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Redetermination of bis(l-asparaginato)copper(II)
(2004) Vencato, Ivo; Lariucci, Carlito; Ferreira, Katiúscia Daiane; Santana, Ricardo Costa de; Carvalho, Jesiel Freitas
The Cu atom of the title compound, [Cu(C4H7N2O3)2], is in a tetragonally distorted octahedral environment. A carboxyl O atom and the α-amino N atom from each ligand coordinate to Cu in a trans square-planar configuration [Cu—O = 2.003 (3) and 2.020 (3) Å, and Cu—N = 2.029 (4) and 2.049 (4) Å]. The octahedron is completed by bridging amide O atoms from adjacent molecules [Cu—O = 2.229 (4) Å and Cu—O = 2.885 (4) Å] separated by c translations. This arrangement creates infinite chains linked in the [001] direction, stabilized by intra- and intermolecular N—H...O bonds. The structure has already been published [Stephens, Vagg & Williams (1975). Acta Cryst. B31, 841–845]; this redetermination reports the structure with higher precision.
Vanadium-doped photorefractive titanosillenite crystal
(2009) Montenegro, R.; Shumelyuk, A.; Kumamoto, R.; Carvalho, Jesiel Freitas; Santana, Ricardo Costa de; Frejlich, Jaime
Holographic recording in a vanadium-doped B12TiO20 (BTO) photorefractive crystal puts into evidence a large hole–electron competition showing a fast and a slow hologram components. From the fast component evolution, some material parameters for the electron-donor photoactive centers are computed. The wavelength-resolved photoconductivity is shown to be strongly modified by V-doping compared to undoped and doped BTO with other elements. The increase of photoconductivity by green light preexposure is almost negligible here if compared with undoped BTO. Activation energy for dark conductivity measured for BTO:V is similar to that for undoped BTO, as measured close to room temperature, but sensibly lower than the value reported in the literature for a much higher temperature range. Optical absorption and EPR spectra do confirm already published results and suggestions about the possible role of vanadium in the sillenite structure.
EPR study of Cu(II) dopant ions in single crystals of bis(l-asparaginato)Zn(II)
(2006) Santana, Ricardo Costa de; Santos, Marcilene Gomes dos; Cunha, R. O.; Ferreira, Katiúscia Daiane; Carvalho, Jesiel Freitas; Calvo, Rafael; Peña Garcia, Ramón Raudel
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 ICu=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.
Magnetic properties and EPR spectra of [Cu(L-arginine)2](NO3)2·3H2O
(2007) Gerard, M. F.; Aiassa, C.; Casado, N. M. C.; Santana, Ricardo Costa de; Perec, Mireille; Rapp, Raul Edgardo; Calvo, Rafael
Magnetic and EPR data have been collected for complex [Cu(L-Arg)2](NO3)2 3H2O (Arg ¼ arginine). Magnetic susceptibility w in the temperature range 2–160 K, and a magnetization isotherm at T ¼ 2.29(1) K with magnetic fields between 0 and 9 T were measured. The observed variation of wT with T indicates predominant antiferromagnetic interactions between Cu(II) ions coupled in 1D chains along the b axis. Fitting a molecular field model to the susceptibility data allows to evaluate g ¼ 2.10(1) for the average g-factor and J ¼ 0.42(6) cm1 for the nearest neighbor exchange coupling (defined as Hex ¼ PJijSi Sj). This coupling is assigned to syn–anti equatorial–apical carboxylate bridges connecting Cu(II) ion neighbors at 5.682 A˚ , with a total bond length of 6.989 A˚ and is consistent with the magnetization isotherm results. It is discussed and compared with couplings observed in other compounds with similar exchange bridges. EPR spectra at 9.77 were obtained in powder samples and at 9.77 and at 34.1 GHz in the three orthogonal planes of single crystals. At both microwave frequencies, and for all magnetic field orientations a single signal arising from the collapse due to exchange interaction of resonances corresponding to two rotated Cu(II) sites is observed. From the EPR results the molecular g-tensors corresponding to the two copper sites in the unit cell were evaluated, allowing an estimated lower limit |J |40.1 cm1 for the exchange interaction between Cu(II) neighbors, consistent with the magnetic measurements. The observed angular variation of the line width is attributed to dipolar coupling between Cu(II) ions in the lattice. r 2007 Elsevier Ltd. All rights reserved.
Synthesis, crystal structure and magnetic properties of a new dinuclear copper(II) amino acid complex [Cu2(l-arg)2(μ-HPO4-O)(μ-HPO4-O,O′)(μ-OH)]− · (H3O)+ · (H2O)6
(2007) Santana, Ricardo Costa de; Carvalho, Jesiel Freitas; Vencato, Ivo; Napolitano, Hamilton Barbosa; Bortoluzzi, Adailton João; Barberis, Gaston Eduardo; Rapp, Raul Edgardo; Passeggi, Mario C .G.; Calvo, Rafael
A novel dinuclear copper(II) complex with the amino acid l-arginine (l-arg), with mono and bidentate HPO42− oxoanions and an OH− anion. [Cu2(l-arg)2(μ-HPO4-O)(μ-HPO4-O,O′)(μ-OH)]− · (H3O)+ · 6H2O (1) was prepared and its structure was determined by X-ray diffraction methods. The two independent copper ions are in a distorted square pyramidal coordination, each bonded to one l-arginine molecule. These two Cu(l-arg) units are bridged by two monoatomic equatorial–apical oxygen ligands belonging to a monodentate hydrogenphosphate group, and to the hydroxyl group. The copper ions in the dinuclear unit at d = 3.1948(8) Å are also connected by two equatorial oxygen belonging to a bidentate hydrogenphosphate. This dinuclear character and bridging scheme, not common for metal–amino acid compounds, is a consequence of the properties of the phosphate anions. The magnetic susceptibility at temperatures between 2 and 300 K and the isothermal magnetization curves at T = 2.29(1) K with applied fields up to 9 T were measured. The magnetic data indicate an antiferromagnetic intradinuclear exchange coupling J/kB = −3.7(1) K and using a molecular field approximation we estimated a weaker ferromagnetic interaction J′/kB ∼ 0.3 K between neighbour dinuclear units.
Single crystal EPR study of electronic structure and exchange interactions for copper(II)(l-arginine)2(SO4) · (H2O)6: a model system to study exchange interactions between unpaired spins in proteins
(2005) Santana, Ricardo Costa de; Cunha, R. O.; Carvalho, Jesiel Freitas; Vencato, Ivo; Calvo, Rafael
We report EPR measurements at 9.77 and 34.1 GHz in powder and single crystal samples of the ternary copper amino acid complex Cu(l-arginine)2(SO4) · (H2O)6. The single crystal Electron Paramagnetic Resonance spectra display a single resonance for all magnetic field orientations in the ca and cb crystal planes. In the ab plane they display two resonances for most orientations of the magnetic field, and only one resonance for orientations close to the crystal axes. This behavior is a result of the selective collapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchange interactions between copper ions. From the characteristics of the collapse and the angular dependences of the position and width of the resonances we evaluate the g-tensors of the copper molecules and estimate exchange interactions |J1/kB| = 0.9 K and |J2/kB| = 0.009 K between copper neighbors at 5.908 Å and at 15.684 Å, respectively. J1 is assigned to a syn–anti equatorial–apical carboxylate bridge with a total bond length of 7.133 Å. J2 is assigned to a long bridge of 12 atoms with a total bond length of 19.789 Å, that includes two hydrogen bonds. The results are discussed in terms of the crystal and electronic structure of Cu(l-arginine)2(SO4) · (H2O)6. We show that J2 is in excellent agreement with the observed magnetic interaction between the reduced quinone acceptors in the photosynthetic reaction center protein of the bacterium Rb. sphaeroides, which is transmitted along a similar chemical path containing two hydrogen bonds. Our findings indicate that it is valid to estimate values for the exchange interactions between redox centers in proteins transmitted along long chemical paths containing sigma and H-bonds, from data obtained in model systems, and emphasize the importance of measuring exchange interactions in biologically relevant model systems.
Electron spin resonance of Cu2+ impurities in l-arginine phosphate monohydrate single crystals
(2022) Santana, Ricardo Costa de; Carvalho, Jesiel Freitas; Amaral, S. R.; Vencato, Ivo; Pelegrini, Fernando; Terrile, Maria Cristina; Hernandes, Antonio Carlos; Calvo, Rafael
Using electron paramagnetic resonance spectroscopy we studied single crystals of l-arginine phosphate monohydrate doped with copper. The angular dependence of the spectrum in three orthogonal planes shows that the Cu2+ ions occupy two symmetry related sites in the structure. The Zeeman and hyperfine coupling tensors were determined. The principal values are: g1=2.053, g2=2.054, g3=2.251, A1≈0, and Superhyperfine interactions with 14N ligands, well resolved for certain orientations of the magnetic field, suggest that Cu impurities have three N ligands. In terms of the molecular structure and based in similar systems, our findings are discussed and suggestions are made.
Alternate Cu2 and Er2 spin carriers in a carboxylate-bridged chain: EPR study
(2009) Calvo, Rafael; Rapp, Raul Edgardo; Sartoris, Rosana P.; Santana, Ricardo Costa de; Perec, Mireille
We report powder EPR measurements at 9.48 GHz and temperatures of 4 K ≤ T ≤ 300 K and at 33.86 GHz and T = 300 K for the polymeric compound {[Cu2Er2(L)10(H2O)4]·3H2O}n (HL = trans-2-butenoic acid) having alternate Cu2 and Er2 dinuclear units bridged by carboxylates along a chain. Above 70 K, when the ErIII resonance is unobservable and uncoupled from the CuII ions, the spectrum arises from the excited triplet state of antiferromagnetic Cu2 units, decreasing in intensity as T decreases, and disappearing when these units condensate into the singlet ground state. Fit of a model to the spectra at 9.48 and 33.86 GHz and 300 K gives gCu∥ = 2.379, gCu⊥ = 2.065, DCu = −0.340 cm−1, and ECu ∼ 0 for the g-factors and zero field splitting parameters. From the T dependence of the intensity of the spectrum above 70 K, we obtain JCu−Cu = −336(11) cm−1 for the intradinuclear exchange interaction. Below 50 K, a spectrum attributed to Er2 units appears, narrows, and resolves as T decreases, due to the increase of the spin−lattice relaxation time T1. The spectrum at 4 K allows calculating g values g1 = 1.489, g2 = 2.163, and g3 = 5.587 and zero field splitting parameters DEr = −0.237 cm−1 and EEr = 0.020 cm−1. The results are discussed in terms of the properties of the Cu and Er ions, and the crystal structure of the compound.
Electron paramagnetic resonance (EPR) of antiferromagnetic nanoparticles of La1−xSrxCrO3 (0.000 ≤ x ≤ 0.020) synthesized by combustion reaction
(2010) Franco Junior, Adolfo; Santana, Ricardo Costa de
Nanocrystalline particles of La1−xSrxCrO3 (0.000 ≤ x ≤ 0.020) compounds were synthesized in order to investigate the antiferromagnetic (AFM) to paramagnetic (PM) phase transition temperature, g-factor, line width and intensity by electron paramagnetic resonance (EPR). All samples were synthesized by combustion reaction method using strontium nitrate, lanthanum nitrate, chromium nitrate and urea as fuel without subsequent heat treatment. X-ray diffraction patterns of all systems showed broad peaks consistent with orthorhombic structure of LaCrO3. The absence of extra reflections in the diffraction patterns of as-prepared materials ensures the phase purity. The average crystallite sizes determined from the prominent (1 1 2) peak of the diffraction using Scherrer's equation was independent of the addition of Sr2+ ions; being ca. 31–29 nm for x = 0.000 and 0.020, respectively. The EPR line width and intensity were found to be dependent on Sr2+ addition and temperature. However, the AFM–PM transition temperature was found to be independent of strontium concentration, being ca. 296 K. In the PM phase, g-factor was nearly temperature independent with increasing of x. The EPR results indicated that the addition of Sr2+ ions may induce creation of Cr3+–Cr4+ clusters.
Specific heat measurements in pure and in (Cu, Mn, Fe, Ni)-doped single-crystals of l-arginine phosphate monohydrate
(2010) Machado, Fernando Luis de Araujo; Sousa, Lázaro Luis de Lima; Cunha, R. O.; Cabral, F. A. O.; Rodrigues, Alexandre Ricalde; Carvalho, Jesiel Freitas; Santana, Ricardo Costa de
The influence of highly diluted impurities (Cu, Mn, Fe, Ni) on the temperature (T) dependence of the specific heat (cp) of l-arginine phosphate monohydrate (LAP) was investigated in the temperature range 1.8–300 K. The doped samples yielded values for cp in excess to those obtained for a pure LAP sample. The melting temperatures (Tm≈420 K) obtained by differential scanning calorimetry for pure and doped LAP samples were found not to be significantly affected by the impurities. The T-dependence of cp was fully accounted for by taking into consideration the Debye contribution, an Einstein term and a contribution due to both Frenkel and Schottky defects. The model fit all cp versus T data using a single value for both the Debye (θD=160 K) and the Einstein (TE=376.8 K) temperatures, and for the energy (εd=157.9 meV) required to create the defects.
Single crystal electron paramagnetic resonance spectra of CuII ions in Cu(tyrosine)2: a study of weak exchange interactions mediated by resonance assisted hydrogen bonds (RAHB)
(2012) Paredes García, Veronica; Santana, Ricardo Costa de; Madrid, Rosa; Baldo, Bianca; Vega, Andrés; Spodine, Evgenia
EPR measurements have been performed on single crystals of [Cu(L‐tyrosine)2]∞ at 33.8 GHz and at room temperature. The EPR spectra display partially resolved EPR lines for most orientations of the magnetic field in the ab plane, and only one resonance for orientations close to the crystal axes, while only a single line is observed along any direction in the ca and cb crystal planes. This behavior is a result of the selective collapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchange interactions between the copper ions. The magnitudes of the exchange interactions between the copper ions were evaluated from the angular variation of the line width and the collapse of the EPR lines. The value |JAD/kB| = 0.8 K between neighboring copper atoms at 4.942 Å is assigned to a syn−anti equatorial−apical carboxylate bridge with a total bond length of 6.822 Å, while the small value |JAB/kB| = 0.004 K is assigned to a long bridge of 11 atoms with a total bond length of 19.186 Å, that includes one resonance assisted hydrogen bond (RAHB). This finding is discussed in terms of values obtained for similar paths in other model compounds and in proteins.
Structure and magnetism of catena-poly[copper(II)-μ-dichloro-l-lysine]hemihydrate: copper chains with monochloride bridges
(2012) Santana, Ricardo Costa de; Ferreira, Bruno N.; Sabino, José Ricardo; Carvalho, Jesiel Freitas; Peña, Octavio; Calvo, Rafael
We report the preparation and characterization by elemental analysis, infrared (IR), single crystal X-ray diffraction, magnetic measurements and electron paramagnetic resonance (EPR) of the new compound catena-poly[copper(II)-l-dichloro-L-lysine]hemihydrate, with formula C6H14Cl2CuN2O2(H2O)0.5, here after called [Cu(lys)Cl2]. The compound crystallizes in the space group C2 with copper ions in a square pyramidal coordination, and the structure consists of zigzag chains along the b axis bridged by equatorial-apical chlorine bonds with Cu–Cu distance of 3.5414(4) Å, and a small Cu–Cl–Cu angle of 85.47(6). Magnetic susceptibility and isothermal magnetization curves reveals a weak antiferromagnetic interaction 2J = 0.30(2) cm1 supported by the Cu–Cl–Cu bridge connecting CuII neighbors in the chains. EPR spectra at 34.0 GHz were obtained in powdered samples and in a single crystal with B0 in three orthogonal planes. Assuming an axially symmetric molecular g-matrix, we obtained principal g-values g// = 2.2794(9) and g? ¼ 2:0489ð3Þ from the crystal g-matrix calculated using the observed angular variation of the EPR line position. The angular variation of the line width, largest along the chain direction, is attributed to dipolar couplings between CuII ions in the lattice. The magnetic and EPR results are discussed in terms of the exchange couplings.
Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu2+ ions
(2007) Santana, Ricardo Costa de; Santos, Marcilene Gomes dos; Cunha, R. O.; Ferreira, Katiúscia Daiane; Carvalho, Jesiel Freitas; Calvo, Rafael
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 ICu= 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 ACu tensors calculated from the EPR data are g1=2.051(1), g2=2.062(2), g3=2.260(2), ACu,1=16.9(5)×10−4 cm−1, ACu,2=21.8(6)×10−4 cm−1, ACu,3=180.0(5)×10−4 cm−1. The principal directions corresponding to g3 and to ACu,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.
Collapse of the EPR fine structure of a one-dimensional array of weakly interacting binuclear units: a dimensional quantum phase transition
(2011) Calvo, Rafael; Elías, Abud Julián; Sartoris, Rosana P.; Santana, Ricardo Costa de
Binuclear (also called dimeric) compounds with pairs of antiferromagnetically coupled spins ½, 𝐒𝟏 and 𝐒𝟐 (𝐻ex = −𝐽0 𝐒𝟏.S𝟐, with 𝐽0<0 for antiferromagnets), have been around for ∼60 years, providing roots to the field of molecular magnetism. In addition, as reported in recent years, weak interactions between binuclear units in a crystalline network give rise to interesting systems of interacting bosons having an energy gap, which are important in the study of quantum phenomena in many body systems coupled by stochastic distributions of interactions. Binuclear compounds have gained new relevance in the last decade with the observation of Bose-Einstein condensation. In this work, we use electron paramagnetic resonance (EPR) to study the role of weak inter-binuclear exchange couplings J’ (|J’| ≪|⁢𝐽0|) in the spectra, elementary excitations, and spin dynamics of a one-dimensional (1-D) array of antiferromagnetic (AFM) binuclear units in the hybrid (organic-inorganic) Cu𝐼𝐼 compound [Cu(CH3COO)(phen)(H2O)]2·(NO3)2·4H2O. In this material, the acetate (CH3COO)− anion supports the intra-binuclear exchange coupling 𝐽0, and the stacking of the (phen) = 1,10-phenanthroline rings of neighbor units supports the inter-binuclear interactions J′, giving rise to well-isolated chains. This has advantages over other binuclear compounds studied previously because magnetically equal units are arranged in a 1-D spatial arrangement along the direction of their symmetry axis, simplifying the analysis of the data and allowing a simpler treatment. In addition, single crystals of good quality allow detailed EPR experiments. EPR spectra were collected at ∼33.8 and ∼9.4–9.8 GHz in oriented single crystals at room temperature and in powder samples at temperatures (T) between 10 and 300 K. By varying the energy levels of the binuclear units with the magnetic field orientation, or changing the population of the excited triplet state with temperature and, consequently, the effective coupling between units, we observe in single-crystal samples sudden merges of the fine structure peaks, accompanied by a large narrowing, when the inter-binuclear coupling becomes larger than the splitting of the triplet state. In addition, and because of this collapse of the fine structure peaks, the spectra of powder samples display a strong and unexpected central peak that decreases in intensity with decreasing temperature, as it occurs with the binuclear signals. We first discuss the dimensional quantum phase transition indicated by the spectral changes using Anderson-Kubo's theory of exchange narrowing. The data allow evaluation of the binuclear exchange coupling 𝐽0 = (−74 ± 3) cm−1, and the interactions between neighbor binuclear units |J′| = (0.04 ± 0.01) cm−1. We also consider the magnetic excitations (triplet excitons or triplons) arising from the inter-binuclear couplings, and introduce a model explaining qualitatively the observed collapse and narrowing of the EPR spectra in terms of these excitons. We analyze the role of temperature in the inter-binuclear interactions and the exchange correlation times of the binuclear systems and compare our results with those in binuclear compounds in which Bose-Einstein condensation occurs.
Zirconia-supported cobalt catalysts: activity and selectivity in NO reduction by CO
(2017) Lima, Thiago de Melo; Pereira, Cristiane Alves; Nova Castelblanco, William; Santos, Camila Messias Barbosa; Silva, Sebastião William da; Santana, Ricardo Costa de; Urquieta Gonzalez, Ernesto Antonio; Sartoratto, Patrícia Pommé Confessori
Cobalt catalysts supported on tetragonal and monoclinic zirconia were prepared using a one-pot procedure through the incorporation of a Co3O4 colloidal suspension during the support synthesis and evaluated in the reaction of NO reduction by CO. A better dispersion of the active phase was obtained by this method when compared to the conventional impregnation one. Furthermore, the crystalline phase of zirconia in the supports had a crucial effect in the dispersion of the cobalt species, being that the pure tetragonal phase led to a higher dispersion of the active phase. The one-pot prepared tetragonal zirconia led to cobalt active species that consisted in well-dispersed small agglomerates of Co3O4 and Co2+ species as oxo-ions which displayed the best catalytic performance among the studied catalysts, with the highest NO conversion (75%) and high selectivity to N2 (85%) at 300 °C. These results show that the one-pot methodology employed in this work has a strong potential to produce suitable catalysts for the abatement of NOx emissions.
Magnetic behaviour of bimetallic layered phases M′0.2Mn0.8PS3·0.25 H2O (M′ = ZnII, CuII, NiII, CoII)
(2017) Fuentealba, Pablo; Cortes, C.; Manzur, J.; Paredes García, Veronica; Venegas-Yazigi, Diego; Silva, I. D. A.; Santana, Ricardo Costa de; Magon, Claudio José; Spodine, Evgenia
In this work the magnetic properties of bimetallic phases M′0.2Mn0.8PS3·0.25H2O (M′ = CoII, NiII, CuII or ZnII) have been explored and compared with those of the pristine phase MnPS3. Magnetic susceptibility, high field magnetization and electron paramagnetic resonance (EPR) studies reveal that the transition temperature between the antiferromagnetic and paramagnetic order for the pristine phase is shifted to lower values in the bimetallic phases. From magnetization measurements the critical field of the spin-flop transition is found to be dependent on the nature of the added secondary transition metal ion. EPR spectra of all compounds in the temperature range of 8–300 K present a single resonance line shape. Temperature dependence of the EPR parameters, like line width, g values and double integrated area (IDIN), are obtained from the spectra and present a scenario compatible with the magnetization results. The temperature dependence of the first derivative of the product (IDINT) shows two maxima for all samples, with exception of the CoII phase, indicating two critical temperatures, while these critical temperatures could not be clearly determined by dc susceptibility.
Structural and EPR studies of pyrophosphate-bridged dinuclear CuII complexes
(2014) Sartoris, Rosana P.; Calvo, Rafael; Santana, Ricardo Costa de; Nascimento, Otaciro Rangel; Perec, Mireille; Baggio, Ricardo F.
Two new pyrophosphate-bridged copperII complexes, [Cu(dpa)(H2O7P2)]2 1 and [Cu2(terpy)2(HO7P2)·(H2O4P)·(H3O4P)·(H2O)] 2 (dpa = 2,2′-dipyridylamine and terpy = 2,2′:6′,2″-terpyridine) were isolated and their crystal structures determined by single-crystal X-ray diffraction. The compounds are triclinic and contain dinuclear copperII units bridged by pyrophosphate anions. The EPR spectra observed in three planes of single crystal samples as a function of field orientation at 293 K for compounds 1 and 2, and also for two other pyrophosphate compounds already reported, [Cu(bipy)(cis-H2O7P2)]2·3(H2O) 3 and [Cu(bipy)(trans-H2P2O7)]2 4 display a single resonance for any field orientation and temperatures T between 4 and 293 K, as in mononuclear spin systems, without hyperfine structure, and their g-factors and line widths were measured. The relations between the principal directions of the g-matrices and the molecular structures are discussed and compared with related compounds. The temperature dependences of the intensity of the EPR signals observed for 1–4 above 4 K indicate a paramagnetic Curie behavior, with no indication of intradinuclear exchange interactions (so, |J| < 2 K). The absence of dinuclear splitting and of hyperfine structure of the dinuclear units is explained in terms of averaging out by the interdinuclear interactions, allowing to set a lower limit of their magnitudes.
Structure and magnetism of a binuclear CuII pyrophosphate: transition to a 3D magnetic behaviour studied by single crystal EPR
(2015) Rosana, Sartoris P.; Nascimento, Otaciro Rangel; Santana, Ricardo Costa de; Perec, Mireille; Baggio, Ricardo F.; Perec, Mireille; Baggio, Ricardo F.; Calvo, Rafael
A binuclear CuII compound [Cu2(bpa)2(P2O7)(H2O)2]·2.5H2O, 1, (bpa = 2,2′-bipyridylamine), with pairs of CuII ions bridged by one pyrophosphate tetra-anion, was synthesized and crystallized. Its triclinic structure was determined by single-crystal X-ray diffraction. Electron paramagnetic resonance (EPR) spectra of single crystal samples of 1 were recorded for a fixed orientation of the magnetic field (B0) as a function of temperature (T) between 4.7 and 293 K, and at T = 4.7, 50 and 293 K, as a function of the orientation of B0. Below ∼8 K, the spectra are assigned to two types of mononuclear crystal defects hyperfine-coupled to one copper and two nitrogen nuclei. The g-matrices and hyperfine couplings at these T provide information about the structures of these defects. Above 10 K, the spectrum is dominated by the response of the bulk binuclear CuII material, showing hyperfine interactions with two copper nuclei, collapsing to a single peak above 18 K when the units are magnetically connected, and the magnetic behaviour becomes 3D. We attribute the results above 10 K to the interplay of an AFM intrabinuclear exchange interaction J0 = −28(3) cm−1 (defined as Hex = −J0S1·S2), and three orders of magnitude weaker exchange coupling with average magnitude |J1| ≥ 0.022 cm−1 between CuII ions in neighbouring binuclear units. The interplays between structure, exchange couplings, magnetic dimension and spin dynamics in the binuclear compound are discussed. A previously unreported situation, where the structure of the spectra arising from the anisotropic spin–spin interaction term (D) within the binuclear unit is averaged out, but the forbidden half field transition is not, is observed and explained.
Pyrophosphate-bridged Cu(II) chain magnet: {[Na3Cu(P2O7)(NO3)].3H2O}n
(2010) Sartoris, Rosana P.; Santana, Ricardo Costa de; Baggio, Ricardo F.; Peña, Octavio; Perec, Mireille; Calvo, Rafael
A Cu(II)...Cu(II) pyrophosphate-bridged compound of formula {[Na(3)Cu(P(2)O(7))(NO(3))].3H(2)O}(n) (1) has been characterized. X-ray diffraction measurements show that it crystallizes in the monoclinic space group P2(1)/m, with unit cell dimensions a = 7.2492(5) A, b = 8.2446(6) A, c = 9.9050(7) A, beta = 107.123(1) degrees, and Z = 2. The structure consists of chains of Cu(II) cations at inversion symmetry sites bound to four equatorial oxygen atoms provided by two pyrophosphate anions halved by a symmetry plane and two axial oxygen atoms of nitrate anions. The molar magnetic susceptibility chi(0) of a powdered sample was measured in the temperature range 2 K < T < 273 K, and an isothermal magnetization curve, M(B(0),T), was obtained at T = 30 K, with the magnetic field B(0) between 0 and 5 T. Fitting a spin-chain model to the susceptibility data, we evaluate an antiferromagnetic exchange coupling 2J = -24.3(1) cm(-1) (defined as H(ex) = -2JS(i)S(j)) between Cu(II) neighbors. For any orientation of B(0), single-crystal electron paramagnetic resonance (EPR) spectra obtained at 9.8 and 33.9 GHz at 300 K display a single signal having a g matrix with orthorhombic symmetry, arising from the merger produced by the exchange interaction of the resonances corresponding to the two rotated Cu(II) sites. The g matrices of the individual molecules calculated assuming axial symmetry yielded principal values g(parallel) = 2.367(1) and g(perpendicular) = 2.074(1) at both frequencies, indicating a d(x(2)-y(2)) ground-state orbital for the Cu(II) ions. The angular variation of the EPR line width suggests exchange narrowing in a system with one-dimensional spin dynamics, as expected from the structure and susceptibility data. The results, discussed in terms of the crystal and electronic structures and of the spin dynamics of the compound, are compared with those obtained in other materials.
The NuMI neutrino beam
(2016) Adamson, P.; Anderson, K.; Andrews, M.; Andrews, R.; Anghel, I.; Augustine, D.; Aurisano, A.; Avvakumov, S.; Ayres, D. S.; Baller, B.; Gomes, Ricardo Avelino
This paper describes the hardware and operations of the Neutrinos at the Main Injector (NuMI) beam at Fermilab. It elaborates on the design considerations for the beam as a whole and for individual elements. The most important design details of individual components are described. Beam monitoring systems and procedures, including the tuning and alignment of the beam and NuMI long-term performance, are also discussed.

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