Complete Determination of Nitrogen Quadrupole and Hyperfine Tensors in an Oxovanadium Complex by Simultaneous Fitting of Multifrequency ESEEM Powder Spectra

One-dimensional C- and X-band as well as two-dimensional X-band ESEEM experiments were performed on the complex oxobis(2-methylquinolin-8-olato) vanadium(IV) in frozen solution. A¹⁴N ESEEM simulation strategy based on initial first- and second-order perturbation analysis of peak positions in orientationally selected ESEEM spectra is presented. The constraint parameters extracted enable one to reduce the number of free fitting parameters for each nitrogen from 10 to 4. These are the α, β resp. the φ, θ Euler angles of the NQI and the HFI tensor defined in the coordinate system of the axialgtensor. The local symmetry of the complex allows one to reduce the number of free parameters to two angles only. Subsequently, a grid search in the remaining Euler space produced the starting parameters for the final fit of the¹⁴N hyperfine and quadrupole tensors. The anisotropic nitrogen hyperfine interaction tensor was found to be strongly nonaxial (0.06, 0.51, −0.57) MHz with the components significantly smaller than the isotropic hyperfine constant −6.18 MHz. In contrast, the quadrupole tensor withK= 0.58 MHz is close to axial (η = 0.13). These tensors share the principal axis normal to the ligand plane (as imposed by the local symmetry). The axes in the ligand plane are, however, rotated 50° with respect to each other. The orientation of the quadrupole tensor axes correlate within 10° with the orientation of the ligand plane following from the X-ray structure.     

Orientation of the imidazole type ligands in copper complexes studied by ESEEM

Effects of nitrogen quadrupole coupling tensors on ESEEM spectra are discussed. It is shown from theoretical considerations that the spectra, especially the double quantum transition line, are strongly affected by changes of tensor orientation in the magnetic field. Model calculations for angle-selected nitrogen ESEEM spectra of copper complexes in the disordered state show that the double quantum transition line changes its intensity with the resonance magnetic field set position in the EPR spectrum. These model calculations also show that estimation of the orientation of the imidazole type ligands with respect to theg principal axes may be easily made without any tedious processes of computer simulation of the whole ESEEM spectral pattern. The usefulness of the method is experimentally shown by applying it to the structural studies of the copper complexes of 2-(2′-pyridyl)imidazole and 2(2′-pyridyl) benzimidazole and their six coordinated complexes with nitrogeneous bases.     

A pulsed EPR study of the catalytic oxidation of CO over Cu/SnO2

The role of the Cu(II) in the catalytic oxidation of CO over Cu/SnO₂ with low Cu(II) content was studied by continuous wave EPR, electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectroscopes. Three methods were employed for introducing the copper: (i) by coprecipitation, (ii) impregnation onto SnO₂ gel and (iii) impregnation onto calcined SnO₂. Two types of Cu(II) species were identified in these calcined Cu/SnO₂ materials. Those belonging to the first type, termed B and C, exhibit highly resolved EPR spectra with well defined EPR parameters and are located within the bulk of the oxide. The other group comprises a distribution of surface Cu(II) species with unresolved EPR features and are referred to as S. While the latter were readily reduced by CO the former required long exposures at high temperatures (> 673 K). The specific interactions of the different Cu(II) species with CO were investigated through the determination of the¹³C hyperfine coupling of enriched¹³CO. The ESEEM spectra of calcined samples, generated either by coprecipitation or impregnation, show after the adsorption of CO signals at the Larmor frequencies of^{117, 119}Sn and¹³C and at twice these Larmor frequencies. Although these signals indicate that^{117, 119}Sn and¹³C are in the close vicinity of Cu(II), they cannot provide the hyperfine couplings of these nuclei. This problem was overcome by the application of the HYSCORE experiment. The 2D HYSCORE spectra show well resolved cross peaks which provide the hyperfine interaction of these nuclei. Simulations of the HYSCORE spectra yield for^{117, 119}Sn an isotropic hyperfine constant,a _iso, of ±4.0 MHz and an anisotropic component,T _?, of ±2.0 MHz. Pulsed ENDOR spectra also showed^{117, 119}Sn signals which agree with the above values. The¹³C cross peaks yielda _iso=±1.0 MHz andT _?=±2.0 MHz. Similar C cross peaks were observed in spectra of calcined Cu/SnO₂ after the adsorption of CO₂. Based on the same hyperfine couplings in the samples exposed to¹³CO and¹³CO₂ the signals were assigned to surface carbonate species generated by part of the Cu(II) S type species rather then by species B and the role of the Cu(II) in the oxidation process is discussed.     

EPR study of Cu(II) dopant ions in single crystals of bis(l-asparaginato)Zn(II)

We report an electron paramagnetic resonance (EPR) study at 33.9 GHz and room temperature of oriented single crystal samples of bis(l-asparaginato)Zn(II) doped with Cu(II). The variation of the spectra with magnetic field orientation was measured in three crystal planes (a*b, bc and a*c, with a*=b×c). These spectra display two groups of four peaks arising from the hyperfine interaction with the I_Cu=3/2 nuclear spins of copper. They were assigned to Cu(II) ions in two lattice sites related by a 180° rotation around the b-crystal axis. The g and hyperfine coupling (A) tensors of the Cu(II) ions were evaluated from the single crystal data. Some indeterminacy in the assignment of the signals was avoided measuring the EPR spectrum of a powder sample. Their principal values are g₁=2.060(1), g₂=2.068(2), g₃=2.283(2), and A₁≈0.1×10⁻⁴, A₂=13×10⁻⁴ and A₃=165×10⁻⁴ cm⁻¹. The eigenvectors corresponding to g₃ and A₃ are coincident within the experimental error; the other eigenvectors are rotated 5.6° in the perpendicular plane. Considering the crystal structure of bis(l-asparaginato)Zn(II), our EPR results indicate that the Cu(II) impurities replace Zn(II) ions in the host crystal. We propose a molecular model based on the EPR data and the structural information, and analyse the results comparing the measured values with those obtained in similar systems.     

A new spectral resonance of metallic nanorods

The extinction, integrated scattering, and absorption spectra of gold and silver nanorods with random and regular orientation are studied. The calculations are performed for spheroids, circular cylinders, circular cylinders with hemispherical ends (T-matrix method), and rectangular prisms (discrete dipole approximation). A new quadrupole resonance is discovered that arises between the usual plasmon dipole resonances excited by a field longitudinal or transverse with respect to the symmetry axis. The new resonance can be excited only by a TM incident wave and is the greatest for orientation of the symmetry axis of the particle at an angle of 54° with respect to the light beam.     

Potential pockets in the238U+238U system and their possible consequences

Within the double-folding model the separation, shape, and orientation dependence of the interaction potential is studied for two heavy ions. An effective nucleon-nucleon interaction (M3Y) derived fromG-matrix elements and based upon the Reid soft-core potential is used. Deformed Fermi-type matter densities with static quadrupole and hexadecapole deformations were utilized. The model is applied to the²³⁸U+²³⁸U system and shows dramatic dependence on the deformations and orientations.     

Lattice sum calculations and a Mössbauer study on electric field gradient in phlogopite

Lattice sum contributions have been calculated at the two octahedral sites in a phlogopite mica assuming a systematic distribution of octahedral cationic charges. This, unlike the case of a random distribution of charges, is able to reproduce broad features of quadrupole doublet spectra in Mössbauer experiments suggesting that the doublet assignments in terms of the two structural sites,M ₁ (trans) andM ₂(cis), are quite valid. Angle dependence of Mössbauer spectra has also been studied for a ferric-rich phlogopite mica sheet to determine the orientation of the EFG principal axis component and the sign of the quadrupole coupling constant. Quadrupole splitting values and the positive sign of coupling constants match the theoretical predictions quite well but the model fails to predict the observed EFG orientation.     

E.S.R. Study of copper and silver N,N-dialkyldiselenocarbamates

Single crystal E.S.R. studies of Cu(II) and Ag(II) dialkyldiselenocarbamates, diluted in the corresponding Ni(II) complexes, are reported. A method has been developed which removes the necessity to measure the spectra on the same single crystal and in three mutually orthogonal planes. Equations have been derived for the resonance fields and the transition probabilities, taking into account the full spin hamiltonian and without assuming coinciding tensor axes. Applying these equations the g tensor and the metal hyperfine and quadrupole coupling tensors are obtained in high precision. In contrast to the situation in the corresponding dithiocarbamates, it was found for all diselenocarbamates studied that the axes of the g and metal hyperfine tensor do not coincide and that the relative orientation depends on the central metal ion and on the molecular symmetry. The highest g value was always found in the molecular plane, whereas in the dithiocarbamates the highest value is found perpendicular to the molecular plane. The spectra reveal the presence of two sets of two equivalent Se atoms, so that the molecule has inversion symmetry. Especially from the measured ⁷⁷Se hyperfine splittings it could be concluded that the built in guest molecule accepts the structure of the host crystal. Also measurements are reported on powders, glasses and liquid solutions. Evidence has been obtained that in the latter two media the g and metal hyperfine principal axes coincide.     

Transient EPR at 240 GHz of the excited triplet state of free-base tetra-phenyl porphyrin

The electron paramagnetic resonance (EPR) spectrum measured at 240 GHz of the lowest excited triplet state of the free-base tetra-phenyl porphyrin (H₂-TPP) molecule is presented. The high frequency that is employed allows for the full resolution of theg-anisotropy (2.00354(2), 2.00272(15), and 2.00194(5) forx, y, andz zero-field axes respectively), and the obtained values are compared to those of other porphyrin-based systems. These results are one of the first addressing theg-anisotropy in the photoexcited state of porphyrins. The usefulness of high-field transient EPR in elucidating not only the principal values of theg-tensor but also the orientation of theg-tensor principal axes with respect to those of the zero-field splitting tensor in porphyrin disordered systems is also discussed.     

Studies of EPR and ENDOR spectra of14N and15N label bis(2-hydroxyacetophenyl ketoxime)-63Cu(II) and-65Cu(II) complexes in disordered system

The EPR spectra of isotopic label bis(2-hydroxyacetophenyl ketoxime)-Cu(II) [N?Cu-HAP] complexes, such as¹⁴N?⁶³Cu-HAP,¹⁵N?⁶³Cu-HAP and¹⁴N?⁶⁵Cu-HAP in frozen THF solution below 77 K, and their ENDOR spectra in frozen DMSO/EtOH (5∶1) solution below 20 K were studied. The exact values of the components ofg-tensor, of hyperfine tensors of copper isotopes, and of superhyperfine interaction tensors of copper with nuclei of nitrogen isotopes and¹H nuclei, and of the¹⁴N nuclear quadrupolar moment coupling tensor were obtained. The bond parameters α, β, δ, γ and the corresponding energy levels of N?Cu-HAP complexes were calculated by using EPR and ENDOR data. It was shown that the unpaired, electron is delocalized not only to the nearest N atom but also to the H atom, of ligands, which is more far from the Cu ion.     

Probing electronic and structural properties of the reduced [2Fe−2S] cluster by orientation-selective1H ENDOR spectroscopy: Adrenodoxin versus rieske iron-sulfur protein

The¹H electron-nuclear double resonance (ENDOR) spectra in frozen buffer solutions of the reduced [2Fe?2S] clusters in adrenodoxin (Adx) and in the “Rieske” iron-sulfur protein (ISP) from the bovine mitochondrial bc1 complex were measured at low temperatures (5–20 K) and analyzed by spectra reconstruction. A single paramagnetic species with iron valence states (II) and (III) connected uniquely to the cluster irons was found in both proteins. For Adx, the experimental spectra from 23 field positions across the nearly axial (g _max=2.0241,g _int=1.9347, andg _min=1.9331) electron paramagnetic resonance (EPR) spectrum were analyzed. Four larger hyperfine couplings were assigned to the cysteine β-protons near the Fe(III) ion. Transfer into the crystal structure showed that the Fe(III) ion was coordinated to the residues Cys55 and Cys92. The spin density was estimated as +1.60 for the Fe(III) and ?0.6 for the Fe(II) ion, respectively. Theg-tensor direction with respect to the cluster showed strong similarities with the earlier assignment inArthospira platensis ferredoxin (Canne C., Ebelshauser M., Gay E., Shergill J.K., Cammack R., Kappl R., Hüttermann J.: J. Biol. Inorg. Chem. 5, 514, 2000). An Adx mutant (T54A) exhibiting a change (70 mV) in redox potential showed no significant influence at the [2Fe?2S] cluster. The Rieske ISP was subjected to the same analysis. The ENDOR spectra from 35 field positions across the rhombic (g _max=2.028,g _int=1.891, andg _min=1.757) EPR spectrum were simulated. Three major proton contributions were identified from the orientation behavior. Two were assigned to cysteine β-protons and one to a β-proton of His141. In contrast to Adx, the direction of theg _max-component was found to lie roughly in the FeS-core plane and the largest proton coupling occurred alongg _int. The spin population was estimated as about +1.6 for the oxidized and ?0.55 for the reduced iron.     

Characteristics of ESEEM and HYSCORE spectra of S>1/2 centers in orientationally disordered systems

One- and two-dimensional electron-spin echo envelope modulation (ESEEM) spectra of Kramers’ multiplets in orientationally disordered systems are simulated using a simple mathematical model. A fairly general high-field spin Hamiltonian is considered with a general g-tensor and arbitrary relative orientations between all tensors involving the electron-spin S, the nuclear spin I, and their interaction. The zero field splitting (ZFS) and the nuclear quadrupole interactions are, however, approximated by their respective secular part in a way that retains all orientation dependencies and it is assumed that the nuclear quadrupole interaction is smaller than the hyperfine interaction. These approximations yield an effective sublevel nuclear Hamiltonian for each EPR transition and are sufficient to account for the most important characteristics of the ESEEM spectra of high electronic multiplets in orientationally disordered systems. Moreover, they allow to obtain some analytical expressions that for I=1/2 illuminate important aspects of 2D hyperfine sublevel correlation (HYSCORE) experiments in S=3/2, 5/2 systems. The pulses are considered as ideal and selective with respect to the different EPR transitions. The contributions of the latter to the echo intensity are weighed according to their different nutation angles and equilibrium Boltzmann populations. For simple axial cases with I=1/2, analytical expressions, analogous to the S=1/2 case, were derived for: (i) the modulation depth, (ii) the lineshapes of the HYSCORE cross-correlation ridges, and (iii) ENDOR powder pattern. Experimental results obtained from Mn(D₂O)₆²⁺ and VO(D₂O)₅²⁺ in frozen solutions are presented, compared, and analyzed in light of the theoretical part.     

Strong dependence of multichannel ballistic transport on the geometric symmetry

Ballistic electron transport in Aharonov–Bohm-type ring structures is investigated where the single-channel problem is nontrivially extended to the multichannel one in which the important interchannel scattering effect is considered. It is found that theS-matrix of a ring structure should reflect the geometric symmetry if the interchannel scattering effect is properly accounted for and that the symmetry relationships of theS-matrix plays a crucial role in the conductance oscillation behavior in ballistic two-dimensional rings. The magnetostatic as well as the electrostatic Aharonov–Bohm effects are studied for two ring structures of different symmetry.     

Comparison of the V-H and V-D phase diagrams by means of NMR

The phase diagrams of vanadium-hydrogen (V-H) and vanadium-deuterium (V-D) have been investigated by measuring the nuclear magnetic resonance (NMR) spectra of vanadium and deuterium in strongly textured vanadium foils. In the cubic α- and α′-phases both the V and D lines are unsplit. From the orientation dependence of the quadrupole splitting of the two resonances in the tetragonal β-phase it is found that for both atoms the electric field gradient is axially symmetric with its largest component parallel to the c-axis. In contrast to V-H, the β-phase of V-D exists only in a small region near D/V ratios of about 0·4 at 300°K; for higher D concentrations the α′-phase remains stable down to about 200°K, below which the orthorhombic δ-phase is formed. In spite of the nearly cubic arrangement of the V atoms in the δ-phase, the intensity of the vanadium line is reduced corresponding to first-order quadrupole splitting. From the absence of any V satellites in the δ-phase, it follows that the orientation of the principal axes of the field gradient at the V sites deviate strongly from the cube edges of the V lattice. For deuterium the largest component is parallel to one cube edge, which must be the c-axis of the orthorhombic cell. Thus the field gradient at the D sites has the same symmetry in the β- and δ-phase, with quadrupole frequencies of 100 and 55 kHz, respectively. This difference can be attributed to the fact that in the two phases different sites are occupied: octahedral sites in the β-phase and tetrahedral sites in the δ-phase.     

Two-dimensional ENDOR-ESEEM correlation spectroscopy

A two-dimensional (2D) experiment that correlates electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) frequencies, useful for unraveling and assigning ENDOR and ESEEM spectra from different paramagnetic centers with overlapping EPR spectra, is presented. The pulse sequence employed is similar to the Davies ENDOR experiment with the exception that the two-pulse echo detection is replaced by a stimulated echo detection in order to enhance the resolution in the ESEEM dimension. The two-dimensional data set is acquired by measuring the ENDOR spectrum as a function of the time interval T between the last two microwave pulses of the stimulated echo detection scheme. This produces a series of ENDOR spectra with amplitudes that are modulated with T. Fourier transformation (FT) with respect to T then generates a 2D spectrum with cross peaks connecting spectral lines of the ESEEM and ENDOR spectra that belong to the same paramagnetic center. Projections along the vertical and horizontal axes give the three-pulse FT-ESEEM and ENDOR spectra, respectively. The feasibility of the experiment was tested by simulating 2D ENDOR-ESEEM correlation spectra of a system consisting of an electron spin (S = (1/2)) coupled to two nuclei (I(1) = I(2) = (1/2)), taking into account experimental conditions such as pulse durations and off-resonance irradiation frequencies. The experiment is demonstrated on a single crystal of Cu(2+) doped l-histidine (Cu-His), containing two symmetrically related Cu(2+) sites that at an arbitrary orientation exhibit overlapping ESEEM and ENDOR spectra. While the ESEEM spectrum is relatively simple and arises primarily from one weakly coupled (14)N, the ENDOR spectrum is very crowded due to contributions from two nonequivalent nitrogens, two chlorides, and a relatively large number of protons. The simple ESEEM projection of the 2D ENDOR-ESEEM correlation spectrum is then used to disentangle the ENDOR spectrum and resolve two sets of lines corresponding to the different sites. Copyright 2000 Academic Press.     

ESR study of some Cu(II)-4-substituted-2-Thiazolylhydrazone complexes

The Cu(II) complexes with 2-N-acetyl-salicylidene-hydrazino-4-chlor-methyl thiazole (L.) and 2-N-acetyl salicylidene-hydrazioo-4-thiazolyl acetic ester (L_II) were prepared and investigated by ESR measurements. The powder ESR spectrum at room temperature of CuL_IICl is quasi-isotropic (g=2.113), while for CuL_IICl is characteristic of monomeric species with axial symmetry (g _II)=2.234,g ₁=2.073). The isotropic ESR spectra of the CuLCl compounds in DMSO solution suggest the presence of pseudo-tetrahedral monomeric species. The anisotropic spectra were obtained for adsorbed CuLCl DMSO solutions on NaY zeolite. The parallel hyperfine structure shows the coexistence of two magnetic nonequivalent monomeric species. The estimation of some LCAO-MO coefficients using Kivelson and Neiman’s approximation reveals a dominant ionic character for copper-ligand bonds.     

Electron-lattice interaction of Ag− and Cu− centers in alkali halides

Absorption, emission, and excitation spectra of Ag^? centers in KCl, RbCl, CsCl, and CsBr are measured at low temperatures. The positions of theA emission bands are slightly different afterC andA band excitation, respectively. This is believed to be due to the existence of two different types of minima in the adiabatic potential energy surface of the³ T _1u state. The symmetry of the energy minima in the¹ T _1u state is trigonal for KCl∶Ag^? and Cu^?, but tetragonal for CsBr∶Ag^?. This becomes evident from the polarization properties of the emission. The energy and temperature dependence of the polarization is discussed. Uniaxial stress causes polarized emission of Ag^? and Cu^? centers measured from LHeT to 100 K. This is due to a splitting and mixing of the relaxed excited states by the stress. The effects are used to calculate the coupling constants between thep electron and theE _g andT _2g lattice modes. They are compared with predictions from the point-charge model for different lattice structures. A new assignment of the absorption bands of KCl∶Cu^? to the excited states of Cu^? is established on measurements of emission spectra and lifetimes.     

Hyperfine Interaction Tensors of 13C Nuclei for Ring Carbons of Ubisemiquinone-10 Hydrogen Bonded in Alcohol Solvents

The anion radicals of ubiquinones-10 ¹³C chemically labeled at the C₅ or C₆ ring positions in alcohol have been studied by 1D and 2D ESEEM to define the hyperfine interaction tensors with the ¹³C nuclei. Analysis of the cross-peak line shapes and simulations of the spectra allowed us to conclude that the hyperfine tensors are characterized by an anisotropic component T ~6 MHz and an isotropic coupling a ~?3 MHz with support from DFT calculations. However, these values were found to be inconsistent with the shift of the sum combination harmonic in the four-pulse ESEEM spectra. Simulations resolve this apparent discrepancy by showing that the shift of the sum combination to lower frequency and its broadening can be accounted for by a distribution of the hyperfine couplings. A spread of the methoxy group conformations, as supported by previous experimental observations, is suggested as the mechanism influencing the distribution of the hyperfine couplings for the ring carbons.     

Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu ions

We investigated the crystal growth, electron paramagnetic resonance (EPR) and optical absorption spectra of l-threonine doped with Cu²⁺. The quality, size and habit of the single crystals grown from aqueous solution by the slow solvent evaporation and by the cooling methods vary when the impurities are introduced during the growth process. The variations with the magnetic field orientation of the EPR spectra of single-crystal samples at room temperature and 9.77 GHz in three crystal planes (ab, bc and ac) show the presence of copper impurities in four symmetry-related sites of the unit cell. These spectra display well resolved hyperfine couplings of the of Cu²⁺ with the I_Cu= of the copper nuclei. Additional hyperfine splittings, well-resolved only for specific orientations of the magnetic field, indicate that the copper impurity ions in the interstitial sites have two N ligands with similar hyperfine couplings. The principal values of the g and A_Cu tensors calculated from the EPR data are g₁=2.051(1), g₂=2.062(2), g₃=2.260(2), A_Cu,1=16.9(5)×10⁻⁴ cm⁻¹, A_Cu,2=21.8(6)×10⁻⁴ cm⁻¹, A_Cu,3=180.0(5)×10⁻⁴ cm⁻¹. The principal directions corresponding to g₃ and to A_Cu,3 are coincident within the experimental errors, reflecting the orientation of the bonding planes of the copper ions in the crystal. The values of the crystal field energies are evaluated from the optical absorption spectrum, and the crystal field and bonding parameters of the Cu impurities in the crystal are calculated and analyzed. The EPR and optical absorption results are discussed in terms of the crystal structure of l-threonine and the electronic structure of the Cu²⁺ ions, and compared with data reported for other systems. The effects of the impurities in the growth and habit of the crystals are also discussed.     

Photoelectron spectroscopy of sulfur heterocycles adsorbed onto a Cu(110) surface

Photoelectron spectra of Cu(110) after exposure to thiacyclobutane, thiacyclopentane, and thiacyclohexane, as well as of the clean Cu(110) surface, have been obtained using He(I) radiation. Comparison of the spectra of the absorbates with gas-phase spectra suggests a strong sulfur-to-copper dative bond. Sulfur lone-pair orbital stabilization energies were obtained for thiacyclobutane (1.4 eV), thiacyclopentane (1.1 eV), and thiacyclohexane (0.6 eV), and these stabilization energies are correlated with the dative sulfur-to-surface bond energies. Ring-strain changes are suggested as the reason for additional small shifts of absorbate-bond energies in the thiacyclobutane spectrum. Relaxation energies were found to decrease in the order 4- > 5- > 6-membered ring. An empirical linear relation was found between relaxation shifts and lone-pair orbital stabilization energies for monoheteroatom heterocycles bonded to Cu(110) by a lone-pair dative bond. All adsorbates studied showed marked angular dependence, suggesting nonrandom orientation on the surface. Al Kα X-ray data were used to calculate percentage surface coverages. Other sulfur-containing adsorbates are discussed, including thiophene, allyl mercaptan, and thiacyclopropane. Thiacyclopropane was found to decompose upon exposure to copper.