ESR Line Shape Research Articles

Radical formation during the reaction of bis(fluorosulfuryl)peroxide, S 2O 6F 2, with tetrasulfur tetranitride, S 4N 4, and elemental sulfur, S 8

The radical intermediates formed initially in the reaction of S 2O 6F 2 with S 4N 4 and S 8 have been studied by ESR spectroscopy at X-band level. The transient radical formed in the reaction between S 2O 6F 2 and S 4N 4 has the spin Hamiltonian parameters g xx =2.0014, g yy =2.0054, g zz =2.0249, A xx =22.5 MHz, A yy =4.5 MHz, A zz =2.0 MHz and Q zz =4 MHz as determined by simulation of ESR lineshapes obtained from polycrystalline samples. The spin Hamiltonian parameters are very similar to those previously reported for [S 3N 2] +·, and the spectra are interpreted as being most likely due to the [S 3N 2] +· radical. The reaction between S 2O 6F 2 and S 8 under similar conditions produces a radical that has a g-tensor of the form g xx =2.0026, g yy =2.0184, g zz =2.0258, as determined by simulation. The transient species generated is postulated to be a similar S 5 +· radical.

Rotation of deuterated methylene groups in the diffusional regime. Isotope effect of anisotropic α-deuterons on ESR lineshapes of ⋅CD 2-COO − radical in irradiated ZnAC dihydrate single crystal

A nonperturbative simulation of the temperature-dependent ESR lineshape of the anisotropic ⋅CD 2-COO − radical in the presence of exchange of the two deuterons has been performed in the diffusional region for this radical, e.g. 170–300 K. Both anisotropic hyperfine (hfi) tensors and considerable nuclear quadrupolar interactions (nqi) were necessary in the simulation of the rigid limit lineshape and the dynamics. By comparison with the earlier reported protonated case and for temperatures higher than 170 K no isotope effect was observed in the Arrhenius barrier of the hindered rotation, being 7.1 kcal/mol in both cases.

Bucky(basket)ball: Stabilization of Electrogenerated C60.bul.- Radical Monoanion in Water by Means of Cyclodextrin Inclusion Chemistry

A water-soluble magenta [gamma]-cyclodextrin inclusion complex of C[sub 60] ([gamma]-CD/C[sub 60]) was prepared by a mild method which involved solubilizing C[sub 60] in carbon disulfide followed by extracting C[sub 60] with a [gamma]-CD aqueous ethanolic solution. The UV-visible spectroscopy of [gamma]-CD/C[sub 60] and spectroelectrochemistry of the electrogenerated [gamma]-CD/C[sub 60][sup [minus]] radical monoanion in water resemble those of C[sub 60] in n-hexane and C[sub 60][sup [minus]] in dichloromethane, respectively, indicating that the electronic properties of the C[sub 60] and C[sub 60][sup [minus]] guests are practically unaffected by molecular inclusion. The FT-IR spectrum of solid [gamma]-CD/C[sub 60] is only slightly different from that of [gamma]-CD. Cyclic voltammetry of the [gamma]-CD/C[sub 60] complex in aqueous 0.15 M LiClO[sub 4] solution shows three one-electron electroreductions of which only the first two, of E[sub 1/2] values equal to [minus]0.57 and [minus]1.03 V vs SCE, are reversible. The ESR spectrum in a frozen 0.2 M LiClO[sub 4] aqueous solution of electrogenerated [gamma]-CD/C[sub 60][sup [minus]] is similar to that of C[sub 60][sup [minus]] in aprotic solvents. The ESR line shape and linewidth are dependent upon the temperature with an activation energy, E[sub a] = 0.043 eV, while the g value, equal to 1.998, is temperature independent overmore » the 77-220 K range. 54 refs., 7 figs.« less

Application of a new rotation matrix in dynamic solid state ESR and NMR lineshape simulation

A new rotation matrix was applied to the computer simulation of dynamic ESR and NMR lineshapes. A cubic jump model was chosen to illustrate the use of this rotation matrix.

ESR investigations of different forms of TTT-TCNQ CT complex in reticulate-doped polymer films

Magnetic properties of tetrathiotetracene-tetracyanoquinodimethane (TTT-TCNQ) CT complex microcrystallites forming conductive networks in reticulate-doped polymer films are investigated using the ESR technique. ESR lineshape, temperature dependence of spin susceptibility, linewidth and g factor are analysed. TTT-TCNQ is found to crystallize in at least three forms in reticulate-doped films. It is shown that magnetic properties are related to morphology of the conductive network. Properties of complexes with stoichiometries 1:2 and 1:1 forming needle-like networks are similar to those previously described, while properties of whisker-like microcrystals forming dendritic networks are different and their stoichiometry is probably 3:2 or 2:1. Non-equilibrium crystallization during film casting leads to relatively high defect concentration, but the effect of disorder on magnetic properties of the networks seems to be reduced by concentration of defects in some highly imperfect regions.

Molecular time scale generalized Langevin equation theory for coupled spin systems

The MTGLE theory is formulated for spin systems using the exact low temperature limit solution of the one dimensional ferromagnetic Heisenberg model as a guide. The final equations are solved to reproduce the exact time correlation function and GLE response function for the spin 1/2 XY model in the high temperature limit [Sur et al., Phys. Rev. B 12, 3845 (1975)]. The advantage of the method is that it can be applied to complex systems with relative ease and in the case of spin systems provides an avenue for investigating effects of anisotropic dipolar spin interactions. One such application to exchange narrowing of the ESR line shape is discussed.

Electron spin resonance characterization of the anion radicals of singly, doubly, and triply reduced C 60 in liquid solutions

The results of ESR studies as a function of temperature in liquid solution for several anion radicals of C 60 −n (n = 1, 2, and 3), in the solvents benzonitrile, dimethylformamide, dimethylsulfoxide, and tetrahydrofuran are reported and discussed. The anion radicals are prepared by electrochemical reduction techniques which allow the preparation of samples which contain, quantitatively, the desired anionic species. Generally, the ESR lineshapes and linewidths of the mono- and tri-anions are dependent upon the solvent. Whereas, the linewidths are independent or weakly dependent upon temperature. The g-values are solvent dependent but show no temperature dependence in the range studied. The ESR studies in liquid solution were performed to provide insight into previously reported ESR studies in frozen solution which found the ESR linewidths and lineshapes strongly dependent upon both solvent and temperature. These latter phenomena are interpreted in terms of external fields due to the frozen matrix which cause the partial removal of the expected three-fold orbital degeneracy of the C 60 anion radicals. The liquid solution ESR studies were performed to answer the question of whether there are also internal factors which contribute to the partial removal of the degeneracy or whether it is entirely external in nature.

Efficient characterization of the orientational ordering of ESR-active probes in supermolecular fluids

Supermolecular fluids often exhibit organized arrangements which order the orientations of solved ESR-active probe molecules. Due to the high microviscosity, the ESR line-shape of the probe virtually reflects the distribution of the resonating frequencies of the involved spin packets and then the degree of orientational order of the probe. An efficient strategy to get precise estimates of both the magnetic parameters of the probe molecule and the width of the spin packet from the ESR spectrum has been developed. This allows us to adjust only the strength of the effective orienting potential in the fit procedure. A study of the orientational ordering of cholestane probe molecule dissolved in side-chain polymeric mesophases provides accurate testing of the procedure.

Multifrequency ESR and NMR analysis of the metal–ATP interaction

AbstractMultifrequency ESR analysis allows a new interpretation of the correlation times involved in nuclear relaxation. In this study the metal‐ATP interaction, so important in ecosystem energy mechanisms, was interpreted in the light of new multifrequency ESR and NMR experimental data. ESR spectra of the Mn(II)‐ATP complex were recorded at 2–4 GHz (S‐band), 9 GHz (X‐band) and 35 GHz (Q‐band). A loop‐gap resonator was used to enhance the ηQ0 product at low frequencies. The ESR line shapes at different temperatures and frequencies led to the interpretation of the predominant relaxation mechanism involved. The NMR relaxation rates, also recorded at different temperatures and frequencies, are discussed in terms of a motional model taking into account a distribution of correlation times. Structural and dynamic features were analysed in order to determine the effective correlation time contributing to nuclear paramagnetic relaxation. A comprehensive analysis of the multifrequency ESR spectra is crucial for the correct interpretation of the NMR spectra and of the metal–ATP interaction.

In situ ESR of polyaniline by electrochemical doping: Polaron to Pauli-like instead of polaron to bipolaron

Polyaniline film was synthesized electrochemically with HBF 4 as an electrolyte. A potential was changed step by step to measure ESR spectra. At low doping levels ESR line shape was found to be a single Lorentzian. At higher doping levels, however, it was composed of double Lorentzians; narrow line and broad one. An integral intensity of the broad line becomes larger than that of the sharp one at the higher potentials. Temperature dependence of spin susceptibility suggested that the sharp line behaves as a Curie spin and the broad one does as a Pauli one, which is consistent with low frequency ESR result on the chemically prepared emeraldine salt with BF 4 −. The sharp line can be interpreted as a polaron created in an initial stage of the electrochemical doping. Presumable origin of the broad line was discussed.

Rotational dynamics and conformational kinetics in liquid crystals

Abstract Two particular aspects of solute dynamics in ordered media are analysed on the basis of the solution of multivariate diffusion equations: the effects of the solvation dynamics on the rotational motions of dipolar probes in liquid crystal solvents, and the alteration of reaction pathways in isomerization kinetics caused by the solvent order. The introduction of a suitable solvent coordinate allows the interpretation of high frequency contributions in the rotational correlation functions observed by spectroscopic techniques, namely dielectric dispersion, IR and Raman spectroscopy, ESR lineshapes and optical Kerr effect. For molecular systems undergoing conformational changes, a method is offered to evaluate the modification of the torsional barriers resulting from the anisotropic torques modulated by the molecular shape changes along the reaction coordinate.

ESR line shapes and methyl rotation in the acetic acid anion radical

The CH 3COOD − and CD 3COOD − radical anions have been studied by ESR in irradiated crystal acetic acid in the temperature region 77–170 K. The line-shape variations which are due to methyl rotation are simulated with a program based on the Liouville direct method. The activation energy to the motion is evaluated and is found not to be appreciably affected by susbtitution of the methyl protons by deuterons. The activation energies are determined to be 2.2 and 2.5 kcal/mol respectively. Indications of hydrogen bond effects on the activation barrier are also found.

ESR line shape analysis for strongly distorted signals due to an intense paramagnetic sample: Application to spin polarized atomic hydrogen

We present a method suitable for a real time computer analysis of ESR signals strongly distorted due to an intense paramagnetic sample. This occurs typically in experiments with highQ cavities and narrow lines carried out at low temperatures. We have applied this method to ESR of gaseous spin polarized atomic hydrogen. During an experiment the density of gas atoms in a cavity may decrease a few orders of magnitude. Thus the ESR line shape evolves correspondingly from a regime of heavily distorted signals to that of undistorted ones. We have compared our analysis with independent measurements of the hydrogen density by a sensitive pressure gauge.

A small protein in model membranes: a time-resolved fluorescence and ESR study on the interaction of M13 coat protein with lipid bilayers.

Model membranes with unsaturated lipid chains containing various amounts of M13 coat protein in the alpha-helical form were studied using time-resolved fluorescence and ESR spectroscopy. The lipid-to-protein (L/P) ratios used were > 12 to avoid protein-protein contacts and irreversible aggregation leading to beta-polymeric coat protein. In the ESR spectra of the 12-SASL probe in dioleoyl phosphatidylcholine (DOPC) bilayers no second protein induced component is observed upon incorporation of M13 coat protein. However, strong effects are detected on the ESR lineshapes upon changing the protein concentration. The ESR lineshapes are simulated by assuming a fixed ratio between the parallel (D parallel) and perpendicular (D perpendicular) diffusion coefficients of 4, and an order parameter equal to zero. It is found that increasing the protein concentration from L/P infinity to L/P 15 results in a decrease of the rotational diffusion coefficient D perpendicular from 3.4 x 10(7) to 1.9 x 10(7) s-1. In the time-resolved fluorescence experiments with DPH-propionic acid as a probe, it is observed that increasing the M13 coat protein concentration causes an increase of the two fluorescent lifetimes, indicating an increase in bilayer order. Analysis of the time-resolved fluorescence anisotropy decay allows one to quantitatively determine the order parameters <P2> and <P4>, and the rotational diffusion coefficient D perpendicular of the fluorescent probe. The order parameters <P2> and <P4> increase from 0.34 to 0.55 and from 0.59 to 0.77, respectively, upon adding M13 coat protein to DOPC bilayers with an L/P ratio of 35.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroreduction of buckminsterfullerene, C60, in aprotic solvents: electron spin resonance characterization of singly, doubly, and triply reduced C60 in frozen solutions

The results of ESR studies on frozen solutions of buckminsterfullerene anion radicals, C{sub 60}{sup n{minus}}(n = 1, 2, and 3), in the solvents benzonitrile, dichloromethane, pyridine, and tetrahydrofuran are reported and discussed. The samples are prepared by electrochemical reduction techniques which allow the preparation of samples which contain, quantitatively and selectively, the desired anionic species. The ESR spectral features of the anions in frozen solutions were studied as a function of temperature. The ESR line shapes and linewidths are dependent upon the solvent and the temperatures whereas the g values are independent of solvent and only weakly dependent upon temperature. The C{sub 60}{sup {minus}}(S = 1/2)g value averages 1.998 over the four investigated solvents and the temperature range of 77-210 K. The resonances of C{sub 60}{sup 2 {minus}} are centered at about g = 2.00 and indicate a triplet ground state with an average separation of 12.6 {Angstrom} between the two unpaired electrons. The C{sub 60}{sup 3{minus}} resonance has a g value of 2.001-2.002 which is closer to that of the free electron as compared to C{sub 60}{sup {minus}} and C{sub 60}{sup 2{minus}}. It also exhibits features similar to the monoanion, suggesting a spin of 1/2. 25 refs., 7 figs.,more » 3 tabs.« less

A new approach for the simulation of ESR lineshapes over a large range of correlation times

A new approach for the simulation of ESR lineshapes over a range of correlation times extending from the Redfield limit to the rigid limit is described. The use of a separable operator for the dynamics of a spin label in a uniaxial liquid crystal (“Kangaroo dynamics”) allows an algebraic solution of the stochastic Liouville equation. This procedure avoids the numerical instability resulting from clustering of eigenvalues inherent in the conventional solution using an eigenfunction expansion.

MICROWAVE ABSORPTION STUDIES ON HIGH-Tc SUPERCONDUCTORS AND RELATED MATERIALS VIII — ESR OF DPPH COATED ON BiSrCaCuO FILMS FABRICATED ON MgO (100) SUBSTRATES

ESR of DPPH coated on Bi - Sr - Ca - Cu - O films fabricated on MgO (100) substrates by MOCVD have been studied for samples with different thicknesses, 1000 Å and 100 Å. Temperature dependence of the ESR peak-to-peak linewidth, ΔH pp , revealed that ΔH pp , increases with film thickness. The excess ESR linewidth, δ (ΔH pp ) was also analyzed in terms of (1-t)α, with t=T/T c , giving for example α=4±1 for the 1000 Å thick sample. The ESR lineshapes were distorted by rotating the samples in applied magnetic fields. Severe distortion was found for the 1000 Å sample below about 30 K, but the distortion almost disappears at temperatures above 30 K. The applied magnetic field effects were also examined in both field-cooled and zero-field-cooled cases.

ESR Studies of the Anion Radicals of the Fullerene: C60n- (n = 1, 2, and 3).

ABSTRACTThe results of ESR studies on several anion radicals of the fullerene C60, C60n-(n = 1, 2, and 3), in the solvents benzonitrile, dichloromethane, pyridine and tetrahydrofuran containing 0.1 M TBAClO4 are reported and discussed. The samples are prepared by electrochemical reduction techniques which allow the preparation of samples which contain, quantitatively, the desired anionie species. The results of ESR spectral studies on the anions in frozen solutions as a function of temperature are presented. The ESR lineshapes and linewidths are dependent upon the solvent and the temperature. The g-values are weakly dependent upon temperature. The results obtained from this study are presented and discussed in terms of the unique physical structure and properties of C60.

Theoretical comparison and experimental test of the secular and nonperturbative approaches on the ESR lineshapes of randomly oriented, anisotropic systems undergoing internal motion

Abstract The nuclear Zeeman and the electronic nonsecular parts of the spin Hamiltonian complicate the ESR lineshape of exchanging anisotropic spin systems by introducing, at high field, “forbidden” transitions and, at low field, additional shift and splitting. We compare the nonperturbative with the secular approach for such systems. The exchange is treated within the Kaplan-Alexander limit and both A and g tensors are included, resulting in spectrum asymmetry, in contrast to previous separate treatments. The two approaches are then used to simulate the powder spectrum of OCH 2 COO − and compare the results to experimental spectra of an irradiated powder of ZnAc. The powder X-band spectra simulations using the secular approach appear to be accurate. For both the low-field (20 to 200 G) and the high-field (Q-band) regions, however, the nonsecular part of the electronic term and the nuclear Zeeman term, respectively, cannot be neglected. On the other hand, the approximate approach is much faster and consequently more appropriate for treating large, multisite exchanging systems.

Intramolecular dynamics of the C 4H 8NH radical cation. An application of the anisotropic exchange theory for powder ESR lineshapes

The temperature variation of ESR spectra attributed to the heterocyclic radical cation C 4H 8NH + stabilised in a CFCl 3 matrix is shown to be due to intramolecular dynamics of the ring. Two new methods for simulating anisotropic exchange broadened ESR spectra, the secular and nonperturbative, were utilised to investigate the dynamics of C 4H 8NH +. The powder ESR lineshape exhibits overall anisotropic features and a differentiated broadening with respect to the nuclear spin quantum numbers, which could be explained by simultaneously including the g- and hyperfine tensor anisotropies in the model for the dynamical motion. The motion was described by an interconversion of the ring between two isodynamic twisted structures having C 2 symmetry. It was concluded that the major changes of the ESR lineshape were governed by the exchange of the isotropic hyperfine coupling constants. The anisotropic interactions were shown to account for the overall anisotropic features and the differentiated broadening of the spectra.