A Multifrequency (X-, Q-, and W-band) EPR and DFT Study of a Photopolymerizable Dental Resin

The free radicals generated during the polymerization process of Z100 (3 M ESPE) dental resin were examined by electron paramagnetic resonance (EPR) in X-, Q- and W-bands. Experimental generation and spectra simulations were associated with density functional theory (DFT) calculations to determine the molecular structure and explain the EPR spectrum formation. It was assumed that the EPR spectrum was formed by the sum of two different types of radicals: “propagating” and allylic. The spectra simulations and DFT calculations showed good agreement, indicating that the proposed model fully explained the nine lines of the EPR spectrum in X-band and showed that the spectrum formation is the sum of “9 + 5” lines, rather than the “5 + 4” lines predicted early. Simulations in Q- and W-bands showed very close correlation and were essential to support the proposed model.     

Berechnung der streuspektren schwach ionisierter plasmen mit hilfe zweizeitiger wahrscheinlichkeitsdichten

ESR linewidth measurements have been performed on the quasi-two-dimensional Heisenberg magnet (C₂H₅NH₃)₂MnCl₄ at room temperature in the X-, K-, Q- and V-band. As expected from theory a logarithmic frequency dependence of the magic angle linewidth has been determined, which represents a characteristic feature of spin diffusion in a two-dimensional lattice. The spin diffusion constant is evaluated to be D/a² = (1700±250) rad GHz.     

High-field ESR in the metallic fullerides RbC60 and CsC60

We discuss field-dependent electron spin resonance (ESR) experiments on the orthorhombic phase of RbC₆₀ and CsC₆₀. X-and W-band measurements have been performed to determine electronic scattering rates in the metallic temperature regime from 50 to 350 K. The insulating low-temperature phase (T < 50 K) has been investigated by X-, Q-, and W-band ESR, i.e., at 9, 34 and 95 GHz, in order to clarify whether collective resonance modes of a possibly magnetic ground state can be observed.     

Investigation of the molecular motion of spin-labeled lysozyme in solution by the use of ESP spectra and electron spin relaxation

The theory of electron spin relaxation in protein solutions taking into account the “two-motion” model of spin label mobility is developed. The relations obtained for the longitudinal and transverse relaxation rates describe the experimental results better than isotropic (“one-motion”) model. The mechanism of longitudinal relaxation in solution of spin-labeled lysozyme is revealed and the correlation time of the inherent motion of spin label is evaluated. The linewidth analysis and study of ESR spectra under viscosity variation were used to obtain the microdynamical parameters characterizing lysozyme molecular mobility. A discrepancy between the correlation time values obtained by viscosity-variation-technique and the results known from other methods is found and ascribed to the manifestation of the intermediate time-scale mobility of protein.     

Paramagnetic proton nuclear spin relaxation theory of low-symmetry complexes for electron spin quantum number S = 52

A generalization of the modified Solomon-Bloembergen-Morgan (MSBM) equations has been derived in order to describe paramagnetic relaxation enhancement (PRE) of paramagnetic complexes characterized by both a transient (DeltaZFSt) and a static (DeltaZFSs) zero-field splitting (ZFS) interaction. The new theory includes the effects of static ZFS, hyperfine coupling, and angular dependence and is presented for the case of electron spin quantum number S = 52, for example, Mn(II) and Fe(III) complexes. The model gives the difference from MSBM theory in terms of a correction term delta which is given in closed analytical form. The theory may be important in analyzing the PRE of proton spin-lattice relaxation dispersion measurements (NMRD profiles) of low-symmetry aqua-metal complexes which are likely to be formed upon transition metal ions associated with charged molecular surfaces of biomacromolecules. The theory has been implemented with a computer program which calculates solvent water proton T1 NMRD profiles using both MSBM and the new theory.     

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.     

A field-cycling NMR investigation of resonant spin-lattice relaxation features arising from tunnelling methyl groups

(1)H nuclear spin-lattice relaxation has been investigated in sodium acetate trihydrate and sorbic acid using field-cycling NMR in the solid state. The relaxation is dominated by the reorientation of the methyl groups. Resonant features arising from coherent tunnelling are observed in both the magnetic field dependence of the spin lattice relaxation rate, T(1)(-1)(B(z)) and in the inverse temperature dependence, T(1)(-1)(1/T). The two systems have different barrier heights and tunnelling frequencies, providing different perspectives on the tunnel resonance phenomena. The magnetic field dependence enables different spectral density components to be separately investigated and in the carboxylic acid, sorbic acid, concerted proton transfer in the hydrogen bonds is also identified at low field and low temperature. The methyl hindering barriers and the correlation times characterising the reorientational dynamics has been accurately determined in both materials.     

Theoretical Investigation of Lasing without Inversion in a Multiple Quantum Well System

In this study, the influence of exciton spins relaxation (ESR) on spatial evolution of amplification without population inversion in a four-level GsAs multiple quantum wells is discussed theoretically. The role of propagation distance on absorption, dispersion and group index of weak probe light in the presence and absence of ESR is then analyzed. It is found that exciton spin relaxation (spin decoherence) which resulted from many-particle interaction in semiconductors have essential roles to adjusting the absorption and dispersion properties of weak probe light. Moreover, it is realized that the propagation distance in the presence of ESR can lead to switching between subluminal and superluminal light propagation. Our study provides a suitable treatment for the next generation of all-optical systems in semiconductor nanostructures.

     

Multifrequency (X-band to W-band) CW EPR of the organic free radical in petroleum asphaltene

Petroleum of Arabian and Colombian origin was studied by electron paramagnetic resonance (EPR) spectroscopy at X- (9 GHz), Q- (34 GHz) and W-bands (94 GHz). The experiments were performed at room temperature (about 300 K) and at 77 K (W-band only). The asymmetry in the lines corresponding to free radicals was observed more intensely in the W-band spectra. The values of the line width ΔH in the spectra increased linearly with the microwave frequency utilized in the EPR experiments. A mathematical simulation of the free radical signal for the EPR spectra in three bands with a set of parameters corresponding to a single species was attempted, but this was not exactly coincident with the experimental signals, suggesting that the hyperfine interaction of the unpaired electron with its neighborhood corresponds to more than one species of radical in the molecular structure of the petroleum asphaltene.     

Magnetic resonance in systems with equivalent spin-1/2 nuclides. Part 1

Electron paramagnetic resonance (EPR) spectra of S=1/2 systems XL(n) with n equivalent nuclei having spin I=1/2 have been simulated for microwave frequencies in the L-, X-, and W-bands. It has been shown that for n>2 nuclei, the EPR spectra have a more complicated form than anticipated from the usual oversimplified analysis, which predicts n+1 lines with intensity ratios given by the coefficients of the binomial expansion. For the XL(n) system with n=3, the EPR spectra in fact consist of six lines. The exact solution of the spin-hamiltonian for this case has been obtained, which gives four levels in zero magnetic field. For n>2 systems, the degeneracy of the energy levels cannot be completely removed by the Zeeman electronic and nuclear interactions. For n>4, certain spin states cannot occur, consistent with the (generalized) Pauli exclusion principle. Discussion of the underlying theory, invoking exchange degeneracy and the appropriate permutation group theory, is included in some detail. Analogous considerations hold for NMR spectroscopy of non-radicals.     

Determination of the parameters of the rotational diffusion of complexes of serum albumins with Triton X-100 from analysis of tryptophan fluorescence

The rotational diffusion of complexes of human serum albumin (HSA) and bovine serum albumin (BSA) with neutral surfactant Triton X-100 is study by analyzing the polarized tryptophan fluorescence and its parameters are determined (rotational relaxation time, diffusion coefficient, effective radius). Similarities in the solubilization of both proteins are revealed: an effective solubilization BSA and HSA in solutions containing neutral surfactant Triton X-100 is achieved at concentration of the latter of 0.3 mM, slightly greater than its critical micelle concentration (0.25 mM), with the most significant effect taking place at pH 5.0, a value close to the isoelectric points of the proteins.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

Linewidth Analysis of Spin Labels in Liquids: II. Experimental

This work demonstrates that homogeneous linewidths can be extracted from continuous wave electron paramagnetic resonance spectra and that they quantitatively agree with the predictions of existing relaxation theory. We suggest that relaxation theory can be used to predict experimental lineshapes provided that the simulations properly include sources of broadening. We have found that the rotational correlation times for spin labels in different percentages of glycerol/water mixtures are best modeled by a power law treatment for the viscosity, similar to that for translational diffusion. The translational diffusion coefficients themselves also have a power law dependence on the viscosity for glycerol/water mixtures. The linewidths were linearly dependent upon both the oxygen and the spin label concentration. The hyperfine splittings of all nuclei were observed to decrease linearly with increasing spin label concentration, completely at odds with existing theory which predicts a quadratic dependence upon concentration. The linear dependence was independent of hyperfine splitting until the magnitude of the hyperfine splitting was less than the homogeneous linewidth.     

Electron spin resonance signal of Luttinger liquids and single-wall carbon nanotubes

A comprehensive theory of electron spin resonance (ESR) for a Luttinger liquid state of correlated metals is presented. The ESR measurables such as the signal intensity and the linewidth are calculated in the framework of Luttinger liquid theory with broken spin rotational symmetry as a function of magnetic field and temperature. We obtain a significant temperature dependent homogeneous line broadening which is related to the spin-symmetry breaking and the electron-electron interaction. The result crosses over smoothly to the ESR of itinerant electrons in the noninteracting limit. These findings explain the absence of the long-sought ESR signal of itinerant electrons in single-wall carbon nanotubes when considering realistic experimental conditions.     

Mössbauer effect study of the temperature and pressure dependence of the singlet-quintet intersystem crossing dynamics in an iron(II) spin crossover complex

The lineshapes of Mössbauer spectra of the iron(II) spin crossover complex [Fe(6-mepy)₃ tren] (PF₆)₂ are affected by the dynamics of the HS?LS equilibrium. The lineshapes are reproduced with a stochastic two-state-relaxation-model yielding rate constants similar to those determined for related complexes in solution. Application of an external pressure of 150 MPa increases the relaxation rate.     

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurement of the relaxation times T₁ and T₂ provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T₁ was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T₂ was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times τ_c for different types of intracellular water were calculated according to a simple two-phase model. The values for T₁, T₂, and proton spin density (i.e., water content) are: nucleus ⪢ animal cytoplasm > vegetal cytoplasm. Based on the estimation of τ_c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.