Electron Spin Polarization and Time-Resolved Electron Paramagnetic Resonance: Applications to the Paradigms of Molecular and Supramolecular Photochemistry

Most molecular and supramolecular organic photochemical reactions involve paramagnetic reactive intermediates (such as molecular triplet states, triplet radical pairs, and free radicals). In a number of cases these species are created with "anomalous" spin populations which are far from thermal equilibrium. Such paramagnetic species are said to be "spin polarized" and may be observed directly by time-resolved electron paramagnetic resonance (TREPR). The TREPR technique can be applied to exploit spin polarization, which, in addition to providing an enormous signal to noise enhancement, also reveals the mechanisms involved in photochemical reactions. TREPR spectroscopy provides a means of tracking the reaction of radicals with molecules and the nonreactive interactions of radicals with other radicals in real time. The latter interactions provide a systematic investigation of supramolecular interactions of geminate radicals in micelles.     

Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.     

Cu(II) Sorption Mechanism on Montmorillonite: An Electron Paramagnetic Resonance Study

The mass of gamma-globulin fouling an Anodisc alumina membrane with a nominal pore diameter of 0.1 μm has been measured at several concentrations and pHs. This fouling resulted from filtering through the membrane in a continuous recirculation device. The low-concentration fouling can be attributed mainly to adsorption. The complete concentration dependence of fouling mass has been obtained and fitted to a Freundlich heterogeneous isotherm, from which the pH dependence of active fouling sites and energies has been also obtained. Adsorption is studied as a function of the electrostatic forces between the solute and the membrane. A sharp maximum in the adsorbed mass for zero electrostatic force is observed. At high concentrations, accumulation plays a relevant role at alkaline pH, as confirmed by flux decay experiments, retention measurements, and AFM (atomic force microscopy) pictures. Copyright 2000 Academic Press.     

Determination of the Nanostructure of Polymer Materials by Electron Paramagnetic Resonance Spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is one the few methods that can characterize structural features in the range between 0.5 and 5 nm in systems that lack long‐range order. Approaches based on EPR spectroscopy provide good structural contrast even in complex materials, as the sites of interest can be selectively labeled or addressed by suitably functionalized spin probes using well established techniques. This article assesses the EPR experiments available for distance measurements on nanoscales in terms of the accessible distance range, precision, and sensitivity. Recommendations are derived for the proper choice of experiment for a given problem. Both simple and sophisticated methods for data analysis are described and their limitations are evaluated. It is discussed which assumptions must be made to extract a pair correlation function from EPR data. Finally, applications to the study of polymer chain conformation and the structure of ionically functionalized diblock copolymers are highlighted.     

Electron Paramagnetic Resonance Spectroscopy of the Iron-Molybdenum Cofactor of Clostridium pasteurianum Nitrogenase

We report a computer simulation study of the electron paramagnetic resonance (EPR) spectral line shape of the iron-molybdenum cofactor of nitrogenase. The unusually broad and asymmetric line shape of the EPR spectrum can be interpreted in terms of a distribution of zero-field splitting parameters called D-strain. The best fit simulations were computed using D = 2.5 cm(-1) and E = 0.317 cm(-1) and distributions in D and E approximated by Gaussians of half-widths 0.446 cm(-1) and 0.108 cm(-1), respectively. The value of D estimated in the present work is smaller than previous estimates by others but consistent with the temperature dependence of the EPR spectrum. The large D-strain is most likely caused by an ensemble of nearly isoenergetic conformational states and should not be considered as being indicative of chemical inhomogeneity.     

Ordering of PCDTBT Revealed by Time‐Resolved Electron Paramagnetic Resonance Spectroscopy of Its Triplet Excitons

Time‐resolved electron paramagnetic resonance (TREPR) spectroscopy is shown to be a powerful tool to characterize triplet excitons of conjugated polymers. The resulting spectra are highly sensitive to the orientation of the molecule. In thin films cast on PET film, the molecules’ orientation with respect to the surface plane can be determined, providing access to sample morphology on a microscopic scale. Surprisingly, the conjugated polymer investigated here, a promising material for organic photovoltaics, exhibits ordering even in bulk samples. Orientation effects may significantly influence the efficiency of solar cells, thus rendering proper control of sample morphology highly important.     

E—112EPR系统中的AFC失控及故障排除

E-112型EPR(Electronic Paramagnetic Resonance)波谱仪是Varian公司生产的。运行及操作中出现几次故障，其中以自动频率控制(AFC)失控较为典型。     

EPR法研究自由基在移植免疫中的作用

用电子顺磁共振（ＥＰＲ）法，化学发光法及荧光光谱法研究了１００例正常人、４２例角膜移值患者泪液中的自由基、超氧化物岐化酶（ＳＯＤ）和水溶性脂质过氧化物（ＷＳＬＰ）等指标．结果表明正常人泪液中自由基的自旋浓度是（３．６±０．５８）×１０１２ｓｐｉｎｓ／ｍＬ，ＳＯＤ浓度为（３．８４±１．４５）μｇ／Ｌ、水溶性脂质过氧化物相对荧光强度为９．５９８～１６．２２５Ｕ／ｍＬ．角膜移植后三项指标变化出现两个规律的峰，这种规律性变化在三项指标间存在显著的相关性．其中５例患者的特殊变化证明移植排斥反应与术后这些变化的进一步持续发展有关     

An introduction to Inelastic Electron Tunneling Spectroscopy (IETS) and its chemical applications

Inelastic Electron Tunnelling Spectroscopy (IETS) provides information concerning the vibrational and excitation modes present in molecular species adsorbed onto an electrically insulating substrate. Our objective is not to cover the entire literature to date. Rather we hope to stimulate an interest in IETS among chemists by outlining some of the systems of chemical interest already studied; and to provide sufficient background information to guide the construction and operation of an inelastic electron tunnelling spectrometer and interpretation of the data.     

Heterometallic Co(III)(4)Fe(III)(2) Schiff Base Complex: Structure, Electron Paramagnetic Resonance, and Alkane Oxidation Catalytic Activity

The heterometallic complex [Co(4)Fe(2)OSae(8)]·4DMF·H(2)O (1) was synthesized by one-pot reaction of cobalt powder with iron chloride in a dimethylformamide solution of salicylidene-2-ethanolamine (H(2)Sae) and characterized by single crystal X-ray diffraction analysis, magnetic measurements, high frequency electron paramagnetic resonance (HF-EPR), and M?ssbauer spectroscopies. The exchange coupling in the Fe(III)-Fe(III) pair is of antiferromagnetic behavior with J/hc = -190 cm(-1). The HF-EPR spectra reveal an unusual pattern with a hardly detectable triplet signal of the Fe(III) dimer. The magnitude of D (ca. 13.9 cm(-1)) was found to be much larger than in related dimers. The catalytic investigations disclosed an outstanding activity of 1 toward oxidation of cycloalkanes with hydrogen peroxide, under mild conditions. The most efficient system showed a turnover number (TON) of 3.57 × 10(3) with the concomitant overall yield of 26% for cyclohexane, and 2.28 × 10(3)/46%, respectively, for cyclooctane. A remarkable turnover frequency (TOF) of 1.12 × 10(4) h(-1) (the highest initial rate W(0) = 3.5 × 10(-4) M s(-1)) was achieved in oxidation of cyclohexane. Kinetic experiments and selectivity parameters led to the conclusion that hydroxyl radicals are active (attacking C-H bonds) species. Kinetic and electrospray ionization mass spectrometry (ESI-MS) data allowed us to assume that the trinuclear heterometallic particle [Co(2)Fe(Sae)(4)](+), originated from 1 in solution, could be responsible for efficient generation of hydroxyl radicals from hydrogen peroxide.     

Aggregation of Sodium 2,4,5-(tri-n-Alkyl)-benzene Sulfonates in Aqueous Solution: Micellization and Microenvironment Characteristics Studied by Electron Paramagnetic Resonance and Steady-State Fluorescence Quenching

The aggregation behaviors and microenvironmental characteristics of five sodium tri-n-alkylbenzene sulfonates (STABS) micelles have been investigated by electron paramagnetic resonance (EPR) and steady-state fluorescence quenching (SSFQ) techniques. The results indicated that the micropolarity of STABS micelles was less sensitive to the increase of the alkyl chain length, and showed slight decrease with the increasing solution concentration. The microviscosity of STABS micelles increased with the increase of the long alkyl chain length, decreased with the increase of the short alkyl chain length, and exhibited little increase when increase the solution concentration. The salinity of added inorganic salt exhibited obvious effect on the decrease of micropolarity and increase of microviscosity of STABS. These results suggested that these different alkyl chains showed different effects on the micellization of STABS and the salinity's effect was even bigger.     

From Planar Toward Tetrahedral Copper(Il) Complexes: Structural and Electron Paramagnetic Resonance Studies of Substituent Steric Effects in an Extended Class of Pyrrolate-Imine Ligands

A series (1a-1h) of crystalline bis-pyrrolate-imine copper(II) complexes was isolated in good yield (typically 60-90%) by reaction of the Schiff base 2-N-(R)-pyrrolecarbaldimine (where R is an alkyl group of variable steric capacity) with a copper(II) salt and base in methanol or water. For new complexes 1f-1h, pyrrolate-imine coordination is inferred by loss of ν(N-H) and a shift (ca. 40 cm^?1) to lower energy for ν(C=N) in the infrared spectra relative to free Schiff base. The X-ray crystal structures of 1f, where R = benzyl, and 1g, where R = diphenylmethyl, show trans geometry of the pyrrolate-imine moieties around Cu(II). The distortion from planarity of the CuN₄ coordination sphere, defined as the dihedral angle between the two chelating N(imine)-Cu-N(pyrrolate) planes, is 33.13(5)° for 1f and 29.3° for 1g. X-band electron paramagnetic resonance (EPR) spectra for 1a-1h in glassy solutions (ca. 100 K) are approximately axial, with an inverse correlation between A _z and g _z . At room temperature in fluid solution there is an inverse correlation between A _iso and g _iso. Plots of A _z , A _iso, g _z , or g _iso as a function of the dihedral angles between the ligand planes in 1f and 1g, as well as the previously characterized 1a (R = H) and 1d (R = tert-butyl) were used to determine the dihedral angles of the four complexes of unknown geometry. A red-shift in the ligand field bands in the electronic absorption spectra in chloroform also correlates with increasing dihedral angle. For each of these correlations, the data point for the diphenylmethyl derivative 1g deviates slightly from the line based on the other complexes; this is attributed to crystal packing forces causing a smaller dihedral angle for 1g in the crystal than in solution.     

Electron Paramagnetic Resonance and optical absorption spectra of Cr3+ in zinc maleate tetrahydrate single crystals

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cr³⁺ ions doped in single crystals of zinc maleate tetrahydrate (ZMTH) have been studied at room temperature (300 K). The EPR spectra exhibit a group of three fine structure transitions, characteristic of the Cr³⁺ ion. From the observed EPR spectra, the spin-Hamiltonian and zero-field splitting parameters have been determined. The optical absorption spectrum exhibits two broad bands characteristic of Cr³⁺ ions in octahedral symmetry. From the observed spectrum, the crystal field parameters have been evaluated.     

Resonance Raman Spectroscopy,and Its Application to Inorganic Chemistry. New Analytical Methods (27)

Resonance Raman spectra are obtained when the wave number of the exciting radiation is close to, or coincident with, that of an electronic transition of the scattering species. Such spectra are usually characterized by a very large enhancement of the intensities of particular Raman bands, sometimes with the appearance of intense overtone and combination tone progressions. The technique provides detailed information about excited electronic states because it is only the vibrational modes associated with the chromophore that are resonance-Raman active. Additionally, the high sensitivity is such that compounds at concentrations as low as 10^?6 mol/L may be detected, enabling resonance Raman spectroscopy to be used as an analytical tool and for the study of chromophores in molecules of biological interest.     

Electron Paramagnetic Resonance Study of Radical Pairs and Isolated Radicals Trapped in Irradiated Long-Chain Hydrocarbons at Low Temperatures

The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH₃CH₂.CH₂CH₃) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min^?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations.