Local31P and distant31P and195Pt ENDOR studies of powdered samples of Bis(O,O′-disubstituted-dithiophosphato)copper(II) complexes magnetically diluted in the corresponding Pd(II) and Pt(II) host lattices

ENDOR studies on bis(dithiophosphato)copper(II) complexes magnetically diluted in the corresponding Pd(II) and Pt(II) host lattices performed with the use of the method of “polycrystalline ENDOR crystallography” are reported. In these samples well resolved local³¹P (A_iso ca. 29 MHz and A_aniso ca. 0.9 MHz) as wel1 as distant³¹P (A _iso ca. 1.3 and 5.5 MHz and A_aniso ca. 0.2 and 0.4 MHz) and¹⁹⁵Pt (A_iso ca. 3.1 MHz and A_aniso ca. 0.2 MHz) ENDOR transitions are recorded. It is shown that the directions of the main values of A_aniso for all local³¹P and distant³¹P and¹⁹⁵Pt ENDOR transitions are parallel. Comparison of these results with those reported about the same complexes but diluted in other host lattices shows that there is no difference in the EPR and local³¹P ENDOR parameters thus suggesting that the structure of the paramagnetic complex remains unchanged. It is found that distant ENDOR transitions appear only in the case when the host lattice contains one molecule in the unit cell, i.e., when all molecules are parallel to each other. Using the obtained data from local and distant³¹P ENDOR transitions some structural features of Cu(dtp)₂ as well as Pd(dtp)₂ and Pt(dtp)₂ complexes are found for the first time.     

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.     

Ligand exchange reactions between copper(II)- and nickel(II)-chelates of different sulphur- and selenium-containing ligands

A single-crystal E.S.R. and multi-nuclear ENDOR study of the mixed ligand complex tetra-n-butylammonium(maleonitriledithiolato)(monoethyldithiophosphato)cuprate(II), diamagnetically diluted by the corresponding Ni(II) complex, and the crystal structure of the host complex are reported. n-Bu₄N[Ni(mnt)(HEtdtp)].0·7 acetone is triclinic, space group P1, Z = 2 with a = 9·029(2), b = 13·432(4), c = 17·246(5) Å, α = 108·9(1), β = 90·2(1) and γ = 109·4(1)°. The spin-hamiltonian parameters are calculated from the results of an extended Hückel molecular orbital calculation. The experimental g-tensor and the copper and the four sulphur hyperfine coupling tensors are reporduced well. The almost isotropic and unexpectedly large coupling of the phosphorous atom is negative, which can be understood qualitatively from a spin polarization mechanism.

The hyperfine coupling tensors of eight protons, three of the acetone molecule, four of the tetra-n-butylammonium cation and one intramolecular one, could be determined. The E.S.R. linewidths are unusually small for Cu complexes. Unresolved proton hyperfine interactions are found to give the largest contribution.     

Transfer and fragmentation reactions of14N at 60 MeV/u

Inclusive energy spectra and cross sections of reaction products close to the¹⁴N projectile (¹¹C,¹²C,¹³C,¹⁵N and¹⁵O) have been measured in the angular rangeθ _L =1.2°–4.2° at an incident energy of 60 MeV/u for five different target nuclei (¹²C,²⁷Al,⁵⁸Ni,⁹⁰Zr and²⁰⁸Pb). Two components are found to be systematically present in the energy spectra of the carbon isotopes, the first similar to that observed at relativistic energies and the second shifted down in energy characteristic for a very dissipative process. The dependence of the differential and integrated cross sections on the target mass indicates that the two-body final channels (¹⁴N,¹³C), (¹⁴N,¹⁵N) and (¹⁴N,¹⁵O) are strongly correlated to the fragmentation channels.     

Presence of Jahn-Teller distortions in a novel six-coordinate Ag(II) complex: temperature dependent EPR,optical and magnetic susceptibility measurements

Single-crystal variable temperature EPR, optical and polycrystalline magnetic susceptibility studies have been made on a novel six-coordinate Ag(II) complex. Temperature dependent EPR studies on pure single crystals of this compound reveal that dynamic Jahn-Teller distortion operates above 230 K, between 230 K and 120K static Jahn-Teller distortion sets in and below 110 K there is evidence of exchange interaction. Crystal g values were obtained by least-squares fitting with the data obtained from the orientation dependent EPR spectra of the undiluted single crystal of this complex at 300 K and 77 K. From an optical study the Jahn-Teller stabilization energy is found to be ～2250cm^?1. Comparison of Abs_max values for other silver(II) compounds enables us to conclude that the formal geometry of this complex is a tetragonally distorted octahedral. Infrared spectra of this complex were also recorded over a wide range of temperature. Magnetic susceptibility measurements over a wide range of temperature on the powder sample of this compound reveal that the complex is antiferromagnetically coupled in the temperature range 5–40 K with 2J = 0.906cm^?1, and above 40K it is ferromagnetically coupled with 2J = +7.4cm^?1. The effective magnetic moment (μ_eff) of this complex has been compared with that of a series of other silver(II) complexes available in the literature. Finally, the spectral and magnetic data of this complex have been compared with those of a corresponding isostructural and isomorphous copper(II) complex.     

Temperature Dependence of Hyperfine Interaction for 15N Nitroxide in a Glassy Matrix at 10–210 K

Principal ¹⁵N hyperfine interaction (hfi) values in ¹⁵N-substituted nitroxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl dissolved in nematic liquid crystal 4-pentyl-4′-cyanobiphenyl (5CB) were measured in a wide temperature range of 10–210 K, for 5CB frozen to a glassy state. X-band continuous-wave electron paramagnetic resonance (CW EPR) and pulse X- and Q-band ¹⁵N electron-nuclear double resonance (ENDOR) techniques were employed. To avoid microwave saturation at low temperatures in CW EPR studies, a holmium complex Ho(Dbm)₃Bpy (where Dbm is dibenzoylmethane and Bpy is 2,2′-bipyridine) was added. X- and Q-band ¹⁵N-ENDOR data have shown that the nitroxide hfi tensor is axially symmetric. The combination of data from all techniques allowed us to obtain the temperature dependence of isotropic and anisotropic parts of the nitroxide hfi tensor. Above ~100 K, a linear dependence of the anisotropic hfi value was observed, whereas below 30 K it was found to be nearly temperature independent. Such a behavior can be interpreted using the model of restricted orientational motions (librations) of a spin probe in a glassy matrix, with quantum effects occurring at low temperature (“freezing” of the librations). The energy quantum for the libration motion estimated from the temperature dependence of hfi of the spin probe is 84 cm^?1. Low-frequency Raman spectra of 5CB were also obtained, which provided the mean vibrational frequency of 76 cm^?1 for glassy 5CB.     

Revealing the Spin-Hamiltonian and Characteristic Parameters of Paramagnetic Centers in Cu2+-Doped Potassium Metabisulfite (K2S2O5) Single Crystal by Electron Paramagnetic Resonance Technique

The present study is mainly related to finding out spin-Hamiltonian parameters of Cu²⁺-doped potassium metabisulfite [K₂S₂O₅] single crystals using electron paramagnetic resonance (EPR) technique which has been applied in the temperature range from 297 to 113 K for single and powder crystals of the title compound. The existence of two complexes and two Cu²⁺ sites for each complex was concluded from the angular variation of the EPR spectra. Having constructed the g and the hyperfine tensors, the spin-Hamiltonian parameters have been obtained for these two complexes. Using these parameters, the covalency parameter (α ²), mixing coefficients (α and β) and Fermi contact term (K) have been calculated.     

A Q-band pulsed ENDOR spectrometer for the study of transition metal ion complexes in solids

We describe the design of a pulsed electron nuclear double resonance (ENDOR) spectrometer operating at Q-band frequencies (35 GHz) for studies of transition metal ion complexes in the temperature range between 4.2 and 297 K. Specific features of the spectrometer are a microwave IMPATT generator, a homebuilt cavity, and a commercial Bruker magnet. Standard Davies and Mims ENDOR sequences have been implemented. The performance of the spectrometer is demonstrated for a broad radio frequency range by¹H,¹⁴N,³¹P,¹³³Cs, and²⁰⁷Pb pulsed ENDOR experiments of Cu²⁺, Cr⁵⁺, and V⁴⁺ transition metal ion complexes in both single crystals and disordered materials.     

Proton ENDOR study of copper(II) bis(dithiocarbamate)

The proton hyperfine coupling tensors of the methylene protons in methyl-deuterated copper(II) bis(N,N-diethyldithiocarbamate) in a diamagnetic host crystal of the corresponding nickel complex have been measured by ENDOR spectroscopy. Two intermolecular and all four intramolecular proton coupling tensors could be determined. With the aid of spin densities, obtained from extended Hückel molecular orbital calculations, the anisotropic part of the tensors can be reproduced quantitatively, taking into account all two- and three-centre contributions. Comparison of the transition frequencies which are computed from the theoretical tensors with the experimental transitions enables the tracing of another five tensors which cannot be completely determined experimentally.     

Resolution enhancement of nitrogen hyperfine patterns in the EPR spectra of Cu(II) complexes: FT Analysis of Cu(II)(His-Gly)2

Fourier analysis of EPR spectra is used to achieve straightforward discrimination between an even or odd number of nitrogen nuclei bound to copper in solution. The method is tested on a copper (II) dipeptide (Cu-HistidylGlycine) complex to confirm the room temperature structural arrangement previously hypothesized by conventional techniques. Any residual uncertainty about ligand coordination around copper in solution is eliminated by the present approach. This simple straightforward method is particularly useful for extracting information from poorly resolved patterns of room temperature EPR spectra.     

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.     

EPR and ENDOR studies of the distal site coordination of shark heart myoglobin

The coordination of small molecules and ions at the sixth position of ferric myoglobins (Mb) from the heart muscle of two species of shark,Lamna ditropis (LD) andPrionace glauca (PG), was studied in frozen solution by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. The results were compared with those of horse heart Mb (EQMb). From our preliminary studies of the absorption spectra and partial amino acid sequences of PGMb and LDMb, we expected PGMb to have glutamine (Gln E7) in place of the distal His E7 that is highly conserved in most animals and present in LDMb. Here, we have used proton ENDOR to show that a water molecule coordinates to the ferric ion in both LDMb and PGMb at pH 6.3. The observed proton hyperfine coupling constant of the coordinating water molecule of PGMb is smaller than those of LDMb and EQMb, which suggests weaker coordination of the water molecule in PGMb, consistent with the absent distal His (E7). Coordination of several exogenous ligands at the distal site was examined by EPR. PGMb showed a coordination behavior different from that of LDMb and EQMb for 1-methylimidazole, 4-methylimidazole and SCN^?, suggesting that its distal site is less sterically hindered. On the basis of the ENDOR and EPR results, we assigned the distal residues of PGMb and LDMb as Gln and His, respectively.     

13Cu、65Cu和15N标记的二(2-羟基苯乙酮肟)-铜(Ⅱ)配合物的ESR波谱研究

本文解析了同位素标记的⁶³Cu-¹⁴N-HAP,⁶⁵Cu-¹⁴N-HAP,⁶³Cu-¹⁵N-HAP。⁶⁵Cu-¹⁵N-HAP,和Cu-¹⁵N-HAP,等五种配合物在四氢呋喃中,在77K温度下测得的分辨较好的ESR波谱。从图上不仅得到了⁶³Cu和⁶³Cu的超精细分裂,而且还得出分辨较好的¹⁴N和¹⁵N的超超精细分裂。比较合理地确定了g_x,g_y,g_z;A_x⁶⁵,A_y⁶⁵,A_z⁶⁵和A_x⁶³,A_y⁶³,A_z⁶³以及A_⊥¹⁵,A_⊥¹⁵和A_⊥¹⁴,A_⊥¹⁴等张量参数。利用测得的波谱参数计算了键参数,并讨论了配合物的电子结构及成键特性,得到了较满意的结果。     

Liquid-crystalline dendrimer Cu(II) complexes and Cu(0) nanoclusters based on the Cu(II) complexes: An electron paramagnetic resonance investigation

New nanostructured materials, namely, the liquid-crystalline copper(II) complexes that contain poly(propylene imine) dendrimer ligands of the first (ligand 1) and second (ligand 2) generations and which have a columnar mesophase and different copper contents (x = Cu/L), are investigated by EPR spectroscopy. The influence of water molecules and nitrate counterions on the magnetic properties of complex 2 (x = 7.3) is studied. It is demonstrated that water molecules can extract some of the copper ions from dendrimer complexes and form hexaaqua copper complexes with free ions. The dimer spectra of fully hydrated complex 2 (x = 7.3) are observed at temperatures T < 10 K. For this complex, the structure is identified and the distance between the copper ions is determined. It is shown that the nitrate counterion plays the role of a bridge between the hexaaqua copper(II) complex and the dendrimer copper(II) complex. The temperature-induced valence tautomerism attended by electron transport is revealed for the first time in blue dendrimer complexes 1 (x = 1.9) with a dimer structure. The activation energy for electron transport is estimated to be 0.35 meV. The coordination of the copper ion site (NO₄) and the structural arrangement of green complexes 1 (x = 1.9) in the columnar mesophase are determined. Complexes of this type form linear chains in which nitrate counterions serve as bridges between copper centers. It is revealed that green complexes 1 (x = 1.9) dissolved in isotropic inert solvents can be oriented in the magnetic field (B ₀ = 8000 G). The degree of orientation of these complexes is rather high (S _z = 0.76) and close to that of systems with a complete ordering (S _z = 1) in the magnetic field. Copper(0) nanoclusters prepared by reduction of complex 2 (x = 7.3) in two reducing agents (NaBH₄, N₂H₄ · H₂O) are examined. A model is proposed for a possible location of Cu(0) nanoclusters in a dendrimer matrix.     

Theory of63Cu and17O quadrupole interactions in copper oxide high-T c systems

Electronic wave-functions obtained by the first-principles Unrestricted Hartree-Fock Cluster procedure are utilized to evaluate the electric field-gradient tensors at the⁶³Cu and¹⁷O nuclei in YBa₂Cu₃O₇. Our results provide satisfactory explanations of the available nuclear quadrupole interaction data for this system.     

Variable temperature EPR spectra of Copper (II) ions in kainite crystals

X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in KMgClSO₄·3H₂O single crystals have been performed at low temperature (123 K). The angular variation of the EPR spectra reveals the presence of two Cu²⁺ sites, which have different orientations. The spin-Hamiltonian parameters of this six-coordinated cupric ion have been evaluated from the EPR spectra at 123 K. The forbidden lines due to Δm_I=±1 transitions are observed in between allowed transitions. The temperature variation EPR studies have also been performed both for a single crystal and a polycrystalline sample. The ground state wavefunction of Cu²⁺ ions has been estimated and is found to be an admixture of d_3z²−r² and d_x²−y². The temperature variation of the EPR spectra reveals that Cu²⁺ ions exhibit dynamic Jahn-Teller effect. From the polycrystalline EPR data, the temperature dependent magnetic susceptibilities are evaluated and discussed.     

EPR of Er3+, Nd3+, and Ce3+ ions in YAlO3 single crystals

EPR spectra of the Er³⁺, Nd³⁺, and Ce³⁺ ions substituting for the Y³⁺ ion in the YAlO₃ yttrium orthoaluminate lattice are studied. The EPR spectra of these rare-earth ions are described by a spin Hamiltonian of rhombic symmetry with an effective spin S=1/2. The principal values of the g tensors were determined from an analysis of the angular dependences of the EPR spectra. The orientation of the local magnetic axes of paramagnetic centers relative to the YAlO₃ crystallographic directions are shown to depend on the actual rare-earth species. The EPR spectra exhibit a hyperfine structure due to the ¹⁶⁷Er, ¹⁴³Nd, and ¹⁴⁵Nd odd isotopes, which permitted unambiguous identification of these spectra. The hyperfine coupling constants for the odd erbium and neodymium isotopes are determined.     

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 interactionJS ₁ S ₂ 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 asJ _zz =?0.02 cm^?1,J _xx.yy =0.01 cm^?1 for compound I.     

Simulation of the Electron Paramagnetic Resonance Spectrum of the Triplet Ground State Dimer Di-μ-(Pyridine N-Oxide)Bis[Bisnitrato(Pyridine N-Oxide)Copper(II)]

The EPR spectrum of the triplet ground state dimer di-μ-(pyridine N-oxide)bis[bisnitrato(pyridine N-oxide)copper(II)] has been reported recently¹. Of the very few triplet ground state copper(II) dimers with resolved metal hyperfine structure^2,3, the EPR spectrum of this complex is most complete. Previously, the analysis of the spectra of triplet ground state copper(II) complexes, in order to extract magnetic parameters, has been made using the equations reported by Wasserman et al.⁴ The best magnetic parameters should be obtained from a simulation of the experimental spectrum. We wish to report here the computer simulation of the EPR spectrum of a powdered sample of [Cu(II) (PYO)₂ (NO₃)₂]₂.