Frequency dependence of EPR signal intensity, 248 MHz to 1.4 GHz

The electron paramagnetic resonance pulsed free induction decay (FID) of a degassed solution of a triaryl methyl radical, methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2-d:4,5-d']bis(1,3)dithiol-4-yl) tripotassium salt, 0.2 mM in H2O, was measured at VHF (247.5 MHz) and L-band (1.40 GHz). The calculated and observed FID signal amplitudes (in millivolts) agreed within 1 and 6%, and the ratio of the normalized FID signals at the two frequencies agreed within 5%. The FID decay time constant was 2.7 micros at both frequencies.     

In vivo oximetry by a pulsed longitudinally detected ESR spectrometer

By using a narrow single electron spin resonance (ESR) line agent, triarylmethyl, tris(8-carboxy-2,2,6,6-tetrahydroxyethylbenzo[1,2-d:4,5-d′] bis(1,3)dithiole-4-yl)methyl sodium salt (TAM OX063), pulsed longitudinally detected ESR (LODESR) measurements of a phantom or the chest of a living mouse at the operating frequency of ca. 300 MHz were taken and the effective longitudinal relaxation time (T ₁^*) was estimated for oximetry. Under irradiation of a pair of π-pulses with a variable interval between pulses (τ), in-phase LODESR signal intensities were obtained from the phantoms containing TAM dissolved in a physiological saline solution at a concentration of 1 mM and various concentrations of oxygen. TheT ₁^* of the phantom was calculated from the plotted curve of the LODESR signal intensity against τ. It was found that the reciprocal ofT ₁^*, i.e., the longitudinal relaxation rate, increased with the concentration of oxygen. In vivo pulsed LODESR measurements of the chest of living mice that had received a TAM injection via the intraperitoneal route were made. While the LODESR measurements were being made, the mice in one group breathed normal air and those in another group breathed 100% oxygen. It was found that the longitudinal relaxation rate of the mice breathing 100% oxygen was significantly greater than that of mice breathing normal air, indicating that breathing 100% oxygen elevates the thoracic longitudinal relaxation rate.     

Quinoxaline derivatives as corrosion inhibitors for mild steel in hydrochloric acid medium: Electrochemical and quantum chemical studies

The corrosion inhibition potential of four quinoxaline derivatives namely, 1-[3-(4-methylphenyl)-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl]butan-1-one (Me-4-PQPB), 1-(3-(4-methoxyphenyl)-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl)butan-1-one (Mt-4-PQPB), 1-[3-(3-methoxyphenyl)-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl]butan-1-one (Mt-3-PQPB) and 1-[3-(2H-1,3-benzodioxol-5-yl)-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl]butan-1-one (Oxo-1,3-PQPB) was studied for mild steel corrosion in 1 M HCl solution using electrochemical, spectroscopic techniques and quantum chemical calculations. The results of both potentiodynamic polarization and electrochemical impedance spectroscopic studies revealed that the compounds are mixed-type inhibitors and the order of corrosion inhibition efficiency at 100 ppm is Me-4-PQPB>Mt-3-PQPB>Oxo-1,3-PQPB>Mt-4-PQPB. Fourier transform infrared (FTIR) and ultraviolet–visible (UV–vis) spectroscopic analyses confirmed the presence of chemical interactions between the inhibitors and mild steel surface. The adsorption of the inhibitor molecules on mild steel surface was found to be both physisorption and chemisorption but predominantly chemisorption. The experimental data obey Langmuir adsorption isotherm. Scanning electron microscopy studies revealed the formation of protective films of the inhibitors on mild steel surface. Quantum chemical parameters obtained from density functional theory (DFT) calculations support experimental results.     

Synthesis and Characterization of Molecules Containing Thiazole and Oxazole Moieties and Study of ESIPT Phenomenon

Two novel ESIPT molecules, 2-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxazol-2-yl]phenol 9a and 4-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxazol-2-yl]benzene-1,3-diol 9b were synthesized by condensing 1-amino-3-(1,3-benzothiazol-2-yl)naphthalen-2-ol with 2-hydroxybenzoic acid and 2,4-dihydroxybenzoic acid respectively. The novel compounds were characterized by FT-IR, (1)H NMR, Mass spectral and elemental analysis. Effect of polarity on photo physical properties, absorption and emission were studied. Compounds showed single absorption and dual emission due to ESIPT phenomenon. The structural changes due to ESIPT phenomenon in terms of bond angle, bond distances and geometry were investigated by using Gaussian 03 software. These two novel ESIPT molecules are thermally stable up to 200?°C.     

The exploration of magnetic photo-excited states using organometallic spin couplers based on the silole ring

The photoexcited states of the 1,1-dimethyl-2,3,4,5-tetraphenylsilole (TPS) and of the bis(tertiobutylnitroxide)-substituted TPS (TPS-TNO) have been studied using optical and time-resolved electron spin resonance (TRESR) spectroscopic methods. The TPS-TNO was designed and synthesized with in mind to achieve the photo-induced spin alignment which has been observed with anthracene-based radicals [3] and [4]. Although a photoexcited triplet state has been characterized for the TPS coupler, TPS-TNO did not show any signal arising from photoexcited states. This result may come from a very short lifetime of the photoexcited high spin states as a result of the attachment of radicals to TPS or from the very low efficiency of the intersystem crossing.     

Development and testing of a CW-EPR apparatus for imaging of short-lifetime nitroxyl radicals in mouse head

This article describes a method for reducing the acquisition time in three-dimensional (3D) continuous-wave electron paramagnetic resonance (CW-EPR) imaging. To visualize nitroxyl spin probes, which have a short lifetime in living organisms, the acquisition time for a data set of spectral projections should be shorter than the lifetime of the spin probes. To decrease the total time required for data acquisition, the duration of magnetic field scanning was reduced to 0.5 s. Moreover, the number of projections was decreased by using the concept of a uniform distribution. To demonstrate this faster data acquisition, two kinds of nitroxyl radicals with different decay rates were measured in mice. 3D EPR imaging of 4-hydroxy-2,2,6,6-tetramethylpiperidine-d₁₇-1-¹⁵N-1-oxyl in mouse head was successfully carried out. 3D EPR imaging of nitroxyl spin probes with a half-life of a few minutes was achieved for the first time in live animals.     

Solid state NMR strategies for the structural characterization of new hybrid materials based on the intercalation of nitroxide radicals into CdPS3

The radical cations 2-(3-N-butylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide (m-BuPYNN) and 4-(ethylammonium)-2,2,6,6-tetramethylpiperidin-1-oxide (EATEP) are successfully intercalated into the layered host structure of CdPS(3) via ion exchange. The reaction proceeds either directly from ethanolic solutions of the radical iodide salt or via a two-stage process involving first the formation of an intermediate tetramethylammonium intercalate. The resulting materials, which are described by the compositional formula Cd(1-x)PS(3){Rad}(2x), are characterized by chemical analysis, X-ray powder diffraction, bulk susceptibility measurements and EPR spectroscopy. Modern single and double resonance solid state NMR techniques are introduced successfully to study the structural modifications of the host lattice and the details of the intermolecular guest/host interactions. (1)H MAS-NMR spectra reveal substantial differences in the unpaired electron spin density distributions within the radical ions intercalated into the host lattice compared to those obtained for the pure radical ion salts, leading to different bulk magnetic properties. The results of (1)H/(31)P double resonance experiments indicate that the orientation of the guest molecules is dominated by Columbic interactions between the radical cations and the negatively charged cadmium vacancies in the host lattice.     

Evaluation of spin labels for in-cell EPR by analysis of nitroxide reduction in cell extract of Xenopus laevis oocytes

Spin-label electron paramagnetic resonance (SL-EPR) spectroscopy has become a powerful and useful tool for studying structure and dynamics of biomacromolecules. However, utilizing these methods at physiological temperatures for in-cell studies is hampered by reduction of the nitroxide spin labels and thus short half-lives in the cellular environment. Consequently, reduction kinetics of two structurally different nitroxides was investigated in cell extracts of Xenopus laevis oocytes using rapid-scan cw-experiments at X-band. The five member heterocyclic ring nitroxide PCA (3-carboxy-2,2,5,5-tetramethylpyrrolidinyl-1-oxy) under investigation features much higher stability against intracellular reduction than the six member ring analog TOAC (2,2,6,6-tetramethylpiperidine-N-oxyl-4-amino-4-carboxilic acid) and is therefore a suitable spin label type for in-cell EPR. The kinetic data can be described according to the Michaelis–Menten model and thus suggest an enzymatic or enzyme-mediated reduction process.     

E.S.R. of sulphur-substituted purines and nucleosides; carbon-centred radicals in mercaptopurine crystals at 77 K

Single crystals of the sulphur-containing DNA-base analogue 6-methylmercaptopurine (6MeMP) and its riboside 6-methylmercaptopurine riboside (6MeMPR) have been prepared and irradiated by 4·0 MeV electrons at 77 K. Electron spin resonance techniques have been used to study the radiation-induced radicals at 77 K. The primary carbon-centred radical, common to both molecules, has been identified as a species formed by hydrogen atom abstraction from the methyl group. The principal values of the two α-proton hyperfine coupling tensors and the g-tensors were almost the same for both molecules and for 6MeMPR were: α1, -28·8, -17·9 and -6·4 G; α2, -28·2, -15·7 and -9·8 G; and g, 2·0063, 2·0024 and 2·0018. These data indicate a spin density of 0·77 on the methyl carbon atom. Molecular orbitals determined from CNDO/2 methods were used in calculations of the directions and magnitude of the g-tensor principal values. Comparison of these calculated values and experimental data suggests that contribution of spin density in d-orbitals on the sulphur atom is important in describing the g-tensor. Methyl H-abstraction radicals trapped in pairs were also detected in 6MeMP and the data are consistent with an effective interspin distance of 4·67 Å.     

Asymmetric line broadening in the electron resonance spectra of biradicals

At high temperatures the dominant relaxation process which determines the linewidths in the electron resonance spectra of flexible biradicals is modulation of the scalar electron-electron exchange interaction. In systems of high viscosity, the modulation of the exchange interaction is often quenched, and the rotational modulation of the anisotropic magnetic interactions now constitutes the principal relaxation mechanism. In this paper we derive a theoretical expression for the broadening which results from this relaxation process. The applications of the theory to the determination of molecular configurations, electron-electron separations and the sign of the exchange interaction are illustrated by comparison with the electron resonance spectrum of bis(2,2,6,6-tetramethyl-piperidinol-1-oxyl)carbonate. The theory is also of value in understanding the spectra of partially immobilized biradical spin labels.     

The Temperature Dependence of Nitroxide Spin–Label Interaction Parameters: a High-Field EPR Study of Intramolecular Motional Contributions

High-field W-band electron paramagnetic resonance (EPR) spectroscopy was utilized to study the temperature dependence of the magnetic interaction parameters (g-, hyperfine-, quadrupole tensors) of two types of doublet-state nitroxide spin probes in glass-forming ortho-terphenyl solution: a five-membered ring system of pyrroline type (model for the commonly used methane thiosulfonate spin label) and a six-membered ring system of piperidine type (model for the commonly used TOAC spin label). The analysis of the g- and hyperfine tensors in terms of their isotropic and anisotropic parts reveals at least two mechanisms of motion that are responsible for the temperature dependence of the interaction parameters. The first mechanism is attributed to the overall small-angle motion of the nitroxide molecule in the glassy matrix; it leads to an averaging of the anisotropies of the EPR parameters. The second mechanism originates in an intramolecular out-of-plane motion of oxygen in the nitroxide group. This type of motion is evidenced by comparing the experimental findings for the spin-interaction parameters with the results of density functional theory calculations. The harmonic oxygen out-of-plane vibrations result in a variation of both the isotropic and anisotropic parts of the g- and hyperfine tensors. In contrast, the quadrupole tensor is not influenced by this vibration mechanism in the temperature range under study (90–240 K). Consequences of the applicability of such typical nitroxide radicals for probing details of their protein environment and for studying librational dynamics in frozen solutions are discussed.     

Inherent Microporosity and Photostability of Fluoroacrylic Polymer Films Studied by Electron Paramagnetic Resonance of Nitroxide Spin Probes

Efficient and straightforward methods for characterization of polymers with inherent microporosity are demanded in their targeted design for particular applications. Among critical parameters to be obtained are the size of the pores and polymer stability against photoirradiation. Herewith, we demonstrate the efficiency of electron paramagnetic resonance (EPR) spectroscopy applied to this task. We use stable nitroxide radicals (2,2,6,6-tetramethylpyperidine-1-oxyl) (TEMPO) as reporter spin probes for EPR and investigate a series of perspective polymers with inherent microporosity developed for pressure sensitive paints (PSP), namely, poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate) (FIB), its two modifications poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate-co-1-(4-(4-chloro-2,3,5,6-tetrafluorophenyl)piperazin-1-yl)prop-2-en-1-one) (NS4) and poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate-co-1-(4-(4-tert-butylphenylsulfonyl)piperazin-1-yl)prop-2-en-1-one) (NS5), as well as poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1,1,2,2-perfluorooctylmethacrylate) (PFOMA). Nitroxides were incorporated into the pores of the polymers post-synthetically via a gas-phase sorption, and the mobility of nitroxides tracked by EPR yielded information on the pore sizes and polymer degradation under ultraviolet light. The conclusions obtained by EPR have been supported by a variety of other techniques, thus demonstrating EPR to be a very convenient tool for express analysis of porous polymers.     

Spin Localization in Poly(3-Dodecylthiophen)/PCBM Composite

Charge carriers photoinduced in poly(3-dodecylthiophene)/([6,6]-phenyl-C₆₁-butanoic acid methyl ester) (P3DDT/PCBM) by photons with the energy of 1.88–2.75 eV were investigated by X-band light-induced electron paramagnetic resonance (LEPR). LEPR spectra were attributed to non-interacting polarons and methanofullerene anion radicals with different magnetic and relaxation parameters. A part of these charge carriers are trapped in a polymer matrix. Paramagnetic susceptibility and spin–spin relaxation of mobile charge carriers were shown to follow the activation law.     

Electron spin relaxation of radicals in γ-irradiated malonic acid and methyl malonic acid

Radicals generated by γ-irradiation of malonic acid and methyl malonic acid were studied as a function of temperature by inversion recovery, echo-detected saturation recovery and electron-electron double resonance (ELDOR) at X-band, and by continuous-wave saturation recovery at X-band and S-band. ELDOR reductions for malonic acid radical in polycrystalline and single-crystal samples indicate that nuclear spin relaxation is faster than both electron spin relaxation and cross relaxation between 93 and 233 K. Deuteration of the carboxylate protons caused the maximum ELDOR reduction to shift from about 110 to 150 K, consistent with the assignment of the rapid nuclear spin relaxation to hydrogen-bonded proton dynamics. ELDOR enhancements for both radicals formed in methyl malonic acid indicate that cross relaxation is faster than both electron spin relaxation and nuclear spin relaxation between 77 and 220 K. Enhanced cross relaxation and electron spin relaxation are attributed to the rotation of methyl groups at a rate comparable to the electron Larmor frequency. The temperature dependence of the enhancement of 1/T _1e was analyzed to determine the activation energies for methyl rotation. The same radical is formed in irradiated methyl malonic acid and L-alanine, but the barrier to rotation of the α-methyl is 500 K in the methyl malonic acid host and 1500 K in the L-alanine host, which indicates a large impact of the lattice on the barrier to rotation.     

Evidence for free radicals produced in aqueous solutions by diagnostic ultrasound

Recent theoretical calculations have shown that small gas nuclei in water exposed to microsecond ultrasonic pulses above an intensity threshold may grow into transient cavities that collapse violently, leading to the formation of .OH radicals and .H atoms. We have detected these free radicals in aqueous solutions exposed to microsecond pulsed ultrasound using spin trapping and electron spin resonance (ESR). The public health implications of our results are discussed.     

Bipolar Alq3-based complexes: Effect of hole-transporting substituent on the properties of Alq3-center

Two bipolar Alq₃-based complexes, tris{5-[(carbazole-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(CzHQ)₃) and tris{5-[(phenothiazine-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(PHQ)₃), involving an Alq₃-center and three hole-transporting substituents (carbazole or phenothiazine), were prepared and characterized. Effects of hole-transporting substituent on the properties of Alq₃-center were investigated in detail. It is found that the two complexes have improved hole-transporting performance and appropriate thermal stability (the 5%-weight-loss temperatures T_5%>260 ^oC). Photoluminescence (PL) spectra indicate that both energy transfer and electron transfer can take place simultaneously in the PL process of these complexes. Both thermodynamics and dynamics of the electron transfer were studied and corresponding parameters were calculated. Energy transfer is favorable for the PL of Alq₃-center, while electron transfer is unfavorable for the PL of Alq₃-center. These results will be useful to explore novel OLEDs material with increased efficiency.     

Synthesis of spin-labelled poly(acrylic acid)s and their segmental motion study

Understanding the segmental dynamics of polymer chains is cardinal to decipher the microscopic behaviour in order to modulate the bulk properties of polymers. The study of electron spin resonance (ESR) spectroscopy of spin-labelled polymers is useful to understand the segmental dynamics of polymer chains in solution. In this paper, poly(acrylic acid)s (PAAs) were spin labelled with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl radicals. Spin-labelled PAAs (SL-PAAs) were characterised by Fourier-transform infrared spectroscopy, ultraviolet–visible spectroscopy, thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry (CV), and ESR analyses. The polyelectrolyte complexes of SL-PAAs were prepared by employing poly(diallyldimethylammonium chloride) (PDADMAC) as the polycation and analysed by transmission electron microscopy, dynamic light scattering (DLS), and ESR spectroscopies. The effect of molar mass on the segmental dynamics of SL-PAAs in pristine as well as in the form of polyelectrolyte complexes (PECs) was studied. The results indicated that SL-PAAs show a differential complexation behaviour with PDADMAC in the PECs depending on their molar mass.     

Semiempirical molecular orbital calculations of anisotropic 1H, 13C and 19F hyperfine coupling constants in hydrocarbon and fluorocarbon radicals

The anisotropic hyperfine coupling constants (AHCC) from the electron spin resonance (E.S.R.) spectra of a variety of atoms in organic radicals have been calculated by means of semiempirical molecular orbital wavefunctions in the INDO approximation. Hyperfine tensors involving ¹H, ¹³C and ¹⁹F nuclei are obtained for the ?H, ?H₃, CH₃?H₂, (CH₃)₃? hydrocarbon radicals, malonic acid radical, ?H₂F, ?F₂H, ?F₃ and CF₃?H₂ radicals. The calculated values are compared with available experimental, non-empirical and semiempirical values for these radicals. All integrals of the operator entering the electronic contributions have been evaluated over Slater type orbitals. The introduction of deorthogonalized wavefunctions gives generally better calculated results. In particular, the tensor components of the ¹⁹F AHCC are in good agreement with the experimental results without the necessity of readjusting the effective nuclear charges.     

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).     

Synthesis of and structure–property relationships in zinc complexes of bis‐metaphenylene semiquinone biradical species

The design of novel, functionalized bis‐metaphenylene semiquinone (SQ) ligands and their corresponding metal complexes which combine conformational flexibility and electron‐withdrawing, electron‐donating, and conjugating substituents enable investigation of multiple structure–property relationships. Along these lines, we report the synthesis of three new bis(Zn^II(SQ)Tp^Cum,Me) complexes containing the bis‐metaphenylene coupling fragment. Using electron paramagnetic resonance spectroscopy, ab initio computations and superconducting quantum interference device magnetometry, we show how spin‐density is affected by the bis‐metaphenylene system substituents. Copyright © 2011 John Wiley & Sons, Ltd.