EPR and ENDOR Study of a series of free base porphycenes in the photoexcited triplet state

Porphycene is a structural isomer of porphyrin. The photoexcited triplet states of porphycene, 2,7,12,17-tetra-n-propylporphycene and 9,10,19,20-tetra-n-propylporphycene in disordered solid solution were studied by EPR and ENDOR. The ENDOR spectra yield the hyperfine tensor elementsA _zz for each of the different groups of equivalent protons. The dipolar contribution toA _zz is estimated and spin densities are derived from the isotropic contribution. They are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/SP and RHF-INDO/S).     

The effect of spin states of iron[II] on the XPS of its mixed complexes

A series of mixed iron[II] phenanthroline complexes having analogous coordination spheres but containing the central iron atom in differing spin states (high-spin, ⁵T₂; low-spin, ¹A₁; and spin triplet, ³A₂) have been investigated by XPS. The data reflect the effects of the spin state of iron on the XPS pattern and on the electron binding energies. Fe[4,7-(C₂H₅OOC)₂phen] ₂(NCS)₂ is the first iron[II] complex in the ³A₂ spin state that has been studied by XPS.     

Optically detected E.P.R. and low-field ENDOR of triplet benzophenone

The ENDOR spectrum of ¹³C substituted (carbonyl carbon) triplet excited benzophenone was studied at ca. 100 G applied magnetic field by means of optical detection. The benzophenone was substitutionally dissolved in dibromodiphenylether (DDE). The ENDOR transitions and the E.P.R. transitions were studied as a function of applied field direction in the ab and bc crystal planes. The angle ? between the line joining the phenyl group centres and a principal axis of the fine structure tensor was found to be 23·6° ± 0·02° (standard deviation limited to one set of measurements). The principal axes of the ¹³C hyperfine tensor were within experimental error (±10°) coincident with the fine structure axes. The principal values of the hyperfine tensor were found to be : |A_xx /h| = 43·16 ± 1·84, |A_yy /th| = 22·66 ± 2·50, |A_zz /h| = 10·25 ± 1·30 MHz. The low-field ENDOR does not provide an indication of the signs of these tensor elements, so the value of the isotropic coupling constant cannot be measured. Two values of the anisotropic (dipole) hyperfine tensor elements were deduced on the assumption that the transitions frequencies are mainly determined by interactions between the nuclear spin and electron density on the carbonyl carbon atom. These values were (i) A _xx / ^d /h = 17·82, A_yy /h = -22·68, A_zz /h = -15·14 MHz and (ii) A_xx /h = 39·73, A_yy /h = -26·09, A_zz /h = -13·63 MHz. Set (ii) is consistent with recent calculations of the spin density distribution in triplet benzophenone in which the orbital spin densities are 0·64 for the carbonyl carbon π-orbital, 0·17 for the oxygen π-orbital, and 0·88 for the non-bonding orbital on oxygen (all expressed as fractions of one electron).

Isotope effects on the fine structure constants of triplet benzophenone were measured and found to be consistent with the changes occurring in the spin-orbit coupling with the ground state.

The kinetic parameters for the excitation and de-excitation of the triplet substates were deduced from transient ODMR studies of benzophenone in DDE. The T_z spin state is mainly populated (85–88 per cent) and this is also the most strongly radiative state (k_z ^r = 0·88). The steady-state populations of the three triplet levels are similar.     

Structural elucidation of Cu(II) ion doped in hexaaquozincdiaquobis(malonato)zincate host by EPR spectroscopy

In order to understand the structural behaviour of Cu(II) in a variety of ligand environments, single crystal electron paramagnetic resonance studies of Cu(II) doped in hexaaquazincdiaquabis(malonato)zincate [Zn(H₂O)₆][Zn(mal)₂(H₂O)₂] are carried out at 300 K. Angular variation of copper hyperfine lines in three orthogonal planes shows the presence of single site, with spin Hamiltonian parameters as g_xx=2.034, g_yy=2.159, g_zz=2.388, A_xx=3.39 mT, A_yy=4.89 mT and A_zz=13.72 mT. The g/A tensor direction cosines are compared with various Zn-O directions in the host lattice, which confirm that Cu(II) enters substitutionally in the lattice. The low value of A_zz has been explained by considering admixture of d_²x−²y ground state with d_²z excited state. EPR powder spectra at 300 and 77 K give identical spin Hamiltonian parameters (g_∥=2.367, g_⊥=2.088, A_∥=11.47 mT, A_⊥=2.63 mT). IR, UV-vis and powder XRD data confirm the structure and symmetry of the Cu(II) ion in the host lattice.     

Azurin-Solvent interaction: an ESR spin labeling investigation

We have investigated by means of electron spin resonance (ESR) spectroscopy using two spin labels, Iodoacetamido-proxyl and 3-Maleimido-proxyl, the dynamics of two different regious around the active site of azurin, a copper containing blue protein. The ESR measurements of spin labeled azurin have been carried out in the 110–300 K temperature range on wet (H₂O, D₂O and ethanol/water mixtures) and lyophilized samples. The behaviours of the outer hyperfine splitting separation, 2A′ _zz , of the ESR spectravs temperature of the lyophilized, and fully hydrated azurin in H₂O and D₂O suggest that the two spin labels are located in regions of the protein surface with different dynamics and polarity. Moreover, all differences in the 2A′ _zz values shown by the spin labeled azurin in normal and heavy water as well as the temperature behaviour disappear when azurin is in ethanol/water mixtures. The results are discussed in terms of a close correlation between the molecular dynamics of the protein fragments to which the two spin labels are bound and the properties of the solvent used.     

EPR Investigation of Exchange Interactions Between Neodymium Ions in {[Nd2(α-C4H3OCOO)6(H2O)2]} n

Electron paramagnetic resonance (EPR) of a new compound {[Nd₂(α-C₄H₃OCOO)₆(H₂O)₂]} _n composed of Nd³⁺–Nd³⁺ dimers is reported. The anisotropy parameters of the spin–spin interaction are determined by fitting experimental and simulated spectra in X- and Q-bands. It is shown that the anisotropy of the exchange interaction gives the main contribution to the anisotropy of the spin–spin interaction. The observed anisotropy disagrees with the expected in the model of the isotropic exchange interaction between real spins. A feature of the EPR spectrum not described by the model of the isolated Nd–Nd dimers and reflecting the magnetization transfer between dimers is detected. The magnetization transfer due to both the relaxation transitions and the interdimer interaction is considered.     

Impurity in three substitutional locations: Single-crystal EPR study of VO(II) in zinc maleate tetrahydrate

Single-crystal EPR study of VO(II)-doped zinc maleate tetrahydrate has been carried out at room temperature. Four types of impurities have been identified in the single-crystal spectra with intensity ratio of 11:5:2:1. However, the analysis has been done only for the three most intense resonances. The evaluated spin Hamiltonian parameters (A in units of mT) from single-crystal rotations are site I: g_xx=1.981, g_yy=1.971, g_zz=1.939; A_xx=7.43, A_yy=7.70, A_zz=18.7; site II: g_xx=1.973, g_yy=1.966, g_zz=1.939; A_xx=7.08, A_yy=7.22, A_zz=18.5; site III: g_xx=1.978, g_yy=1.977, g_zz=1.951; A_xx=7.11, A_yy=7.32, A_zz=18.3.The powder spectrum gives only one set of g and A values, confirming the presence of only one chemically equivalent site. The paramagnetic impurity, VO(II), has entered the lattice substitutionally for all the three sites. The superhyperfine structure, in the intensity ratio of 1:4:6:4:1, arising from the protons of the water ligands, has been found in one site. The admixture coefficients have been calculated and the complex is found to be fairly covalent in nature.     

ESR studies of Cu2+ doped in SrC4H4O6·3H2O single crystals

Electron spin resonance spectra of Cu²⁺ doped in single crystals of strontium tartrate trihydrate grown by a diffusion technique have been investigated at 77K. Copper enters the lattice substitutionally and is trapped at two magnetically inequivalent sites. ESR measurements gave the following values for the spin-Hamiltonian parameters. Cu²⁺(I): g_gg = 2.0380, g_yy = 2.1317, g_zz = 2.3918 and A_xx = 26.3 G, A_yy = 56.3 G, A_zz = 110.8 G. Cu²⁺(II): g_xx = 2.0497, g_yy = 2.1297, g_zz = 2.3706 and A_xx = 19.2 G, A_yy = 61.4 G, A_zz = 107.2 G.     

Molecular orbital study of polarity and hydrogen bonding effects on the g and hyperfine tensors of site directed NO spin labelled bacteriorhodopsin

Semiempirical molecular orbital methods (PM3, INDO, ZINDO/S) have been used to calculate the effects of local electric fields and of hydrogen bonding on the g and hyperfine tensors of a nitroxide spin label model system. The results yield a linear correlation between the two principal tensor components g _xx and A ^N _zz at label sites of varying polarity. Hydrogen bonding with a single water molecule produces a constant shift of Δg _xx ? ?4 × 10^?4. These theoretical results are used to interpret recent high field (3.4 T, 95 GHz) electron paramagnetic resonance investigations on site-directed spin labelled bacteriorhodopsin. This protein reveals a close correlation between proticity and polarity at the various label sites. The slope of the g _xx versus A ^N _zz dependence is affected strongly by polarity induced structural strains of the spin label.     

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H_c. Above H_c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C₅H₁₄N₂)₂N₃(PF₆), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.     

EPR studies of Cu2+ in anhydrous sodium oxalate

Electron paramagnetic resonance spectra of Cu²⁺ doped in single crystal of anhydrous sodium oxalate grown by slow evaporation from saturated aqueous solutions have been investigated. EPR measurements gave the following values for the spin Hamiltonian parameters: g_xx = g_yy = 2.0741, g_zz = 2.3253, A_xx = A_yy = ?14.9G and A_zz = ?147.7 G. The principal axes of the hyperfine coupling and g tensors are spatially coincident. Using the optical absorption energy values given for a similar complex we have estimated the values of the bonding parameters and orbital-reduction factors for the system under investigation.     

Preparation and Mössbauer spectra of CoV2O4 spinel doped with Fe2+ on tetrahedral sites

Co²⁺_0.98Fe²⁺_0.02Fe³⁺_?V³⁺_2-? 04 (?? 0.001) was obtained from a sample containing the iron dope mainly in the trivalent state, by reducing it at 910°C in a CO₂/CO atmosphere. Mössbauer spectra of ⁵⁷Fe at the tetrahedral site were taken between 5 and 300 K. Below the Curie temperature (143 K) there is a magnetically induced electric field gradient eQV_zz and a reduction of the hyperfine field H_hf. Values at 5 K are H_hf=141 kG and eQV_zz=5.74 mm/s. The effective splitting of the Fe²⁺⁵E ground-state doublet, as estimated from the temperature dependence of eQV_zz and H_hf, is $\text{2q}\text{δ}\text{/k = 28 K}$. It is derived that in these oxides the coupling of the electronic ⁵E states to local Jahn-Teller active E-modes of vibration is stronger than in comparable sulphides.     

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T_c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T_c (10 ± 1 K for the organic metal (BEDT-TTF)₃Ta₂F₁₁ and 92 to 12 K for YBa₂Cu₃O_7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T_c for the organic metal. Here the exchange field between the aligned AF domains: J_AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ_AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ_AF < 10^-10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ₁ for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T_c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ₁ = C_H /α_H, C_H and α_H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T_c are correlated to electron-phonon interactions.     

ESR studies of Cl2- (VK) centre trapped in irradiated Ca(ClO3)2 . 2H2O single crystals

A Cl₂- centre has been trapped in X or γ-irradiated Ca(ClO₃)₂. 2H₂O single crystals at 298 K, when the irradiated crystals were illuminated with ultra-violet light (360 nm). This centre is formed at the expense of ClO₂ centres in this crystal. This Cl₂ ^- centre is trapped at two magnetically inequivalent sites in the crystal lattice and these sites become equivalent when the static magnetic field is parallel or perpendicular to the b axis. At many orientations this centre reveals ‘super-hyperfine’ interaction with a proton (I = 1/2) of the water of crystallization. The magnetic parameters are close to those observed in alkali chlorides and the E.S.R. spectrum has been fitted to an orthorhombic spin hamiltonian. The principal g values are g_xx = 2·035, g_yy = 2·033 and g_zz = 2·000 and those of the A values are A_xx = 15·0, A_yy = 31·0 and A_zz = 109·0 G. The shfs parameters are A _‖' = 5·0 A _⊥' = 1·0 G. The V_K centre trapped in this lattice is exceptionally stable at room temperature.     

Die D(d,p)H3-Reaktion mit polarisierten Deuteronen beiĒ d =80 keV

The tensor analyzing powersA _zz ,A _xx?tyy andA _xz of the D(d, p)H³ reaction are measured atē _d =80 keV. It is shown, that the polarization effects at this energy are of the same magnitude as the deviation of the isotropy, using unpolarized deuterons to initiate the reaction.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

Spin alignment in the resonant 24Mg+24Mg system

The spin alignment P_zz in large-angle ²⁴Mg+²⁴Mg inelastic scattering, 1α-transfer, and 2α-transfer at the resonance energy E_CM=45.7 MeV is deduced from the out-of-plane anisotropy of the particle-coincident γ radiation. Except for low-lying single excitations, rather large values around P_zz=0.8 are found for all prominent states, in accordance with the expected spin distribution for a fissioning strongly deformed intermediate complex.     

Hydrogen-induced reduction of electric field gradient in La2Fe14BHx studied by NMR on La

The ¹³⁹La NMR spin-echo spectra of La₂Fe₁₄BH_x are reported. Since lanthanum has no 4f electrons, the observed quadrupole splittings are directly proportional to the lattice part of the electric field gradient, V_zz^lat. This quantity decreases as the content of interstitial hydrogen x grows. To describe the fall-off of V_zz^lat, a dimensionless function f₂(x) is introduced, normalised so that f₂(0)=1. Analysis of the new and earlier published data obtained by several experimental techniques demonstrates that the reduction of both the second-order crystal field parameter A₂₀ and the lattice electric field gradient V_zz^lat is described by the same function f₂(x) in all R₂Fe₁₄BH_x. The only distinction that needs to be made is between R being a light and a heavy rare earth.     

An investigation on the defect structures and spin Hamiltonian parameters for the two orthorhombic Ti3+ centers in ZnWO4

By employing the perturbation formulae of the spin Hamiltonian parameters (SHPs) (g factors g_xx, g_yy, g_zz, hyperfine structure constants A_xx, A_yy, A_zz and superhyperfine parameters A_xx׳, A_yy׳, A_zz׳) for a 3d¹ ion in orthorhombically elongated octahedra and tetrahedra, the defect structures and the experimental EPR spectra are theoretically and systematically investigated for the two orthorhombic Ti³⁺ centers C₁ and C₂ in ZnWO₄. Center C₁ is ascribed to the impurity Ti³⁺ at host W⁶⁺ site associated with two nearest neighbor oxygen vacancies due to charge compensation. The resultant tetrahedral [TiO₄]^5– cluster is determined to undergo the local orthorhombic elongation distortion, characterized by the axial distortion angle Δθ (=θ–θ₀≈–6.84°) of the local impurity-ligand bond angle θ related to θ₀ (≈54.74°) and the perpendicular distortion angle Δε (=ε–ε₀≈2.5°) related to ε₀ (≈45°) of an ideal tetrahedron because of the Jahn–Teller effect. Center C₂ is attributed to Ti³⁺ on Zn²⁺ site, and this octahedral [TiO₆]^9– cluster may experience the local axial elongation ΔZ (≈0.001 Ǻ) and the planar bond angle variation Δφ (≈9.1°) due to the Jahn–Teller effect, resulting in a more regular oxygen octahedron. All the calculated SHPs (i.e., g factors for both centers, the hyperfine structure constants for center C₂ and superhyperfine parameters of next nearest neighbor ligand W for center C₁) show good agreement with the observed values. However, the theoretical results based on the previous assignment of center C₁ as Ti³⁺ on W⁶⁺ site with only one nearest planar oxygen vacancy (i.e., five-fold coordinated octahedral [TiO₅]^7– cluster) show much worse agreement with the experimental data. The defect structures and the SHPs (especially the g anisotropies) are discussed for both centers. The present studies on the superhyperfine parameters of ligand W⁶⁺ for center C₁ would be helpful to further investigations on the superhyperfine interactions of cation ligands which were rather scarcely treated before.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.