Electron spin exchange processes in strongly coupled spin triads

The complexes of Cu2+ hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides are the choice systems to study the spin dynamics of strongly exchange-coupled spin triads. The large values of exchange coupling (ca. 100 cm-1) and high-resolution electron paramagnetic resonance (EPR) at Q- and W-bands (35 and 94 GHz) allowed us to observe and interpret specific characteristics of these systems. An electron spin exchange process has been found between different multiplets of the spin triad, which manifests itself as a significant shift of the EPR line position with temperature. We propose that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations. The estimations of the rate of exchange process and model calculations essentially support the observed phenomena. The studied characteristics of strongly coupled spin triads explain previously obtained results, agree with literature, and should be accounted for in future investigations of similar spin systems.     

Temperature-dependent exchange interaction in molecular magnets Cu(hfac)2L(R) studied by EPR: methodology and interpretations

Exchange-coupled spin triads nitroxide-copper(II)-nitroxide are the key building blocks of molecular magnets Cu(hfac)(2)L(R). These compounds exhibit thermally induced structural rearrangements and spin transitions, where the exchange interaction between spins of copper(II) ion and nitroxide radicals changes typically by 1 order of magnitude. We have shown previously that electron paramagnetic resonance (EPR) spectroscopy is sensitive to the observed magnetic anomalies and provides information on both inter- and intracluster exchange interactions. The value of intracluster exchange interaction is temperature-dependent (J(T)), that can be accessed by monitoring the effective g-factor of the spin triad as a function of temperature (g(eff)(T)). This paper describes approaches for studying the g(eff)(T) and J(T) dependences and establishes correlations between them. The experimentally obtained g(eff)(T) dependences are interpreted using three different models for the mechanism of structural rearrangements on the molecular level leading to different meanings of the J(T) function. The contributions from these mechanisms and their manifestations in X-ray, magnetic susceptibility and EPR data are discussed.     

Recent progress in EPR study of spin labeled polymers and spin probed polymer systems

The EPR technique is commonly used for the detection and characterization of paramagnetic centers in chemical science. This method can provides a lot of information, such as identity, structure, dynamics, interaction, orientation, glass transition temperature, adsorption behavior, functionality, phase behavior, nano-inhomogeneities, and conformation of the free-radical portion of the polymer chain. Most polymers intrinsically possess diamagnetic properties, so in order to study polymers with EPR, paramagnetic centers need to be incorporated into the polymer systems. Spin labeling and spin probing are main methods of covalently attaching paramagnetic centers to polymer chains or embedding them in polymer matrices through non-covalent interactions, respectively. Spin labeling and spin probing techniques for polymers and polymer systems (especially with nitroxide radicals) have also been studied, which have a profound impact on polymer science. This review focuses on the continuous wave EPR technique and introduces the recent advances in spin labeled polymers and spin probed polymer systems in EPR research.     

Fast photodriven electron spin coherence transfer: a quantum gate based on a spin exchange j-jump

Photoexcitation of the electron donor (D) within a linear, covalent donor-acceptor-acceptor molecule (D-A(1)-A(2)) in which A(1) = A(2) results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, (1)(D(+?)-A(1)(-?)-A(2)), for which the spin-spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A(1)(-?) during the lifetime of (1)(D(+?)-A(1)(-?)-A(2)) rapidly produces (1)(D(+?)-A(1)-A(2)(-?)), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes (1)(D(+?)-A(1)-A(2)(-?)) with (3)(D(+?)-A(1)-A(2)(-?)), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.     

Low temperature EPR spectra of the mesoscopic cluster V15: the role of antisymmetric exchange

The low temperature EPR spectra of the unique nanometer-scale molecular magnet V(15) are analyzed within the three-spin model preserving trigonal symmetry. The Hamiltonian includes isotropic and antisymmetric (AS) exchange interactions introduced by Dzyaloshinsky (Zh. Eksp. Teor. Fiz. 32, 1547 (1957) [Sov. Phys. JETP 5, 1259 (1957)]) and [Moria Phys. Rev. 120, 91 (1960)]. With the aid of pseudoangular momentum representation the exact selection rules for the EPR transitions are deduced. AS exchange in the frustrated triangular system gives rise to a first order zero-field splitting of two low lying spin doublets and to a second order splitting of the excited quadruplet. This leads to a peculiar series of strong intramultiplet transitions and weak intermultiplet transitions. We show that the intramultiplet transitions are allowed only when the vector of the AS exchange is normal to the plane of vanadium triangle, but the series of weak intermultiplet transitions are a consequence of the in-plane part of the AS exchange. We have revealed a special temperature dependence of the EPR pattern with a peculiar shape of the full spectrum. Experimental data on high-frequency EPR of V15 at ultralow temperature are discussed.     

PELDOR on an exchange coupled nitroxide copper(II) spin pair

Transition metal ions play an important role in the design of macromolecular architectures as well as for the structure and function of proteins and oligonucleotides, which makes them interesting targets for spectroscopic investigations. In combination with site directed spin labelling, pulsed electron–electron double resonance (PELDOR or DEER) could be a well-suited method for their characterization and localization. Here, we report on the synthesis and full characterization of a copper(II) porphyrin/nitroxide model system bearing an extended π-conjugation between the spin centres and demonstrate the possibility to disentangle the dipolar through space interaction from the through bond exchange coupling contribution even in the presence of orientational selectivity and conformational flexibility. The simulations used are based on the known experimental and spin Hamiltonian parameters and on a structural model as previously employed for similar systems. The mean exchange coupling of +4(1) MHz (antiferromagnetic) is in agreement with the value of |J| = 3(1) MHz determined from room temperature continuous wave electron paramagnetic resonance (EPR). Thus, as long as the pulse excitation bandwidths are large versus the spin–spin coupling, X-band PELDOR measurements in combination with explicit time trace simulations allow for disentangling the sign and magnitude of through bond electron–electron exchange from the through space dipolar interaction D.     

EPR spin flip transitions in some metal dithiolenes

Small line widths (ΔB_pp ≅ 0.25–0.40 mT) in the EPR spectra of ⁶³Cu²⁺ in the matrices of some transition metal dithiolenes permit the observation of not-so-easily realizable proton spin flip transitions even at room temperature and at low microwave power. These transitions, studied in four different lattices as a function of crystal orientation, microwave power and temperature, have been found to be from the protons of the counter ions and a few ångströms away from the unpaired electron-containing metal centre. It has been possible to identify the protons and estimate their distances from the metal centre by two different methods. A temperature-dependent evaluation of the distance parameters as well as the observation of the “double” and “triple” flips are reported. The differential saturation of the “main” and “satellite” (due to spin flip) lines studied by the CW saturation method has provided information on the mechanism of spin relaxation in these systems.     

Stearic acid spin labels in lipid bilayers: insight through atomistic simulations

Spin-labeled stearic acid species are commonly used for electron paramagnetic resonance (EPR) studies of cell membranes to investigate phase transitions, fluidity, and other physical properties. In this paper, we use large-scale molecular dynamics simulations to investigate the position and behavior of nitroxide spin labels attached to stearic acid molecules in dipalmitoylphosphatidylcholine (DPPC) bilayers. The results of these studies are potentially very important for the interpretation of EPR spectra, which rely on assumptions about the position of the label in the membrane. Additionally, we investigate the effect of chirality and ionization of the carboxyl group of the label. For a non-ionized species, we observe that spin-label molecules are even able to make flip-flop transitions between the leaflets of the bilayer. Such transitions have been previously observed only in very rare cases in molecular simulations.     

Discrepancy between the spin distribution and the magnetic ground state for a triaminoxyl substituted triphenylphosphine oxide derivative

The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.     

Investigation of the incommensurate and commensurate magnetic superstructures of LiCuVO4 and CuO on the basis of the isotropic spin exchange and classical spin approximations

The spin lattices of magnetic oxides LiCuVO(4) and CuO are made up of CuO(2) ribbon chains. The incommensurate and commensurate magnetic superstructures of these oxides were examined by calculating the total spin exchange interaction energies of their long-range order spin arrangements on the basis of the isotropic spin exchange and classical spin approximations. The incommensurate superstructure (0, 0.532, 0) of LiCuVO(4) was analyzed to find that the next-nearest-neighbor spin exchange interaction J(nnn) is more strongly antiferromagnetic than the nearest-neighbor spin exchange interaction J(nn) in the CuO(2) chains. With this finding, we reassessed the relative strengths of the spin exchange interactions of LiCuVO(4) and CuO and then analyzed the relative energies of their long-range order spin arrangements. The incommensurate superstructure (0, 0.532, 0) of LiCuVO(4) is explained when the J(nn)/J(nnn) ratio is -0.40. Both the incommensurate superstructure (0.506, 0, -0.483) and the commensurate superstructure (0.5, 0, -0.5) of CuO, which occur at 231 and 212.5 K, respectively, are well explained in terms of the calculated total spin exchange interaction energies. The incommensurate superstructure of CuO becomes commensurate by a slight change in one interchain spin exchange interaction, which is due probably to a slight structure change brought about by the temperature lowering.     

Energetics of binuclear spin transition complexes

The electronic structures of five binuclear iron(II) complexes, four of which display spin transitions between the low-spin (LS) and high-spin (HS) electronic states, are studied by density functional theory (DFT) calculations. Three electronic states, corresponding to [LS-LS], [LS-HS], and [HS-HS] electronic configurations, are characterized. The nature of the ground state agrees with the experimentally observed magnetic state of complexes stabilized at low temperatures. The results of the calculations agree with the conclusion of the phenomenological model, that the enthalpy of the [LS-HS] state must be lower than the average enthalpy of the [LS-LS] and [HS-HS] states, to create conditions for a two-step spin transition. The exchange parameters between Fe(II) ions in the [HS-HS] states are evaluated. It is shown that all complexes are weakly antiferromagnetic and the synergy between two spin transition centers is mainly of elastic origin.     

Optically detected ESR and low magnetic field signals from spin triads: 2-imidazoline-1-oxyl derivatives in X-irradiated alkane liquids as a method to study three-spin systems

This contribution reports the design and synthesis of a series of spin-labeled charge acceptors to produce three-spin systems of "radical ion/biradical ion" type in X-irradiated alkane liquids. This opens the way to study spin triads in experimental conditions, in which short-lived radical ion pairs are conventionally studied, thus offering optically detected techniques such as magneto-resonance OD ESR and level-crossing MARY spectroscopy. The structure of the synthesized 2-imidazoline-1-oxyl derivatives is A-Sp-R, where A is a positive or negative charge acceptor, R is a stable radical, and Sp is a hydrocarbon bridge. The set of 20+ compounds represent a convenient tool to construct experimental three-spin systems with various properties, e.g. with the "third" spin introduced into one or the other partner of the radical ion pair. The degree of exchange coupling between the two paramagnetic fragments in the biradical ion has been demonstrated to strongly depend on the type of the radical fragment R and the structure of the bridge Sp. As a result, a series of acceptors with systematically reduced exchange interaction has been synthesized, and optimal systems for the observation of low magnetic field effect have been found. In the most favorable case, an OD ESR signal from a spin triad living as short as ca. 100 ns has been registered as a single unresolved line. The exchange integral for this biradical anion (9) was estimated from OD ESR and ESR experiments to be ca. 10(3) G by the order of magnitude, which is much greater than the hyperfine couplings in the biradical ion but much smaller than the thermal energy kT.     

Using spin dynamics of covalently linked radical ion pairs to probe the impact of structural and energetic changes on charge recombination

We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.     

Isolation and characterization of four isomers of a C(60) bisadduct with a TTF derivative. Study of their radical ions.

A family of triads composed of C(60) attached by a rigid spacer to two identical TTF moieties has been synthesized, and some of the isomers have been isolated and characterized by UV-vis spectroscopy, LDI-TOF-MS, and HMBC NMR spectroscopy. AM1 semiempirical calculations of the dipolar moments and the heats of formation of the different isomers have been carried out in order to verify their assignments. Oxidation and reduction of the triads affords the derived radical ion systems, TTF(+*)-C(60)-TTF(+*) and TTF-C(60)(-*)-TTF, which were studied by EPR spectroscopy. Spin density distributions of these radical cations and radical anions show that the unpaired electron is located mainly on the TTF and fullerene moieties, respectively. However, while the EPR signals obtained from the radical cations are very similar for all the isomers, the structured signals observed for the radical anions arising from the coupling of the unpaired electron with the hydrogen atoms of the methylene bridges in the spacer show that there is a strong influence of the isomerism on the spin distribution. Importantly, the theoretical calculations of the spin density distributions of the radical anions fit well with the experimental EPR results.     

Effects of cation siting and spin–spin interactions on the electron paramagnetic resonance (EPR) of Cu exchanged X Faujasite zeolite

Copper(II) exchanged Na X Faujasite zeolite was cation exchanged at levels from one Cu(II) in 30 unit cells (0.033 Cu(II)/UC) to 38 Cu(II) per unit cell (38 Cu/UC) and was examined by continuous wave and two-pulse and three-pulse electron paramagnetic resonance (EPR) at temperatures from 10 K to 300 K. In this work exchange of Cu²⁺ into X Faujasite zeolite is shown by EPR spectral and pulsed EPR relaxation measurements to begin into site I′, where it lies coordinated to a hexagonal prism face with Si:Al ratios of predominantly 4:2 and 5:1. Spin–spin interactions influence EPR g-value averaging, spin–spin relaxation, and spin spectral diffusion in a manner highly dependent on Cu exchange. Spin–lattice relaxation is relatively independent of exchange. The marked increase observed in spin–spin relaxation and g-value averaging at 8 Cu/UC and an effective Cu–Cu distance of 1.2 nm can be understood in terms of filling sodalite cages with an average of 1 Cu²⁺ each.     

Time evolution of a single spin inhomogeneously coupled to an interacting spin environment

We study the time evolution of a single spin coupled by exchange interaction to an environment of interacting spin bath modeled by the XY Hamiltonian. By evaluating the spin correlator of the single spin, we observed that the decay rate of the spin oscillations strongly depends on the relative magnitude of the exchange coupling between the single spin and its nearest neighbor J(') and coupling among the spins in the environment J. The decoherence time varies significantly based on the relative coupling magnitudes of J and J('). The decay rate law has a Gaussian profile when the two exchange couplings are of the same order J(') approximately J but converts to exponential and then a power law as we move to the regimes of J(')>J and J(')相似文献   

Forbidden matrix proton spin flip satellites in trapped electron EPR spectra

Observation of the 35 GHz EPR spectrum of γ-irradiated 10 M NaOH/H₂O and 10 M NaOD/D₂O glassy ices at 77°K has revealed proton spin flip satellites associated with the trapped elctron EPR line. This suggests that forbidden satellite transitions contribute to the lineshape of the trapped electron line in the 9 GHz spectrum which has commonly been studied.     

Net and multiplet CIDEP of the observer spin in recombination of radical-biradical pair

Net and multiplet chemically induced dynamic electron polarization (CIDEP) of the observer/catalyst spin formed in recombination of the radical-biradical pair is studied theoretically. We obtained analytical expressions for the observer spin CIDEP in the high magnetic field and for the multiplet polarization in zero magnetic field. Polarization in the vicinity of the so-called J resonance and its magnetic field dependence are investigated numerically. The observer spin methodology can be useful for probing magnetic interactions in the short-lived spin triads.     

X- and Q-band EPR studies of the dinuclear Mn(II) complex [(Bpmp)Mn2(mu-OAc)2]+. Determination of the spin parameters for the S = 1 and S = 2 spin states

A new mu-phenoxo-bis-mu-acetato di-Mn(II) complex using the BpmpH ligand was isolated as a perchlorate salt (BpmpH = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenol). The X-ray structure has been solved showing that the two Mn(II) ions are in a distorted octahedral environment. Investigation of the variation of the molar magnetic susceptibility upon temperature reveals an antiferromagnetic exchange interaction between the two high-spin Mn(II) ions. Fitting of the experimental data led to g = 1.99 and J = 9.6 cm(-1) (H(HDvV) = JS(A).S(B)). EPR spectra recorded on a powder sample of [(Bpmp)Mn(2)(mu-OAc)(2)](ClO(4)).0.5H(2)O at X-band between 4.3 K and room temperature and at Q-band between 5 and 298 K are presented. A new method based on a scrupulous examination of the variation upon temperature of these experimental spectra is developed here to first assign the transitions to the relevant spin states and second to determine the associated spin parameters. This approach is compared to the deconvolution process that has been previously applied to dinuclear Mn(II) complexes or metalloenzyme active sites. Crystallographic data is as follows: triclinic, space group P one macro, a = 10.154(2) A, b = 12.0454(2) A, c = 17.743(4) A, alpha = 101.69(3) degrees, beta = 93.62(3) degrees, gamma = 94.67(3) degrees, Z = 2.     

Time resolved electron spin resonance of spin correlated micelle confined radical pairs: shape of the anti-phase structure

A new approach to the theory of the EPR of spin correlated radical pairs (SCRP) is formulated. It is based on spectral exchange methods as applied to the two-site model and takes into account explicitly both the motion of one quantum (transverse magnetization) and two quantum coherences (polarizations) of SCRPs. This innovation allows for the interpretation of anti phase structure (APS) spectral shape asymmetry and for the transformation of the initially created ST0RPM polarization into the APS spectral pattern. The results of this method as applied to micellized spin correlated radical pairs have been compared with the exact numerical solution of the corresponding stochastic Liouville equation and is commented on.