V. K. Voronkova

The reverse shift of the EPR line of paramagnetic centers coupled to species with a fast paramagnetic relaxation

The EPR spectrum of the spin 1/2 paramagnetic centers with a relatively slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under these conditions only the EPR line of paramagnetic centers with a relatively slow relaxation is detectable in experiment. The shape of this line is analyzed by solving numerically kinetic equations for the spin density matrix for simple model systems. Depending on a ratio between the exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxation rates, as the relaxation rates decrease, the EPR line shifts toward the gravity center of the total EPR spectrum. In the case of extremely fast relaxation, as the relaxation rates decrease, the reverse shift of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd2(C4O4)4(H2O)16] · 2H2O when relaxation rates were changed by temperature variation.

Unusually large values of the E.P.R. spectra fine structure parameter of Cu(II) dimers with two-bridge exchange mechanisms

E.P.R. study of related dimeric complexes (C5H12N)2[Cu2(OCH3)2 {2,4-NO2)2C6H3O}4]2CH3OH(I) and (C5H12N)2[Cu2(OCH3)2{2,4-(NO2)2 C6H3O}4](II) in X- and Q-bands in the temperature range 292-1·65 K has been carried out. The unusual magnetic properties : (i) the difference of the ground state isotropic exchange for these complexes, J I ≲ 0, J II = 130 cm-1, in spite of the practically unchanged structure of the dimeric fragment; (ii) nearly coincident values of the spin triplet fine structure parameter, |D I| = 1·27 cm-1, |D II| = 1·26 cm-1; (iii) the largest yet observed |D| values in Cu(II) dimers non-correlating with the ground state isotropic exchange, are reported. The value of the exchange interaction in the excited dimer J x 2-y 2,xy (I) = -330 cm-1 is estimated. The large ferromagnetic value of J x 2-y 2,xy is accounted for by the two-bridge exchange mechanisms associated with the virtual electron transfer along the Cu2O2 ring. The change of the isotropic exchange in the ground state when going from I ...

Membrane-Sugar Interactions Probed by Pulsed Electron Paramagnetic Resonance of Spin Labels.

Sugars can stabilize biological systems under extreme desiccation and freezing conditions. Hypothetical molecular mechanisms suggest that the stabilization effect may be determined either by specific interactions of sugars with biological molecules or by the influence of sugars on the solvating shell of the biomolecule. To explore membrane-sugar interactions, we applied electron spin echo envelope modulation (ESEEM) spectroscopy, a pulsed version of electron paramagnetic resonance (EPR), to phospholipid bilayers with spin-labeled lipids added and solvated by aqueous deuterated sucrose and trehalose solutions. The phospholipids were 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The spin-labeled lipids were 1,2-dipalmitoyl-sn-glycero-3-phospho(TEMPO)choline (T-PCSL), with spin-label TEMPO at the lipid polar headgroup. The deuterium ESEEM amplitude was calibrated using known concentrations of glassy deuterated sugar solvents. The data obtained indicated that the sugar concentration near the membrane surface obeyed a simple Langmuir model of monolayer adsorption, which assumes direct sugar-molecule bonding to the bilayer surface.

MIXED-VALENCE FE(III)-FE(IV) DIMERS. PREPARATION AND PHYSICAL PROPERTIES OF OXIDATION PRODUCTS OF OXO-BRIDGED BINUCLEAR IRON(III) COMPLEXES

Abstract Polycrystalline samples of the oxidation products of nine oxo-bridged binuclear iron(III) complexes [Fe2L2O]+[I3]−X (I–VIII) and [Fe2L2(OH)2]+ (BF4)− (IX), where L2− = some substituted tetraisothiosemicarbazide ligands, X = iodine or iodinechloroform solvates, are investigated over the temperature range 4.2–300 K using EPR, Mossbauer and magnetic susceptibility measurements. The results of the investigation show that oxidation of the μ-oxo [Fe(III)2L2O] dimers gives the mixed-valence Fe(III)Fe(IV) dimers. The 57Fe Mossbauer spectra of these compounds in the temperature range 300-80 K exhibit a single quadrupole doublet which indicates delocalization of the ‘extra’ electron between iron sites, at least within the Mossbauer time scale of about 10−7 s. The presence of the antiferromagnetic exchange interaction in these dimers is evidenced by variable-temperature (4–300K) magnetic susceptibility data. These data are least-squares fit to the theoretical equation for a spin Hamiltonian which includes the isotropic exchange interaction between two iron centers (JS1S2,J ∼ 30–50 cm−1), the axial zero-field splitting of the iron centers with the parameter D ∼ 10 cm−1, and the interdimer exchange interaction (|z′Jinter| ∼ 1–9 cm−1). The ground state with S = 1 2 for all dimers is proved by the value of the magnetic moment at 4.2 K. Practically symmetric signals with g values ∼ 2.0 are observed in X- and Q-band EPR spectra at room temperature. Analysis of the temperature dependence of the spectra detected in the temperature range 100–300 K for I–VIII and 300-40 K for IX compounds showed that the spectra are due to transitions within the excited spin states with the total quantum number S > 1 2 . The linewidth increases as the S value decreases and the signal due to S = 1 2 is not detected. It is proposed that the observed peculiarities of the EPR spectra are connected with the intramolecular electron transfer which is accompanied by some fluctuations of the dimer structure. The absence of the signals at low temperature indicates that the period of this fluctuation coincides with the EPR time scale (10−10 s).

Exchange interaction at the supramolecular level. EPR investigation of two copper (II) compounds: [Cu2(acac)2(phen)2(bpe)](CIO4)2·(bpe)·CH3CN·H2O and [Cu2(acac)2(phen)2(bpp)](CIO4)2·6H2O (bpe=trans-1,2-bis(4-pyridyl)ethylene, bpp=bis(4-pyridyl)propane)

Intra- and intermolecular exchange and dipole-dipole interactions in two supramolecular compounds [Cu2(acac)2(phen)2(bpe)](CIO4)2·(bpe)·CH3CN·H2O (I) and [Cu2(acac)2(phen)2(bpp)]× (CIO4)2·6H2O (II), which are built up of binuclear fragments through π-π stacking interactions, are investigated. The electron paramagnetic resonance (EPR) spectra of the polycrystalline samples of I and II were measured in the X-band in the temperature range of 300–4.2 K, and in the Q-band atT=300 and 4.2 K. The EPR spectra were interpreted as being due to weakly interacting dimer fragments. Triplet and singlet states of dimer fragments arise from a larger interactionJS1S2 between two nearest copper complexes of two neighboring binuclear fragments. The theoretical analysis of the EPR spectrum of the polycrystalline sample for weakly interacting triplet states is carried out. The influence of the weak interaction between triplet states with value ofJ′ is considered in the model of the frequency exchange. A special attention is focused on the presence of the additional signal due to the exchange merging in some orientations where theJ′ value exceeds the fine structure parameters of the spectrum. The analysis of the conditions for the detection of the additional signal and of the influence of this signal on the form of the EPR spectrum allows us to estimate the value of the exchange interactionJ′=o.025±0.005 cm−1 for compounds I and II and anisotropic part of exchange interaction between two nearest copper complexes asJzz=−0.02 cm−1,Jxx.yy=0.01 cm−1 for compound I.

Exchange interaction and spin dynamics in pentanuclear clusters, Cu3Ln2(ClCH2COO)12(H2O)8 (Ln = Nd3+, Sm3+, Pr3+)

New compounds, [Cu3Ln2(ClCH2COO)12(H2O)8]·2H2O with Ln = Nd3+ (I), Sm3+ (II), Pr3+ (III), built up of pentanuclear clusters were synthesized and studied by means of X-ray analysis and electron paramagnetic resonance (EPR). X-ray data show that all compounds are isostructural and the pentanuclear clusteres may be considered as a linear system with alternating Cu(II) and Ln(III) ions: Cu(2)-L1-Ln-L2-Cu(1)-L2-Ln-L2-Cu(2) with L1 and L2 being bridging fragments and Cu(1) and Cu(2) being structurally nonequivalent copper complexes. EPR studies demonstrate that in the temperature range of 100–293 K the signals due to only one type of the copper complexes are observed in the spectra of I–III. AtT<100 K the spectral temperature dependence is nontrivial. AtT<30 K new signals are detected in the spectra of I and II. The temperature dependence of the EPR spectra is interpreted under the assumption that the parameter of the exchange interaction Cu(2)-Ln considerably exceeds the parameter of the interaction Cu(1)-Ln. EPR spectra are calculated for the fragments of five paramagnetic centers in the frames of the model taking into account the nonequivalence of two copper complexes, short longitudinal and transverse paramagnetic relaxation times of the rare-earth ions at room temperature and the change of the relaxation rates when the temperature decreases. The results of the calculations show that it is possible to obtain information about the interactions in the system on the basis of the analysis of the temperature dependence of the EPR spectra of the central copper complex. The parameter of the isotropic part of the exchange interaction between copper and neodymium ions (for the interaction Cu(2)-Nd) is estimated as about 15 cm−1. A considerable rearrangement of the spin states when the temperature changes is found for all complexes.

Paramagnetic resonance and magnetism of copper(II) complexes with N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane

EPR (X- and Q-band) and magnetic susceptibility over the temperature range 4.2–300 K are reported for a number of dinuclear Cu(II) complexes derived from N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc). [Cu2X(tpmc)](ClO4)3·n (X=N3, F (n=2CH3CN), Cl (n=H2O), Br, I (n=H2O), NO2), where the X anions bridge two copper ions and the tpmc ligand has the boat coordination mode, [Cu2tpmc](ClO4)4 and [Cu2X2tpmc](ClO4)2 (X=N3, NCS), where the X anions occupy apical positions and the tpmc ligand has the chair coordination mode. It is established that the properties of the complexes are determined by intramolecular exchange interactions occurring through both the bridging anion X and the tpmc ligand as well as by magnetic dipole–dipole interactions. In the majority of the compounds the exchange is weak, e.g. it corresponds to J equals 2.0 cm−1 for [Cu2N3(tpmc)](ClO4)3 and 3.4 cm−1 for [Cu2(N3)2(tpmc)](ClO4)2. [Cu2OH(tpmc)](ClO4)3·2H2O is an exception in which antiferromagnetic (AF) exchange of about several tens of wave numbers is operating. The EPR spectrum of this complex is described by the following spin Hamiltonian parameters: gz=2.25, gy=2.05, gx=2.04, D=0.06 cm−1, E=0.005 cm−1. Moreover, an extremely strong influence of lattice perturbations on the exchange interactions is recognized for this compound: a partial loss of the H2O molecules (coordinated at a greater distance) by a small amount of dinuclear fragments results in a transformation of an AF exchange into a ferromagnetic or a weak AF one with different values of the anisotropic spin–spin interaction parameters (for example, D=0.172 cm−1, E=0.015 cm−1). The EPR spectra of frozen solutions of all the complexes in NMF are typical for the triplet state, with a distinctly resolved hyperfine structure for the ΔMS=1 and ΔMS=2 signals, and show very similar values of g factors, hyperfine and fine structure parameters (gz=2.255, Az=0.0087 cm−1, gx,y=2.052, D=0.025 cm−1), implying the same molecular structure for dinuclear complexes formed in solution as a result of the loss of one bridging X by dimers with the boat configuration or two X anions by dimers with the chair configuration.

Effects of low symmetry and spin-spin interaction in the centrosymmetric dimer [Cu(PU)5(ClO4)2]2 with out-of-plane bond

Electron paramagnetic resonance (EPR) of single-crystal [Cu(PU)5(ClO4)2]2, built of dimeric fragments with P symmetry, Z = 1, and ClO4 anions, was studied over a range of temperatures at both X and Q band frequencies. EPR spectra show the presence of exchange interaction J(S1 · S2) (J < 0) between two identical Cu2+ ions (S 1 = S 2 = 1/2) in the dimeric fragment. The single-crystal spectra recorded in three different planes were analysed by diagonalization of the full Hamiltonian matrix. The values of g and D were found to have coincident y axes, with the x and z axes rotated by 45°. The tensor of the dipole-dipole interaction was calculated. Two variants for the anisotropic exchange interaction tensor were determined. The data obtained from the analysis of the single-crystal spectra were used to simulate the polycrystalline spectrum. The peculiarities of the polycrystalline spectra for centres S = 1 with nonparallel g and D tensors and the possibilities of their unambiguous interpretation are discussed. ...

Electron Spin Density on the N‑Donor Atoms of Cu(II)− (Bis)oxamidato Complexes As Probed by a Pulse ELDOR Detected NMR

We have applied the pulse ELDOR detected NMR (EDNMR) technique to determine the tensors of the transferred Cu (S = 1/2) - (14)N (I = 1) hyperfine (HF) interaction in single crystals of diamagnetically diluted mononuclear o-phenylenebis(N(R)-oxamide) complexes of [(n)Bu4N]2[Cu(opboR2)] (R = Et 1, (n)Pr 2) (1%) in a host lattice of [(n)Bu4N]2[Ni(opboR2)] (R = Et 3, (n)Pr 4) (99%) (1@3 and 2@4)). To facilitate the analysis of our EDNMR data and to analyze possible manifestations of the nuclear quadrupole interaction in the EDNMR spectra, we have treated a model electron-nuclear system of the coupled S = 1/2 and I = 1 spins using the spin density matrix formalism. It appears that this interaction yields a peculiar asymmetry of the EDMR spectra that manifests not only in the shift of the positions of the EDNMR lines that correspond to the forbidden EPR transitions, as expected, but also in the intensities of the EDNMR lines. The symmetric shape of the experimental spectra suggests the conclusion that, in the studied complexes, the quadrupole interaction is negligible. This has simplified the analysis of the spectra. The HF tensors of all four N donor atoms could be accurately determined. On the basis of the HF tensors, an estimate of the spin density transferred from the central paramagnetic Cu(II) ion to the N donor atoms reveals its unequal distribution. We discuss possible implications of our estimates for the magnetic exchange paths and interaction strengths in respective trinuclear complexes [Cu3(opboR2) (pmdta)2](NO3)2 (R = Et 6, (n)Pr 7).

The first observation of electron spin polarization in the excited triplet states caused by the triplet-triplet annihilation

The spin polarization of excited triplet states caused by mutual annihilation of triplet states was detected by time-resolved electron paramagnetic resonance (EPR) of triplet excitons in two molecular crystals, anthracene-tetracyanobenzene and phenazine-tetracyanoquinodimenthane. The time profile of the two EPR lines of the triplet exciton spectrum have been studied in the time range up to 200 μs after a laser pulse. Besides the initial polarization of the lines, due to intersystem crossing, a long-lasting polarization process is detected which is attributed to the triplet-triplet annihilation. The investigation of the dependence of the phenomenon on the intensity of the light pulse and on the orientation of the crystals in the magnetic field is presented.