Single-crystal ESR studies of a nitroxide radical

Summary ESR spectra are reported for a stable nitroxide radical 4-hydroxy-2, 2, 6, 6-tetramethyl-piperidine-1-oxyl (TANOL) introduced as impurity in a matrix of known structure, 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine atX- andQ-band. The use of the perdeuterated nitroxide showed a considerable improvement in ESR spectra resolution for most orientations in the crystal. The interactions of protons in the neighbourhood of the paramagnetic group N−O were studied and the superhyperfine tensors for the two strongly coupled protons were determined: one of these protons is hydrogen-bonded to the N−O at a distance of 2.09 ?. The principal values of the tensor are −5.2; +5.4; −5.1 G. The second proton is further away from the electron and is certainly not involved in hydrogen bond. The principal values are in this case 2.0, −1.7, −2.7 G. Thea crystal axis is a principal direction for all magnetic tensors for TANOL in this system.

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Riassunto Si riportano gli spettri ESR per un radicale nitrossido stabile 4-idrossi-2, 2, 6, 6-tetrametil-piperidina-1-ossile (TANOL) introdotto come impurità in una matrice di struttura nota, 4-idrossi-2, 2, 6, 6-tetrametilpiperinadi (1) nelle bandeX eQ. L'uso del nitrossido predeuterato ha mostrato un considerabile miglioramento nella risoluzione degli spettri ESR per moltissime orientazioni nel cristallo. Si studiano le interazioni dei protoni nei pressi del gruppo paramagnetico N−O e si determinano i tensori superiperfini per due protoni con accoppiamento forte: uno di questi protoni è legato con l'idrogeno al N−O ad una distanza di 2.09 ?. I valori principali del tensore sono −5.2; +5.4; e −5.1 G. Il secondo protone è molto spostato dall'elettrone ed è certamente non coinvolto in un legame a idrogeno. I valori principali sono in questo caso 2.0, −1.7, −2.7 G. L'assea del cristallo à una direzione principale per tutti i tensori magnetici per il TANOL in questo sistema.

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Резюме Приводятся ESR спектры для устойчивого радикала окиси сзота TANOL, введенного, как примесь, в матрицу известной структуры вX- иQ-зону. Использование передейтерированной окиси азота обнаруживает значительное улучшение разрешения в ESR спектрах для большинства ориентаций в кристалле. Исследуется взаимодействие протонов в окрестности параметрической группы N−O. Определяются сверхтонкие тензоры для двух сильно связанных протонов: один из этих протонов представляет водород, связанный с N−O на расстоянии 2.09 ?. Главные значения этого тензора представляют −5.2, +5.4, −5.1 гаусс. Второй протон находится довольно далеко от электрона и не включается в водородную связь. В этом случае главные значения тензора равны 2.0, −1.7, −2.7 гаусс. Кристаллическая осьa представляет главное направление для всех магнитных тензоров для TANOL в этой системе.

     

Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction

A series of C₆₀ fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid solution, for both the ground electronic state and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radicaltriplet interaction producing spin polarization. Opposite initial sign and polarization patterns are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet decay constants, quartet and doublet spin-lattice relaxation rates and branching ratios.     

The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals

The sign of the exchange interactionJ in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign ofJ depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution toJ arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D* of the RTP, which does not take place in the excited quartet state Q*. The mixing of D* and Q* states lowers the energy of the former spin state and gives antiferromagnetic coupling.     

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.     

EPR and ENDOR study of porphyrins and their covalently linked dimers in the photoexcited triplet state

The triplet states of several substituted porphyrins (Tetraphenylporphyrin (H₂TPP), Zinc-Tetramethylporphyrin (ZnTMP), Octaethylporphyrin (H₂OEP) and the Dication of H₂TPP (H₄TPP²⁺)) and two covalently linked dimers with H₂TPP-subunits in disordered solid solution were studied by EPR and ENDOR at liquid helium temperature. The measurement yields theA _zz component of the hyperfine tensors of all α-protons in the reference frame of the zero field splitting tensor. Dipolar and isotropic contributions toA _zz are discussed and spin densities derived. The spin densities are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/S). One of the dimers shows indications of triplet energy transfer between the porphyrin subunits. The order of magnitude of the transfer rate is estimated to be 5 · 10⁵ s^?1.     

A spatiotemporal study on the distribution of intraperitoneally injected nitroxide radical in the rat head using an in vivo ESR imaging system

We have developed a rapid-scan in vivo electron spin resonance (ESR) imaging system operating at 700 MHz based on an air-cored two-coil Helmholtz designed resistive magnet. Using this system, we performed ESR-CT for the intraperitoneally injected nitroxide radical, 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl, in the rat head. The imaging data were collected over the time course range from 5 to 47 min after injection at an interval of 3 min and a series of ESR-CT images were reconstructed at the same slice plane (1 cm anterior to interaural line). The series of ESR-CT images thus obtained by rapid scans provided detailed spatiotemporal information on the distribution of the injected nitroxide radical in the rat head. The brain was imaged as a nitroxide-deficient area while the blood vessels and/or extracranium tissues as a nitroxide-rich area. During periods when high intensities of ESR signals were maintained, spots of nitroxide-accumulation were imaged at the central part of the brain. The spots were assigned to the middle sized blood vessels in the brain.     

A CT using longitudinally detected ESR (LODESR-CT) of intraperitoneally injected nitroxide radical in a rat's head

We have developed an in vivo longitudinally detected ESR (LODESR) imaging system operating at 700 MHz based on a loop-gap resonator and a pair of saddle-type pickup coils. A good linear relationship between the LODESR signal intensity and the applied power in a range up to 15.8 W was obtained. The detection of LODESR signals was barely affected by variations in the resonant properties. The characteristic of LODESR is suitable for in vivo examination. Using this system, we succeeded in obtaining LODESR-CT images of the head region of a rat after the intraperitoneal injection of a nitroxide radical.     

The S-J exchange interaction under the influence of the crystalline field

A magnetic impurity, having the s-j interaction with conduction electrons, under the crystalline field of host metals is investigated. The Hamann integral equation for the t-matrix can be derived and solved by the method of Zittartz and Müller-Hartmann for the cases of (1) small splittings, yielding the Kondo anomaly and decrease of the Kondo temperatureT _K compared with no splittings, (2) a large splitting from the crystalline ground doublet state, producing the higherT _K by the exicited level, and (3) a large splitting from the ground singlet state giving no Kondo effect. The macroscopic properties are calculated for (1) and (2).     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)₂, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g _‖=2.1025,g ₊=2.031,A=75.1×10⁻⁴ cm⁻¹,B=14.8×10⁻⁴,D=276.0×10⁻⁴ cm⁻¹ andE=46.7×10⁻⁴ cm⁻¹. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm⁻¹. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)₂—direct exchange and superexchange through the bridging sulphurs—are discussed.     

Numerical results on the indirect exchange interaction in crystalline and amorphous metals

Computer simulation of the indirect exchange interaction is carried out for a single tight-binding s-band in ordered (simple cubic, body-centered cubic, face-centered cubic) and amorphous (relaxed dense-random-packed) structures. All cases show very similar features in the spatial dependence of the interaction, with no attenuation being observed in the amorphous structure. The structural dependence of the phase coherence of the electron wavefunctions is analysed. Effects due to alloying are briefly discussed.     

Determination of exchange constants of 2-D magnets from ESR in the paramagnetic state

Values of the exchange constant, J, of 2-dimensional magnets are obtained semi-quantitatively from measurements of the anisotropy of ESR line widths in the paramagnetic phase. The theory was reformulated to facilitate comparison with experiment. For cases in which J is known from the susceptibility, the agreement with our ESR derived values is within 30%. For the monolayer magnet, manganese stearate, a value of J/k = 0.5 K is found. Comparison of J to ordering (Neel) temperatures is made.     

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.     

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

A study on electron spin resonance (ESR) spectroscopic determination of exchange interactions in organic oligoradicals is given. When the intramolecular exchange couplingJ between, unpaired electron spins in nitroxide-based oligoradicals falls within the order of 10 Oe (1 mK or 10^?3 cm^?1 forg=2), which is on the same order as the hyperfine couplingA of magnetic nuclei such as nitrogen atoms of nitroxide radicals, the magnitude ofJ can be determined from the hyperfine splitting pattern of ESR spectra in solutions. This range of the exchange couplingJ is not detectable in conventional magnetic susceptibility measurements. We demonstrate an application of hyperfine ESR spectroscopy as a probe for the exchange coupling to a series of organic oligoradicals, which the authors have recently developed as building blocks for molecule-based magnetic materials.     

Synthesis and electron paramagnetic resonance study of a nitroxide free radical covalently bonded on aminopropyl-silica gel

A solid spin-labeled material was obtained starting from 2-chloro-3,5-dinitro-N-(4-(2,2,6,6-tetramethyl-piperidine-1-oxyl)-benzamide) and aminopropyl-silica gel. Stability tests showed that even after several months the spin-labeled material had the same properties as immediately after synthesis. EPR properties of the TEMPO-derivatized silica were studied as a function of solvent polarity and temperature. Rotational correlation times were calculated from EPR spectra and correlated with solvent characteristics and temperature. Polar solvents induce a fast motion of the spin-label, clearly seen in the EPR spectra by the apparition of the well-known TEMPO radical triplet. The solid spin-labeled (dry) sample showed a high interspin interaction, which can be disrupted not only by different (liquid) solvents, but also by absorption of different solids, like cyclodextrins, dendrimers or polyethyleneglycols. Also, changes induced by the temperature were studied in the case of toluene wet sample. From 150 to 370 K, the spectrum is changing from a slow motion spectrum type to a fast motion regime. The preparative procedures to obtain the spin-labeled silica as well as some of its parameters are described.     

Kinetics of nitroxide spin label removal in biological systems: An in vitro and in vivo ESR study

A systematic study on the disappearance of the electron spin resonance (ESR) signal of nitroxides based on six- or five-membered ring and bearing various charges was carried out in vitro and in vivo. The second-order kinetic rate constants of the reaction of spin probes with ascorbate were measured in vitro at various temperatures in phosphate buffered saline, and the relative activation energies were calculated. Clearance rates of the nitroxide radicals in rat brain homogenates and in blood indicate that the ascorbate contribution to nitroxide removal is about 50–70% in brain and 50–90% in blood. These rates can be easily calculated on the basis of the ascorbate concentration and of the second-order kinetic rate constants measured in phosphate buffered saline. ESR spectra acquired in vivo in rat head and tail, by an L-band resonator, indicated that the nitroxide decay rate is a first-order kinetic process in both domains and that the positively charged nitroxides are not retained in the brain, whereas the anionic and uncharged nitroxides are. Once nitroxides with piperidine ring enter the brain, their decay appears controlled mainly by ascorbate, while the ascorbate has a negligible influence on disappearance in brain of five-membered ring proxyl nitroxides.     

ESR study of the ionic radical species in an irradiated single crystal of isocytosine

The radicals in a single crystal of isocytosine, irradiated with gamma rays at 77 K, were studied with the ESR spectroscopy. Three types of radicals were detected. One of them, whose ESR spectrum exhibits two¹⁴N and two¹H coupling nuclei, is interpreted to be the isocytosine cation radical. The second species, characterized by a doublet structure, is thought to be formed by a proton transfer to the isocytosine anion. The third species could not be analysed in detail. All three species are unstable upon warming or upon illumination with green Ar laser light.     

Dynamical study of the possible molecular state X(3872) with the s-channel one gluon exchange interaction

The recently observed X(3872) resonance, which is difficult to assign a conventional c charmonium state in the quark model, may be interpreted as a molecular state. Such a molecular state is a hidden flavor four quark state because of its charmonium-like quantum numbers. The s-channel one gluon exchange is an interaction which only acts in the hidden flavor multi-quark system. In this paper, we will study the X(3872) and other similiar hidden flavor molecular states in a quark model by taking into account the s-channel one gluon exchange interaction.