩噻嗪-1,4萘醌-乙二醇体系光化学过程的时间分辨ESR研究

用时间分辨ESR方法研究了吩噻嗪(PTH)-1,4-萘醌(NQ)-乙二醇(RH)体系中的光化学过程.只观察到萘醌的中性半醌自由基NQH^*的全发射极化的CIDEP(化学诱导动态极化)信号,并未观察到极化的萘醌负离子NQ·^-*的CIDEP信号,表明RH与三重态NQ间的质子转移反应远比PTH与NQ间的电子转移反应为快.NQH·^*的CIDEP信号随pH值的变化表明体系中同时有极化自由基NQH·^*与NQH·₂^+*存在,并且其间有质子交换.     

An electron spin resonance and polarographic study of the sulphone group

The electron spin resonance spectra of the radical anions of diphenyl sulphone, dibenzothiophene S, S-dioxide and thianthrene S, S, S′, S′-tetroxide have been measured and the proton hyperfine splitting constants assigned to the various protons by measurement of the spectra of deuterio and methyl derivatives.

The experimentally determined spin populations were correlated by molecular orbital calculations, using the formalism of Koch and Moffit to describe the sulphone group, and it is shown that irrespective of the mutual orientation of this group and the conjugated system it may be described as far as the electron spin resonance spectra are concerned as contributing a vacant symmetric orbital to the conjugated system whose coulomb integral (α_SO2 ) and sulphur carbon resonance integral (β_C-SO2 ) are given by α_SO2 =α_C – 2·0β, β_C-SO2 =0·8β where α and β are the carbon coulomb integral and carbon-carbon resonance integrals respectively. The polarographic half wave reduction potentials of these compounds and some of their methyl derivatives dissolved in anhydrous dimethylformamide were also determined and the effect of methyl substitution on this quanttiy is also well described by the above parameters for the sulphone group. Finally a difference between the results obtained here and those previously obtained by Vincow from the electron spin resonance spectrum of thioxanthone S, S-dioxide radical anion is noted and discussed.     

Electron spin-lattice relaxation in radicals containing two methyl groups,generated by gamma-irradiation of polycrystalline solids

The effects of methyl rotation on electron spin-lattice relaxation times were examined by pulsed electron paramagnetic resonance for the major radicals in gamma-irradiated polycrystalline alpha-amino isobutyric acid, dimethyl-malonic acid, and L-valine. The dominant radical is the same in irradiated dimethyl-malonic acid and alpha-amino isobutyric acid. Continuous wave saturation recovery was measured between 10 and 295 K at S-band and X-band. Inversion recovery, echo-detected saturation recovery, and pulsed electron-electron double resonance (ELDOR) data were obtained between 77 and 295 K. For the radicals in the three solids, recovery time constants measured by the various techniques were not the same, because spectral diffusion processes contribute differently for each measurement. Hyperfine splitting due to the protons of two methyl groups is resolved in the EPR spectra for each of the samples. Pulsed ELDOR data were obtained to characterize the spectral diffusion processes that transfer magnetization between hyperfine lines. Time constants were obtained for electron spin-lattice relaxation (T(1e)), nuclear spin relaxation (T(1n)), cross-relaxation (T(x1)), and spin diffusion (T(s)). Between 77 and 295 K rapid cross-relaxation (deltaM(s) = +/- 1, deltaM(I) = -/+ 1) was observed for each sample, which is attributed to methyl rotation at a rate that is approximately equal to the electron Larmor frequency. The large temperature range over which cross-relaxation was observed suggests that methyl groups in the radical and in the lattice, with different activation energies for rotation, contribute to the rapid cross-relaxation. Activation energies for methyl and amino group rotation between 160 and 1900 K (1.3-16 kJ/mol) were obtained by analysis of the temperature dependence of 1/T(1e) at S-band and X-band in the temperature intervals where the dynamic process dominates T(1e).     

EPR study of gamma-irradiated 2-Bromo-4′-methoxyacetophenone single crystals

The gamma-irradiated single crystals of 2-Bromo-4′-methoxyaceto-phenone (2B4MA) were investigated using electron paramagnetic resonance (EPR) technique. Density-functional theory calculations were employed to investigate and identify the radicals that have been assumed to be formed upon irradiation of 2B4MA single crystals. The EPR spectra of 2B4MA were recorded at different orientations in the magnetic field at room temperature. Taking into account the chemical structure and experimental spectra of irradiated single crystal of 2B4MA, it was assumed that at least two different radicals were produced in the sample. Following this assumption, six possible radicals were modeled and EPR parameters were calculated by using the DFT, B3LYP/6-311+G(d), for the modeled radicals individually. The calculated hyperfine coupling constants and g-tensors were used as initial values for simulation studies. The three crystallographic axes on the simulated spectra were well matched with experimental spectra for the two modeled radicals. Thus, we identified the R1 type radical and R4 type radical as paramagnetic species produced in gamma-irradiated 2B4MA.     

Pulsed EPR measurements on reaction rate constants for addition of photo-generated radicals to double bonds of diethyl fumarate and diethyl maleate

Addition reaction of photo-generated radicals to double bonds of diethyl fumarate (deF) and diethyl maleate (deM), which are geometrical isomers, was studied by means of time-resolved- (TR-) and pulsed-electron paramagnetic resonance (EPR). Analysis of TR-EPR spectra indicates that adduct radicals from deF and deM should have the same structure. The double bonds of these monomers are converted to single ones by addition reaction, which allows hindered internal rotation to give the same structure of adduct radical. The rate constants for addition reaction of photo-generated radicals were determined by Stern–Volmer analysis of the decay time of electron spin-echo intensity of these radicals measured by the pulsed EPR method. Rate constants for deF were found to be larger than those for deM. This relation is in good consistent with efficiency of polymerisation of deF and deM. Experimentally determined rate constants were evaluated by introducing the addition reaction model on the basis of two important factors enthalpy and polar effects.     

Matrix ENDOR of ion-exchange systems

Electron nuclear double resonance (ENDOR) has been investigated in sulfocation exchangers, containing free radicals stabilized in polymeric matrix or Cu²⁺ and (VO)²⁺ as counterions. It was shown that the ENDOR signal is mainly due to electron-nuclear dipole-dipole interactions between the unpaired electron and nuclei of polymeric matrix or hydrogen atoms of water molecules which hydrate the charge groups. In order to quantitatively describe the ENDOR line shape and intensity, the theory of matrix ENDOR is developed. The correctness of this theory was tested by comparing the temperature dependence of spin-lattice relaxation times calculated from ENDOR line intensities with the corresponding dependence obtained from stationary saturation electron spin resonance spectra. A good agreement was observed in the temperature range from 200 to 350 K. The structural parameters of surroundings of paramagnetic ions Cu²⁺ and (VO)²⁺, which include four coordinated spheres on the distance from 0.3 to 1.2 nm, were calculated. The motional parameters, correlation time and activation energy of mobile protons were also determined. It is concluded that the activation processes of water self-diffusion and proton exchange take place at high temperature, whereas the proton tunneling transfer is possible at low temperature.     

时间分辨电子自旋共振及其应用

简要介绍了作者研制的高时间分产电子自旋共振（ＴＲＥＳＲ）波谱仪的结构与特点。报道了均相与微复相体系中光解自由基的自旋相关自由基极化;在对苯醌的乙二醇／水／硫酸溶液中,观察到苯醌自由基与溶液中质子发生交换效应。综述了ＴＲＥＳＲ在光诱导极化与光诱导电子转移研究方面的应用。     

用最小电离的质子刻度铅–闪烁光纤夹层电磁量能器

研究利用最小电离的质子和已知能量的电子来刻度铅–闪烁光纤夹层电磁量能器.对最小电离粒子输出响应进行光纤衰减修正后,利用最小电离质子的输出响应对量能器测量单元进行标定,然后对不同入射能量的电子事例进行能量重建,重建能量与入射能量有很好的线性关系.本文还对最小电离的质子与μ子对量能器测量单元的输出响应进行了比较,发现二者在误差范围内是一致的.     

核苷酸和芳香酮的电子转移和质子转移反应研究

利用时间分辨的激光闪光光解方法在1∶1乙腈/水溶液中得到了4种核苷酸和芳香酮的瞬态吸收光谱,通过瞬态吸收光谱的变化研究了鸟苷酸、腺苷酸、胞苷酸、胸腺苷酸猝灭二苯甲酮、呫吨酮激发三重态的反应。由于实验中生成了抽氢自由基和负离子自由基,以及核苷酸正离子在水中的快速抽氢反应,推断出芳香酮和鸟苷酸、腺苷酸的反应机理是先发生电子转移后发生质子转移。而在芳香酮和胞苷酸、胸腺苷酸的反应中没有观察到相应的抽氢自由基和负离子自由基的瞬态吸收峰,由此推断出它们和胞苷酸、胸腺苷酸没有发生电子转移和质子转移反应。对瞬态吸收峰处的时间衰减曲线进行拟合得到了核苷酸猝灭芳香酮的速率常数,可以看到随着反应自由能变ΔG的增大,反应速率常数逐渐减小。     

ESR spectroscopic investigation of gamma-irradiated lithium trifluoromethanesulfonate

ABSTRACT

The effect of gamma radiation on lithium trifluoromethanesulfonate (LiTf) was investigated in detail using electron spin resonance (ESR) spectroscopy. The experimental spectra consisted of complex overlapped signals. Simulations of both the 1st- and 10th-day ESR spectra gave valuable information about the spectroscopic and structural features of radiation-induced radicals. More information about the qualitative and quantitative features of the radicals were gathered from the microwave power, storage time and dosimetric feature findings. In the light of the information obtained, four radicals were accepted to be produced after irradiation of LiTf.     

1H and 2D N.M.R. studies of the radical anions of biphenyl,fluorenone and phenanthrene

The proton N.M.R. spectra of biphenyl^-, fluorenone^- and phenanthrene^- and the deuterium N.M.R. spectra of biphenyl-d10^- and phenanthrene-d10^- have been measured in ethereal solutions at room temperature. Sign and magnitude of the hyperfine splitting constants derived from the measured contact shifts are reported and compared with E.S.R. data and predictions from theory. From the measured proton and deuterium relaxation times values for the electron spin and the rotational correlation times have been obtained.