Variable-temperature Electron Paramagnetic Resonance Research Articles

Ligand Dynamics in Spin-Labeled Au Nanoparticles

Variable temperature electron paramagnetic resonance (EPR) spectra of spin-labeled Au nanoparticles in toluene solution were recorded at X-, Q- and W-bands. The chain length of the nitroxide-based spin-labeled ligand and the surrounding alkanethiol ligands was systematically varied. The spectra were analyzed using the NLSL program (Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A 1996, 120, 155) which simulates slow motion spectra using the stochastic Liouville approach. The simulation results show that the rotational diffusion parameters for particles (∼2.6 nm Au core diameter) are dominated by the local motion of the ligand. The length of the spin label has the biggest effect on the nitroxide dynamics followed by the chain length of the surrounding ligands. The results are consistent with the loose packing of ligands on the nanoparticle surface. The packing density of ligands further decreases with the increasing distance from the Au surface. The Arrhenius analysis of the dynamic ...

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

In order to better understand the dependence of charge recombination rate vs. temperaturek CR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D+-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D isp-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-1,8∶4,5-bis(dicarboximide). The EPR spectra of D+-C exhibit marked changes in their overall shape throughout the 190–295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 104 s−1 at 210 K and speeds up to 1.25 · 107 s−1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D+ ·-C rotamers. The simulations also reveal that inversion of the anilino N center emerges atT > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constantK = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D+ ·-C-A− · accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. Thek CR (T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D+·-C-A− · control the overall charge recombination reaction with this radical ion pair.

Design of Gd(III)-Based Magnetic Resonance Imaging Contrast Agents: Static and Transient Zero-Field Splitting Contributions to the Electronic Relaxation and Their Impact on Relaxivity

A multiple-frequency (9.4-325 GHz) and variable-temperature (276-320 K) electron paramagnetic resonance (EPR) study on low molecular weight gadolinium(III) complexes for potential use as magnetic resonance imaging (MRI) contrast agents has been performed. Peak-to-peak linewidths Delta Hpp and central magnetic fields have been analyzed within the Redfield approximation taking into account the static zero-field splitting (ZFS) up to the sixth order and the transient ZFS up to the second order. Longitudinal electronic relaxation is dominated by the static ZFS contribution at low magnetic fields (B < 0.3 T) and by the transient ZFS at high magnetic fields (B > 1.5 T). Whereas the static ZFS clearly depends on the nature of the chelating ligand, the transient ZFS does not. For the relatively fast rotating molecules studied water proton relaxivity is mainly limited by the fast rotation and electronic relaxation has only a marked influence at frequencies below 30 MHz. From our EPR results we can conclude that electronic relaxation will have no influence on the efficiency of Gd(III)-based MRI contrast agents designed for studies at very high magnetic fields (B > 3T).

Dielectric, Electron Paramagnetic Resonance, and Transport Properties of Spanish Moss

We present preliminary data on the electric resistivity, ac dielectric response, variable temperature electron paramagnetic resonance (EPR) and magnetization measurements on Spanish moss. We find that the four-probe dc resistivity is about 10 ohm-meter, the dc dielectric constant is 76 ± 1, close to that of water (80), and the ac dielectric measurements reveal well-defined peaks whose maxima depend on frequency and temperature, with an Arrhenius activation energy of ∼ 0.6 ± 0.1 eV. Magnetization measurements showed diamagnetism down to about 6K and then a Curie tail. The EPR measurements reveal a level of about 10 to 100 parts per million of isolated Mn2+ ions and a ten-fold higher level of Fe3+ cluster peaks, the later of which were strongly temperature dependent down to 5K. The origin of the Fe3+ structure remains unclear.

Variable temperature EPR spectra of Copper (II) ions in kainite crystals

X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in KMgClSO 4·3H 2O single crystals have been performed at low temperature (123 K). The angular variation of the EPR spectra reveals the presence of two Cu 2+ sites, which have different orientations. The spin-Hamiltonian parameters of this six-coordinated cupric ion have been evaluated from the EPR spectra at 123 K. The forbidden lines due to Δ m I=±1 transitions are observed in between allowed transitions. The temperature variation EPR studies have also been performed both for a single crystal and a polycrystalline sample. The ground state wavefunction of Cu 2+ ions has been estimated and is found to be an admixture of d 3 z 2 − r 2 and d x 2 − y 2 . The temperature variation of the EPR spectra reveals that Cu 2+ ions exhibit dynamic Jahn–Teller effect. From the polycrystalline EPR data, the temperature dependent magnetic susceptibilities are evaluated and discussed.

Alkanethioimidoyl radicals: evaluation of beta-scission rates and of cyclization onto S-alkenyl substituents.

Thioimidoyl radicals were generated by addition of alkylsulfanyl radicals to alkyl isonitriles and were characterized by electron paramagnetic resonance (EPR) spectroscopy. The beta-scissions of their C.S-C bonds were studied by variable-temperature EPR spectroscopy and the fragmentation rate constants and activation energies were calculated. The scission rates depend on the stability of the released alkyl radicals but in any case, at room temperature, the processes were fast. Data collected on similar oxyimidoyls showed that their fragmentations are slightly slower compared to those of analogous thioimidoyls. The scission rates of selenoimidoyls could not be studied by EPR and were evaluated by theoretical calculations. EPR experiments also enabled both beta-scission and 5-exo ring closure rate constants of two S-but-3-enyl-substituted imidoyl radicals to be determined, showing that cyclization prevails only at low temperatures. Density functional theory (DFT) theoretical calculations predicted that the fragmentation process preferentially occurs from the s-cis rotamers (X-C bond) of the imidoyl radicals. Thio- and seleno-imidoyls (but not oxyimidoyls) prefer s-trans conformations so that their fragmentations involve prior rotation about the X-C bond.

Trends in exchange coupling for trimethylenemethane-type Bis(semiquinone) biradicals and correlation of magnetic exchange with mixed valency for cross-conjugated systems.

A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states.

Dynamical Jahn–Teller distortion in single crystals of Cu(II) doped magnesium potassium phosphate hexahydrate: a variable temperature EPR study

Single crystal electron paramagnetic resonance (EPR) studies on Cu(II)-doped magnesium potassium phosphate hexahydrate have been carried out at room temperature. The temperature dependence of g and A values has been obtained for the polycrystalline sample and the ground state is unambiguously identified. These results indicate the existence of a dynamic Jahn–Teller distortion for Cu(II) ion. The g and A tensor direction cosines are evaluated and compared with Mg–O directions, which confirms that Cu(II) enters substitutionally in the lattice.

In situ ESR studies over wide temperature range for conducting polymers

In this work, in situ variable temperature electron spin resonance (ESR) measurements were realized for the first time to verify the spin nature of conducting polymers at controlled potentials. By using mixed solvents consisting of water and propanol in 1:1 volume ratio the temperature window was widened to 100 °C (−40 to +60 °C) for the study of polyaniline. According to the temperature dependence of ESR intensities obtained with this technique, it can be clearly seen that polyaniline in HCl solutions took on the Curie spin behavior at low doping level (0.0 V vs AgCl/Ag), and turned to the Pauli spin behavior at high doping level (0.4 V where conductivity reached about its maximum).

On the mode of hexacoordinated NO-binding to myo- and hemoglobin: Variable-temperature EPR studies at multiple microwave frequencies

Electron paramagnetic resonance (EPR) spectroscopy is performed on NO⋅-ligated hemo- and myoglobin at temperatures between 5 and 300 K. Apart from the standard X-band (9.5 GHz), data are taken at K-(24 GHz), Q-(34 GHz), W-(94 GHz) and Y-band (285 GHz) frequencies, respectively. The spectra are delineated into contributions of two EPR signatures differing ing-tensor symmetry, an axial and a rhombic one. The state contributions are found to differ with temperature and the states show temperature-dependentg-factor variations. In MbNO the axial species (denoted state II) is the only species observed at 300 K with X-band. Analysis of its development with decreasing temperature shows a partial transformation into a rhombic state (I). For this, at an intermediate temperature range (240–100 K), irregular line shapes are apparent, indicating disordered geometries of the Fe-NO-heme group. Regular rhombic lineshapes develop increasingly at temperatures below about 70 K. At 5–10 K, the rhombic state I dominates the EPR spectra but state II is still present. A similar development of spectra is observed in HbNO, for which, however, a rhombic contribution is already present at 300 K. Subunit-associated variations in state contributions and EPR appearance are not resolved at any frequency. Also, lineshape irregularities with decreasing temperature are not manifest in HbNO. These findings are used to present a structure-based picture of states I and II and their transformations and to discuss the origin of the line shape irregularities in MbNO.

Electron Paramagnetic Resonance Study of Oxidized B820 Complexes

Variable temperature electron paramagnetic resonance (EPR) measurements were performed on B820 complexes oxidized with potassium ferricyanide. The EPR signals were assigned to radical cations of the bacteriochlorophyll a (BChla) molecules present in the B820 complexes. In the 80−250 K range, the spectral shape and width corresponded to that of monomeric BChla.+. At temperatures higher than 250 K, the line width exhibited a rapid decrease. We attribute the EPR line narrowing to the activated transport of electrons between the B820 BChlas. The parameters governing the charge transport process were extracted from spectral simulations. The simulations incorporated an electron-transfer model that accounts for the effects of glass transitions in the medium. The results of the B820 experiments and simulations are discussed in relation to the EPR characteristics of oxidized light-harvesting 1 (LH1) antenna complexes and photosynthetic reaction centers (RC).

EPR and pulse radiolysis studies on electron transfer in transition metal dithiolenes at the surface of colloidal TiO2

One-electron reduction of several transition metal bis-dithiolenes at the surface of TiO2 particles and in aqueous solutions were studied. Electron Paramagnetic Resonance (EPR) was used to study the reduction at the particle surface and the pulse radiolysis technique was employed for aqueous solutions. Variable temperature EPR measurements following band-gap excitation of the particles showed the disappearance of signals due to the paramagnetic Ni(III) and Cu(II) complexes of 4,5-dimercaptophthalic acid (DMPA), 3,4-dimercaptotoluene (TDT) and maleonitrile dithiolate (MNT), with the surface electron signals vanishing concomitantly. Similarly, the diamagnetic Cu(III) complex of TDT was reduced to the paramagnetic Cu(II) complex. The bimolecular rate constants for the reaction of Ni(III)DMPA and Ni(II)MNT with ea-qwere determined to be 4.3 × 109 dm3 mol-1 s-1 and 1.3× 1010 dm3 mol-1 s-1, respectively. Pulse radiolysis results were used to distinguish between reduction with electron and · CH2OH radical. It is inferred that ea-q · reduces the metal centre while · CH2OH adds to the ligand.

Variable-Temperature Electron Paramagnetic Resonance Studies of Copper-Exchanged Zeolites

Copper-exchanged zeolites, Beta and ZSM-5, were studied using variable-temperature electron paramagnetic resonance (EPR) spectroscopy to probe changes in the local environment of the Cu2+centers when samples were dehydrated and heated in flowing helium or under reagent flow. Hydrated samples of Cu-ZSM-5 and Cu-Beta exhibited EPR spectra consistent with EPR signals previously assigned to Cu2+in distorted octahedral coordination. EPR spectra of dehydrated Cu-Beta and Cu-ZSM-5 showed the presence of coordination environments that were similar to EPR signals previously assigned to Cu2+in distorted square pyramidal and distorted square planar environments. An empirical model is presented that correlatesg∥andA∥for a series of copper-exchanged zeolites and model compounds and provides additional insight into the coordination environment of Cu2+in copper-exchanged zeolites. The empirical model links a number of past EPR studies on different copper-exchanged zeolites and provides an explanation for the observed trends in EPR parameters related to the charge at the copper center. The EPR spectra for dehydrated Cu-ZSM-5 and Cu-Beta samples exhibited a temperature dependence. The EPR spectrum of Cu-ZSM-5 and Cu-Beta recorded at 673 K showed an increase ing∥and a decrease inA∥when compared with the EPR spectrum recorded at room temperature. These changes in spectral parameters are attributed to changes in the electronic environment of the Cu2+species through modification of the coordination environment.

Spectroscopic and magnetic properties and structure of a five-coordinate, O 2-binding cobalt(II) Schiff base complex and of the copper(II) analogue

The cobalt(II) complex CoL and the copper(II) complex CuL of the pentadentate O 2N 2N′ Salen-type Schiff base ligand H 2L = N, N′-bis(3-t-butyl-2-hydroxy-5-methylbenzyliden)-1, 7-diamino-4-methyl-4-azaheptane were prepared, characterized and subjected to single crystal X-ray analysis. The {MO 2N 2N′} coordination core forms a trigonal bipyramid differing in the degree of distortion, as expressed by the O-M-O angles 138.62(8)° [O-Co-O] and 155.21(7)° [O-Cu-O]. The distances between the metal and the aliphatic tertiary nitrogen atoms N′ are markedly different, 2.123(2) [Co-N′] and 2.300(2)Å[Cu-N′]. In contrast to normal Salen complexes the imine nitrogen atoms are located trans to each other. Solution room temperature electron spectra show a broad absorption in the near-IR range at 6350 (CoL) and 8200 (CuL) cm −1. The temperature dependence of the effective magnetic moment for high-spin CoL ( S =3/2) shows a maximum at 50 K, which is reproduced by the following set of magnetic parameters, reflecting the zero-field splitting; g 1 = 1.68, g 1 = 1.89, g = 3.21, D/hc = −38.9cm −1, E/hc = 1.7cm −1, α 110 = 0, zJ/hc = −1.00cm −1. Variable temperature electron paramagnetic resonance experiments prove that CoL, dissolved in toluene, N, N-dimethylformamide or pyridine, binds dioxygen reversibly.

Electron paramagnetic resonance of Ni(II) doped tris(ethylenediamine)zinc(II) dinitrate

Variable temperature electron paramagnetic resonance spectra of tris(ehtylenediamine)zinc(II) dinitrate single crystals doped with Ni(II) have been measured. The host crystal undergoes a trigonal to monoclinic phase transition at 146 K. Above the transition temperature the zero field splitting tensor is axially symmetric with D = −0.831 cm −1 and below it becomes rhombic with D = −0.785 cm −1, E = −0.088 cm −1. The low temperature spectrum is characterised by the pattern repeating every 60° when the crystal is rotated about the high temperature c axis. The analysis shows that the Zn(II) site retains a C 2 symmetry axis and that the distortion away from the D 3 site symmetry observed for high temperatures is small, the principal axes being tilted by 2.6°. This implies that the phase transition involves the flipping of the CC backbone in one of the ethylenediamine ligands of the complex, resulting in a Λδδδ to Λδδλ type conformational change.

Variable temperature electron paramagnetic resonance investigations of a kerosene-based magnetic fluid

The kerosene-based ferrofluid RP-90 has been examined by electron paramagnetic resonance (EPR) from room temperature to liquid nitrogen temperature at the X-band frequency 9.41 GHz. The variations in the g-value and ( ΔH) with temperature indicate the presence of a magnetic phase transition from a ferromagnetic to a ‘spin glass or cluster glass’ state. This transition is associated with the freezing point of the carrier medium. No evidence of the presence of superparamagnetism was observed. The linewidth variation with temperature indicates a dominant bulk rotation relaxation mechanism ( τ B).

Construction and Performance of Ribbon-Wound TE011Cylindrical EPR/ENDOR Cavities

A robust yet sensitive Q-band (35 GHz) cavity has been designed for routine variable temperature electron paramagnetic resonance and electron nuclear double-resonance measurements down to 2 K. The cavity withstands repeated temperature cycling as well as providing high microwave energy density and magnetic-field modulation at the sample. It consists of an aluminum or brass (plain, silver, or gold-plated) ribbon imbedded in a cylindrical epoxy or epoxy/quartz composite with a tunable piston at the bottom. The cavity has all the advantages of the traditional silver wire-wound cavity often used for Q-band measurements but is much more robust and easier to construct. The cavity suppresses degenerate resonant modes and minimizes wall eddy currents induced by field modulation. With standard Varian Q-band modulation coils, a 100 kHz modulation field of 27 G peak-to-peak is obtained at the sample. The cavityQL(≈2500) and weak pitch signal/noise (≈1000:1) with a Varian/E-110 microwave bridge are comparable to those obtained with either a traditional wire-wound cavity or a cavity constructed with a thin silver wall on an epoxy/quartz substrate. A set of parallel posts along the axis of the cavity (bidirection) form the ENDOR coil. The “ribbon-wound” design should be useful in EPR and ENDOR applications at frequencies higher than Q band as well.

Evaluation of potential ferromagnetic coupling units: the bis(TMM) [bis(trimethylenemethane)] approach to high-spin organic molecules

Four new hydrocarbon tetraradicals, 1-4, each composed of two triplet trimethylenemethane (TMM) subunits linked by a potential ferromagnetic coupling unit (FC), were synthesized and characterized by variable-temperature electron paramagnetic resonance (EPR) spectroscopy. Simulation of the EPR powder spectra and a priori calculation of the zero-field splitting parameters aided spectral assignment. The Heisenberg Hamiltonian appears to quantitatively model relative spin-state energies in 1-4. In three cases ferromagnetic coupling was achieved, as evidenced by quintet ground states in the resulting tetraradicals. In one case, strong evidence for antiferromagnetic coupling was obtained.

Models for intramolecular exchange in organic .pi.-conjugated open-shell systems. A comparison of 1,1-ethenediyl and carbonyl linked bis(arylnitrenes)

Linkage of two phenylnitrene electron-spine-bearing units by exchange coupling linker groups leads to model open-shell π-conjugated systems of the general structure:N-Ph-X-Ph-N: of various possible connectivity types. Use of variable-temperature electron spin resonance (ESR) spectroscopy of the dinitrenes with cross-conjugating linker groups-X-allowed study of the groung-state spin multiplicities of open-shell systems that are related in a connectivity sense to the theoretically important diradicals trimethylenemethane, oxyallyl, and 1,1,2,3,3-pentamethylenepropane

Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V4+ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V2O5 by 1 mol.% Fe2O3 which may be due to a decrease in the V4+V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe2O3 which may be due to possible hopping conduction between Fe2+−Fe3+, V4+−Fe3+ and Fe2+−V5+. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V4+V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V4+V ratio and the microstructure.