Electron Spin Echo Studies Research Articles

Electron Spin Echo Studies of Hydrothermally Reduced Graphene Oxide

Ecofriendly hydrothermal reduction of graphene oxide is widely used for producing hydrogels and aerogels, but it yields partly reduced graphene oxide (prGO) containing oxygen groups and some number...

Out-of-Phase Electron Spin Echo Studies of Light-Induced Charge-Transfer States in P3HT/PCBM Composite.

The light-induced charge-transfer (CT) state in the composite of the conductive polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been studied by electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal corresponding to the spin-correlated radical pair P3HT(+)/PCBM(-) has been observed in this composite material. The time-domain ESE shape for different delays between the laser flash and the microwave pulse sequence has been analyzed. In order to explain the evolution of the out-of-phase ESE signal as a function of the delay between the microwave pulses, a model of the CT state is proposed. The hole is assumed to be delocalized on the P3HT chain over several thiophene subunits, while the point-dipole approximation is used to describe the interaction with the electron on PCBM. The distribution of distances between the positive and negative charges in the CT state has been evaluated.

Electron Paramagnetic Resonance and Electron Spin Echo Studies of Co2+ Coordination by Nicotinamide Adenine Dinucleotide (NAD+) in Water Solution

Co2+ binding to the nicotinamide adenine dinucleotide (NAD+) molecule in water solution was studied by electron paramagnetic resonance (EPR) and electron spin echo at low temperatures. Cobalt is coordinated by NAD+ when the metal is in excess only, but even in such conditions, the Co/NAD+ complexes coexist with Co(H2O)6 complexes. EPR spin-Hamiltonian parameters of the Co/NAD+ complex at 6 K are g z = 2.01, g x = 2.38, g y = 3.06, A z = 94 × 10−4 cm−1, A x = 33 × 10−4 cm−1 and A y = 71 × 10−4 cm−1. They indicate the low-spin Co2+ configuration with S = 1/2. Electron spin echo envelope modulation spectroscopy with Fourier transform of the modulated spin echo decay shows a strong coordination by nitrogen atoms and excludes the coordination by phosphate and/or amide groups. Thus, Co2+ ion is coordinated in pseudo-tetrahedral geometry by four nitrogen atoms of adenine rings of two NAD+ molecules.

Electron spin relaxation governed by Raman processes both for Cu2+ ions and carbonate radicals in KHCO3 crystals: EPR and electron spin echo studies

EPR studies of Cu2+ and two free radicals formed by γ-radiation were performed for KHCO3 single crystal at room temperature. From the rotational EPR results we concluded that Cu2+ is chelated by two carbonate molecules in a square planar configuration with spin-Hamiltonian parameters g||=2.2349 and A||=18.2mT. Free radicals were identified as neutral HOCO with unpaired electron localized on the carbon atom and a radical anion CO3·- with unpaired electron localized on two oxygen atoms. The hyperfine splitting of the EPR lines by an interaction with a single hydrogen atom of HOCO was observed with isotropic coupling constants ao=0.31mT. Two differently oriented radical sites were identified in the crystal unit cell. Electron spin–lattice relaxation measured by electron spin echo methods shows that both Cu2+ and free radicals relax via two-phonon Raman processes with almost the same relaxation rate. The temperature dependence of the relaxation rate 1/T1 is well described with the effective Debye temperature ΘD=175K obtained from a fit to the Debye-type phonon spectrum. We calculated a more realistic Debye temperature value from available elastic constant values of the crystal as ΘD=246K. This ΘD-value and the Debye phonon spectrum approximation give a much worse fit to the experimental results. Possible contributions from a local mode or an optical mode are considered and it is suggested that the real phonon spectrum should be used for the relaxation data interpretation.It is unusual that free radicals in KHCO3 relax similarly to the well localized Cu2+ ions, which suggests a small destruction of the host crystal lattice by the ionizing irradiation allowing well coupling between radical and lattice dynamics.

ChemInform Abstract: Electronic Structure of Retinal and Related Polyenals in the Lowest Triplet State: An Electron Spin Echo Study.

The authors have observed transitions between the spin sublevels of the lowest triplet state T{sub 0} of all-trans isomers of retinal and methyl-substituted polyenals by means of electron spin echo spectroscopy in a magnetic field and in zero field. This triplet state appears to be {pi}{pi}* in nature. the frequencies of the zero-field transitions have been determined. For retinal, the authors have observed the T{sub z}-T{sub x} transition at 2370 MHz and the T{sub z}-T{sub y} transitions at 1990 MHz. The upper spin sublevel T{sub z} dominates the populating of T{sub 0} and decays fastest, but the selectivity in the decay is clearly less than in the populating. The lifetimes of the individual sublevels of the lowest triplet state or retinal are 71 {mu}s for T{sub x}, 17 {mu}s for T{sub y}, and 12 {mu}s for T{sub z} at 1.2 K. The data for retinal correspond to those for a linear polyenal with six double bonds. This shows that the carbon-carbon double bond in the cyclohexene ring of potential is also involved in the excitation, which suggests that the chromophoric part of retinal is approximately planar in the lowest triplet state. 22 refs., 6 figs., 4 tabs.

Small-Angle Orientational Motions of Spin-Labeled Lipids in Cholesterol-Containing Bilayers Studied at Low Temperatures by Electron Spin Echo Spectroscopy

Electron spin echo (ESE) study was performed for a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer containing an admixture of lipids (of 1:100 molar ratio) spin-labeled at the polar head or at different positions along the acyl chain and optionally containing additionally cholesterol that substituted POPC lipids in a 50:50 molar ratio. ESE signal was observed below 150-115 K, depending on the label position. Three-pulse stimulated ESE is sensitive to two types of orientational motion of spin labels at these temperatures. The first type of motion (which is also seen in a two-pulse primary ESE experiment) is fast stochastic librations, with a correlation time in the nanosecond scale. The second type of motion is slow millisecond rotations, which in the accessible microsecond time scale are seen as small-angle reorientations. (Restricted rotations also refer to this type of motion.) We found that in presence of cholesterol, fast librations are enhanced at all label positions. This effect may be related to the cholesterol ordering effect. Rotational motions in the presence of cholesterol are remarkably suppressed near the bilayer surface, which may be readily explained by the cholesterol condensing effect. In contrast to that, in the bilayer center rotations in the presence of cholesterol are remarkably enhanced. This effect may be attributed to topological properties of a cholesterol-containing membrane system in which beyond the steroid core, more freedom appears for small-angle motions, as compared with the cholesterol-free case. The found property may be important for physiological temperatures, as well. In particular, it may imply that cholesterol weakens interaction between two leaflets in the bilayer.

Influence of Cholesterol on Molecular Motions in Spin-Labeled Lipid Bilayers Observed by Stimulated ESE

Electron spin echo (ESE) study was performed for spin-labeled lipids 1-palmitoyl-2-stearoyl-(5-d)-sn-glycero-3-phosphocholine in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phospholipid bilayer. Recently (Isaev and Dzuba in J Phys Chem B 112:13285–13291, 2008), three-pulse stimulated ESE was shown to be sensitive to two types of orientational motion of spin labels in phospholipid bilayers at low temperatures (~100–150 K). The first one is fast stochastic libration, with correlation time on the nanosecond time scale. The second one is slow rotational motion, developing on the accessible for measurements microsecond time scale in a small range of reorientation angles, ~0.1°–1°. These two types of motions may be easily discriminated by dependences of the echo decay rates on the time delays between the pulses. The presence of cholesterol in lipid bilayers is found to suppress remarkably rotational motions, while on the contrary stochastic librations seem to become somewhat enhanced. These results evidence that cholesterol increases the long-time stability of lipid orientations in the bilayer, with simultaneous increase of fast fluctuations of these orientations. The former may be related to the known condensing effect of cholesterol and to raft formations, while the latter to the ordering effect.

Spin‐coupling and hyperfine interaction of the nitrogen donors in 6H‐SiC

AbstractThe X‐, Q‐band field‐sweep electron spin echo (FS ESE) study of n‐type 6H‐SiC wafers shows besides the well known hyperfine (hf) triplet lines of14N on two quasi‐cubic (Nc1, Nc2) and hexagonal (Nh) sites additional triplet lines (Nx) of comparatively low intensity with ag‐tensor of about the average value of the Nc2and Nhspectrum at 4.2 K. The Nxtriplet has half of the hf splitting with respect to the nitrogen residing at cubic sites Nc2. Pulsed electron nuclear double resonance (ENDOR) measurements performed on the NxEPR triplet line demonstrate that there is an efficient spin‐coupling between Nhand Nxnitrogen related centers. The Nxtriplet lines were attributed to distant donor Nc–Nhpairs between nitrogen atoms residing at quasi‐cubic and hexagonal sites having a total electron spinS= 1, which were found before in n‐type 4H‐SiC. Based on Q‐band FS ESE and X‐band pulsed ENDOR measurements the values of the hf interaction for Nc1, Nc2were reassigned with respect to the assignment accepted hitherto.The super hyperfine (shf) interaction with neighboring atoms of nitrogen on the quasi‐cubic lattice sites was studied by FS ESE and pulsed ENDOR. The new shf lines due to29Si and13C atoms with large shf interaction which were not observed previously were found in nitrogen FS ESE and ENDOR spectra. One of them, with the largest shf splitting (23.12 MHz), were attributed to the29Si atoms along thec‐axis in nearest‐neighbor positions of nitrogen on the quasi‐cubic sites which unambiguously confirms that nitrogen substitute carbon lattice site in 6H‐SiC. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and dynamics of S3− radicals in ultramarine-type pigment based on zeolite A: Electron spin resonance and electron spin echo studies

X-band electron spin resonance (ESR) spectra of S(3)(-) radicals in ultramarine analog (pigment) prepared from zeolite A and maintaining the original structure of parent zeolite were recorded in the temperature range of 4.2-380 K. Electron spin echo experiments (echo detected ESR, electron spin-lattice relaxation, and spin echo dephasing) were performed in the temperature range of 4.2-50 K. The rigid lattice g factors are g(x) = 2.0016, g(y) = 2.0505, and g(z) = 2.0355, and they are gradually averaged with temperature to the final collapse into a single line with g = 2.028 above 300 K. This is due to reorientations of S(3)(-) molecule between 12 possible orientations in the sodalite cage through the energy barrier of 2.4 kJ/mol. The low-lying orbital states of the open form of S(3)(-) molecule having C(2v) symmetry are considered and molecular orbital (MO) theory of the g factors is presented. The orbital mixing coefficients were calculated from experimental g factors and available theoretical orbital splitting. They indicate that the unpaired electron spin density in the ground state is localized mainly (about 50%) on the central sulfur atom of S(3)(-) anion radical, whereas in the excited electronic state the density is localized mainly on the lateral sulfur atoms (90%). A strong broadening of the ESR lines in directions around the twofold symmetry axis of the radical S(3)(-) molecule (z-axis) is discovered below 10 K. It is due to a distribution of the S-S-S bond angle value influencing mainly the energy of the (2)B(2)-symmetry MO. This effect is smeared out by molecular dynamics at higher temperatures. A distribution of the g factors is confirmed by the recovery of the spin system magnetization during spin-lattice relaxation measurements, which is described by a stretched exponential function. Both the spin-lattice relaxation and electron spin echo dephasing are governed by localized phonon mode of energy of about 40 cm(-1). Thus, the anion-radical S(3)(-) molecules are weakly bonded to the zeolite framework, and they do not participate in the phonon motion of the host lattice because of their own local dynamics.

Dynamical properties and instability of local fluoriteBaF2 structurearound doped Mn2+ ions—EPR and electron spin echo studies

The electron paramagnetic resonance (EPR) and electron spin echo (ESE) were measured at the X-bandfor Mn2+ in a BaF2 crystal in the temperature range 4.2–300 K. In addition to the cubic symmetry centre, twoother lower concentration tetragonal centres were identified. Temperature variations andcomputer simulation of the EPR spectrum confirm that the cubic symmetry of theMnF8 centre isdeformed to two Td tetrahedra of different dimensions at around 45 K. Electron spin relaxation was measured in thetemperature range 4.2–35 K, where the ESE signal was detectable. For higher temperature theMn2+ dynamics produces homogeneously broadened EPR lines. At the lowest temperaturesthe spin–lattice relaxation is governed by ordinary phonon processes with1/T1∼T5. The efficiency of these processes rapidly decreases and at about 11 K a local mode of energy17 cm−1 becomes the relaxation mechanism. Phase relaxation observed as ESE signaldephasing indicates that after the local deformation jumps (tunnelling with frequency4 × 108 s−1)between the two tetrahedral configurations appear, with the energy barrier being the local modeenergy. This motion is directly visible as a resonance-type enhancement of the ESE dephasing rate1/TM around 11 K. Only the cubic centre displays the above dynamics.

Electron spin relaxation and quantum localization in carbon nanoparticle: Electron spin echo studies

We present pulsed electron spin resonance measurements of activated carbon fibers (ACFs) for pristine samples and after adsorption of nitrobenzene in the fibers' porous structure. Two-level tunneling state (TLS) model is discussed in order to define the nature of paramagnetic centers and to explain the electron spin-lattice relaxation mechanism in the studied system. Electron spin echo decay is dominated by spin diffusion at low temperatures (below $7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) and for higher temperatures the echo dephasing rate is governed by thermal excitations of the TLS model. The model enables us to treat the nanographitic units being the basic structural units of ACF, as quantum wells (dots) separated by potential barriers of the order of $25(2)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ determined from temperature dependence of the spin-lattice relaxation rate. Stimulation of these barriers leads to spin localization within nanographitic units. A possible control of the potential barriers is the crucial point for the problem of nanographitic unit system treated as a quantum dot matrix.

EPR, ESE and Pulsed ENDOR Study of Nitrogen Related Centers in 4H-SiC Wafers Grown by Different Technologies

D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.

Electron spin-echo studies of spin-labelled lipid membranes and free fatty acids interacting with human serum albumin

Human serum albumin (HSA) is an abundant plasma protein that transports fatty acids and also binds a wide variety of hydrophobic pharmacores. Echo-detected (ED) EPR spectra and D 2O-electron spin echo envelope modulation (ESEEM) Fourier-transform spectra of spin-labelled free fatty acids and phospholipids were used jointly to investigate the binding of stearic acid to HSA and the adsorption of the protein on dipalmitoyl phosphatidylcholine (DPPC) membranes. In membranes, torsional librations are detected in the ED-spectra, the intensity of which depends on chain position at low temperature. Water penetration into the membrane is seen in the D 2O-ESEEM spectra, the intensity of which decreases greatly at the middle of the membrane. Both the chain librational motion and the water penetration are only little affected by adsorption of serum albumin at the DPPC membrane surface. In contrast, both the librational motion and the accessibility of the chains to water are very different in the hydrophobic fatty acid binding sites of HSA from those in membranes. Indeed, the librational motion of bound fatty acids is suppressed at low temperature, and is similar for the different chain positions, at all temperatures. Correspondingly, all segments of the bound chains are accessible to water, to rather similar extents.

Free radicals and their electron spin relaxation in cellobiose. X-band and W-band ESR and electron spin echo studies

By use of 9.7 GHz and 94 GHz ESR spectra and electron spin echo (ESE)-detected spectra the six radical centres produced by γ-irradiation of cellobiose were identified. The radicals are localized on different carbon atoms. Use of high-frequency ESR spectra with computer resolution enhancement methods enabled unique radical identification and determination of g-factors and proton hyperfine splitting, A, with high accuracy. For radiation doses below 20 kGy three radicals dominate: on C1 with isotropic doublet A = 1.8 mT; on C2, C3 and C4 with triplet A = 2.9 mT; and localized on CH2 with anisotropic triplet. For doses above 100 kGy the radical on C1 dominates, because of cleavage of the glycosidic bonds. Electron spin–lattice relaxation shows that radiation damage of the cellulose structure around the radical centres is significant and radical molecules do not participate in phonon dynamics of the host lattice. The relaxation is because of tunnelling motions of the ring or OH-groups, with tunnelling splitting 2.4 cm−1. Electron spin echo dephasing results identify cellobiose ring torsions with activation energy 117 cm−1.

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.

Temperature variation of the molecular structure of lithium hydrazinium sulphate—a one-dimensional proton conductor

The paper reports the results of detailed studies of the molecular structure of lithium hydrazinium sulphate single crystal in a wide temperature range with the aim to clarify previously reported anomalies in the molecular dynamics at ∼100 K. The crystal does not undergo any structural phase transition from 8 to 460 K however, the donor-acceptor distance, between the nitrogen atoms involved in the hydrogen bond along the c -orthorhombic axis, starts to increase from the temperature of ∼100 K. This results in an anomaly of the phase memory time observed in electron spin echo studies and anomalies of hydrazinium internal modes observed in Raman scattering. Moreover, the anisotropy of the SO 4 tetrahedra is maximal at ∼100 K and decreases at higher temperatures. The possible contribution of fast proton transport, described by the Grotthuss model, to the proton conductivity along the c -axis at the temperature above ∼400 K is discussed.

Studies on the mechanism of action of xanthine oxidase

Recent studies of the reaction mechanism of the molybdenum-containing enzyme xanthine oxidase are presented. The pH-dependence of both the steady-state and rapid reaction kinetics of the enzyme exhibits is bell-shaped, with pKas for the acid and alkaline limbs of 6.6 and 7.4, respectively. These are assigned to ionizations of an active site base and substrate, respectively, with the implication that enzyme acts on the neutral rather than monoanionic form of the purine substrate. A computational study provides evidence that in the course of the reaction tautomerization of substrate occurs, with a proton moving from N-3 to N-9 in the course of the reaction – enzyme facilitation of this tautomerization may contribute as much as 24 kcal/mol in transition state stabilization for the reaction. Electron spin echo (ESEEM) and electron-nuclear double resonance (ENDOR) studies of the so-called “very rapid” Mo(V) intermediate of the reaction, the latter work using a newly synthesized form of the substrate 2-hydroxy-6-methylpurine that has been selectively isotopically labeled at C-8, indicates that product is bound to the molybdenum of the active site in a simple, end-on fashion, consistent with a reaction mechanism involving nucleophilic attack of a (deprotonated) Mo–OH on the C-8 position of substrate. A kinetic study using a series of purines has failed to identify a correlation between the one-electron reduction potential for substrate and catalytic effectiveness, indicating that a reaction mechanism initiated by one-electron, outer-sphere electron transfer is unlikely. Finally, a consideration of the active site structure in the context of the above work suggests specific amino acid residues to target for site-directed mutagenesis studies. Preliminary experiments with two such mutants are entirely consistent with the proposed catalytic roles of two active site glutamate residues.

Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies

Diffusion and recombination of H atoms were studied in solid hydrogen containing ortho-H2 molecules at relative concentration Xo=0.001→0.75 using electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and electron spin echo (ESE) methods at around 4 K. When the rate-determining step for recombination is assumed to be the diffusion of H atoms, the rate constant for recombination at Xo⩾0.1 is consistent with the diffusion coefficient estimated from the analysis of ENDOR spectra and longitudinal spin relaxation behaviors. The recombination rate constant at Xo<0.1, however, is too slow to be explained using the diffusion coefficient estimated from longitudinal spin relaxation and forbidden spin-flip satellite transition studies. This result suggests that, even if one H atom finds another H in its immediate neighborhood, these H atoms do not react to form a H2 molecule at Xo≪0.1. The absence of recombination of H atoms is due to lack of the energy dispersion path required for the recombination of diatomic molecules. Since the absence of recombination becomes less significant at higher Xo, ortho-H2 molecules are found to play an important role in the energy dispersion which accompanies the recombination reaction.

Electron spin echo study of long-lived radicals which cause mutation in γ-ray irradiated mammalian cells

Long-lived radicals, produced by γ-ray irradiation of mammalian cells at room temperature, cause mutation and morphological transformation in the cells. The local environment near the long-lived radicals in irradiated cells was investigated here by the analysis of electron spin echo envelope modulation (ESEEM) spectra. The number of hydrogen (deuterium) atoms surrounding the long-lived radical, which may correspond to the number of water molecules, was estimated roughly as one or two. It is postulated that the long-lived radicals are generated in the interior of biopolymers. The radicals are not produced by the reaction of OH radicals, but mainly by the decomposition of biopolymer which absorbed directly the energy of the ionizing radiation.

Libration motion of guest spin probe molecules in organic glasses: CW EPR and electron spin echo study

Echo-detected (ED) EPR spectra of nitroxide spin probes dissolved in glassy materials provide evidence that guest molecules in these media undergo fast librational motion. Theory of spin relaxation of a librating molecule is presented. The mean squared amplitude, 〈 α 2〉, of this motion which can be derived from continuous wave (CW) EPR spectral splitting is found to depend linearly on temperature in the low temperature region. This may be ascribed to thermal harmonic vibrations. The slope of the linear dependence varies from glass to glass and seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Above the glass transition temperature 〈 α 2〉 increases sharply. Different applications are discussed: study of molecular properties of glass, intracellular glass formation in plant tissues, structural investigations.