Single crystal ESR study of bis(N-benzylpyridoxaldiminato) copper(II)

Single crystals of bis(N-benzylpyridozaldiminato) Cu(II) were studied by ESR in three perpendicula planes at 300 K and 9.4 GHz. Only one exchange—narrowed line was observed for the two magnetically inequivalent copper ions in the monoclinic symmetry lattice with space group P2₁/a and elementary cell parameters a = 9.561(9), B = 16.605(8), C = 17.561(2) Å and β = 104(1°). Each ligand is bidentate via the phenolic oxygen and the imino nitrogen atoms, leading to tetrahedrally distorted square planar environment around the copper atom. The ESR linewidth remained Lorentzian for all orientations considered. The components of the g_xx = 2.043(4), g_yy = 2.064(4), g_zz = 2.259(4). The linewidth anisotropy was analyzed in terms of dipole-dipole interactions, magnetic inequivalence of molecules and hyperfine interactions.     

EPR of layered magnetic metal—amino acid salts. I. Cu(L-PHE)2

EPR measurements in single crystals of Cu(L-PHE)₂, the copper derivative of the amino acid L-phenylalanine, were performed at 300 K and 9.7 GHz. The gyromagnetic factor, g, and the linewidth of the single EPR line were measured in three perpendicular planes of the sample. The gyromagnetic tensor was obtained, and its principal values are g₁ = 2.211, g₂ = 2.134, and g₃ = 2.075. The single resonance is explained by the collapse of the resonances of the two magnetically non-equivalent copper sites due to the exchange interaction. The molecular g-factor of isolated copper ions is obtained from a model assuming axial symmetry. The results are g_| = 2.266 and g_⊥ = 2.075, indicating a d(x² - y²) ground orbital. The orientations of the molecules in the crystal obtained by this model are in agreement with the crystallographic values. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic structure, an incomplete collapse of the hyperfine structure, and contributions arising from non-isotropic exchange. A mean value ¦J¦=0.19 K is calculated for the isotropic exchange interactions between one copper and its six copper neighbors in Cu(L-PHE)₂. Also, a lower limit ¦J′¦ > 0.02 K for the exchange coupling J′ between non-equivalent copper neighbors is obtained.     

Magnetic characterization, synthesis and crystal structure of a heterodinuclear CuGd Schiff base complex bridged by the two phenolic oxygen atoms

A novel heterodinuclear complex formed by the reaction of gadolinium nitrate with Schiff base complex of copper(II) has been synthesized and characterized. Preparation, crystal structure and magnetic properties of the heterodinuclear complex, LCu(Me₂CO)Gd(NO₃)₃, (L=(N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane) are reported. The complex is consisting of a deca-coordinated Gd^III ion which is bridged to four coordinated Cu^II via both phenolate oxygen atoms of the L Schiff base ligand. The average CuGd separation is 3.475(2) Å. There is also one non-coordinating acetone molecule in the crystal structure. The magnetic susceptibility of the complex was measured over the range 4.5–350 K and the observed data were successfully simulated by the equation based on the spin-Hamiltonian operator H=−JS_Cu·S_Gd. The values of the intrachain interaction parameters have been deduced from the magnetic data: exchange integral J(Cu–Gd)=7.3 cm⁻¹, g_Cu=2.17, g_Gd=2.09. This indicates a weak ferromagnetic spin exchange interaction between Cu^II and Gd^III ions. The nature of the magnetic super-exchange interaction of the title compound is compared with similar Cu^IIGd^III heterodinuclear complexes.     

Origin of additional satellites in electron paramagnetic resonance spectra of rare earth ion pairs

Electron Paramagnetic Resonance (EPR) spectrum of pairs of two identical rare earth ions is considered in the case where the two ions feel slightly different crystal fields giving different g factors. When the differences Δg between the g factors give a Zeeman difference term ΔgβB₀ of the order of magnitude of the interaction between the two ions, the pair spectrum is composed of four lines instead of two: the usual doublet structure, and two additional satellites around the main central transitions. It is shown that for rare earth ions, the shape of the EPR pair spectrum is very sensitive to small g factor differences. This situation is illustrated by the case of neodymium pairs in the SrAl₁₂O₁₉ host.     

High-resolution ODMR- and ODNQR spectroscopy of acridine-d9 in its lowest excited triplet state

High-resolution ODMR- and ODNQR spectroscopy have been performed on acridine-d₉ in its photoexcited triplet state. The nuclear quadrupole constants e²qQ/h = −4.36(3) MHz and η = 0.22(1) have been measured for the ¹⁴N nucleus, where the sign of e²qQ/h is determined relative to the signs of the fine structure parameters, D and E. The ¹⁴N ODNQR spectrum, with linewidths as low as 60 kHz, exhibits a satellite structure attributable to the ²D nucleus located at the meso-position in the molecule. For this nucleus |e²qQ/h| = 180(30) kHz may be estimated. The intensity behaviour of the ODNQR transitions as a function of the NMR rf power level leads to a coarse determination of the in-plane components of the ¹⁴N hyperfine tensor with |A_yy^N| 0.2 MHz and |A _yy^N| 4.7 MHz. Owing to accidental overlap of two transitions in the 2E-ODMR spectrum of acridine, the ODNQR technique had to be extended to a five-pulse sequence.     

Synthesis, chemical characterization and crystal structure of the (oxalato-O,O′)bis(1,10-phenanthroline)copper(II) pentahydrate

Violet single-crystals of the complex [Cu(ox)(phen)₂]·5H₂O (1), where ox²⁻ is oxalate and phen is 1,10-phenanthroline, were obtained by slow evaporation of a solution previously prepared by dissolving Cu(ox)·1/3H₂O in a water–acetonitrile solution of phenantroline. Its crystal structure consists of neutral mononuclear [Cu(ox)(phen)₂] units and crystallization water molecules which are held together by face-to-face stacking interactions between the phenantroline aromatic rings and an extensive three-dimensional network of O_w–HO/O_w hydrogen bonds. The copper atom is hexaco-ordinated to two oxygen atoms of a bidentate oxalato ligand and to four nitrogen atoms belonging to two phen ligands in a distorted octahedral cis arrangement. The e.p.r. Q-band spectrum of 1 shows an axial-type signal with g-tensor values of g=2.28 and g=2.06, which is consistent with the axially elongated octahedral geometry of the copper(II) chromophore found in the structural work. The relation gg>2.0 indicates a d(x²−y²) ground state. Variable temperature susceptibility measurements (5–300 K) reveals the occurrence of magnetically isolated paramagnetic centers in the crystal structure. Thermal degradation of the compound 1 under synthetic air atmosphere starts between 50 and 110°C with an endothermic process attributable to the release of the crystallization water molecules.     

'Genome order index' should not be used for defining compositional constraints in nucleotide sequences

A “genome order index,” defined as S = a² + c² + t² + g², where a, c, t, and g are the nucleotide frequencies of A, C, T, and G, respectively, was used to suggest that there exist genome-specific constraints on nucleotide composition. We show that the “evidence” for constraint, S < 1/3, is in fact a mathematical property that is always true regardless of data. Moreover, we show that S is strictly equivalent to and derivable from the Shannon H-function and has no advantage over it.     

Synthesis, X-ray structure and properties of a new nitrite-bound copper(II) complex with 2-(3,5- dimethylpyrazol-1-ylmethyl)pyridine in a CuN4(O) coordination

Synthesis and characterization of a nitrite-bound copper(II) compound [CuL⁴)₂(ONO)]ClO₄ have been achieved (L⁴ = 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine]. The bidentate ligand L⁴ provides a pyridine and a pyrazole donor site; however, they are separated by a methylene spacer. The complex has been structurally characterized and it belongs to only a handful of complexes having nitrito-bound mononuclear copper(II) centre. The metal atom has a distorted square pyramidal geometry with the copper atom displaced from the equatorial plane by 0.25 Å. In MeCN solution the green complex exhibits a broad ligand-field transition at 655 nm with a shoulder at 675 nm and in dichloromethane-toluene glass (80 K) it exhibits an EPR spectral feature characteristic of the unpaired electron in the d_x²−y² orbital. Variable-temperature (80–300 K) magnetic susceptibility measurements in the solid state as well as room temperature measurement in MeCN solution reveal mononuclear magnetically dilute copper(II) centre. When examined by cyclic voltammetry (MeCN solution) it displays electrochemically irreversible Cu^II---Cu^I response [cathodic peak potential, E_pc (V vs saturated calomel electrode (SCE)): −0.32]. An oxidative response is observed at 1.14 V, probably due to bound-nitrite oxidation and is partially removed to generate a solvated complex at the electrode surface. The latter species gives rise to reversible Cu^II---Cu^I redox response [ ].     

Spectrophotometric determination of palladium with 1-thioglycerol and a study of palladium complex formation with four similar thio-organic compounds

A study of the reactivity of five thio-organic compounds with palladium^II ions has been made and one of them, 1-thioglycerol, selected for use as a spectrophotometric reagent for palladium. Its sensitivity is 0.01 μg of palladium/cm² for log I₀/I = 0.001. The sensitivity of the other four compounds is about the same. Beer's law is obeyed over the palladium concentration range of 0.5 to 9 ppm. The effects of pH, order of addition of reagents, temperature and diverse ions have been investigated.     

Synthesis, crystal structure and magnetic properties of a chloranilate-bridged manganese(II) chain compound: [Mn(bipy)(CA)]n (bipy = 2,2′-bipyridine, CA = chloranilate anion)

The compound [Mn(bipy)(CA)]_n (where bipy is 2,2′-bipyridine and CA²⁻ is the dianion of 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone) has been synthesized and its crystal structure determined by single-crystal X-ray diffraction at room temperature. The crystal is built from infinite chains of chloranilate-bridged manganese(II) affording a zigzag structure, with the bipyridine ligands being stacked between the chains. The variable-temperature (2.0–300 K) magnetic susceptibility and EPR data are reported and a weak antiferromagnetic exchange interaction is observed with the exchange parameter estimated as J = −0.20 cm⁻¹.     

氧硫化碳在230 nm光激发下的S(~3P)解离通道

在230nm激光激发下,氧硫化碳(OCS)分子迅速解离生成振动基态但高转动激发的CO(X~1∑_g~+,v=0,J=42-69)碎片,并通过共振增强多光子电离技术实现其离子化。通过检测处于J=56-69转动激发态CO碎片的离子速度聚焦影像,我们获得了各转动态CO碎片的速度分布和空间角度分布,其中包含了S(1D)+CO的单重态和S(~3P_J)+CO三重态解离通道的贡献。不同的转动态CO碎片对应三重态产物通道的量子产率略有不同,经加权平均我们得到230 nm附近光解OCS分子中S(3P)解离通道的量子产率为4.16%。结合高精度量化计算的OCS分子势能面和吸收截面的信息,我们获得了OCS光解的三重态解离机理,即基态OCS(X~1A')分子吸收一个光子激发到弯曲的A~1A'态之后,通过内转换跃迁回弯曲构型的基电子态,随后在C-S键断裂过程中与2~3A"(c~3A")态强烈耦合并沿后者势能面绝热解离。     

Spectroscopic and magnetic properties of copper(II) complexes derived from pyridine-2-carbaldehyde thiosemicarbazone. Structures of [Cu(NO3)(C7H8N4S)(H2O)](NO3) and [{Cu(NCS)(C7H7N4S)}2]

Complexes with the formula CuX(L) (X=N₃ 1, NCO 2 and NCS 3) and [Cu(NO₃)(HL)(H₂O)](NO₃) 4, where HL=C₇H₈N₄S, (pyridine-2-carbaldehyde thiosemicarbazone), have been characterised. Single-crystal X-ray diffraction studies on compounds 3 and 4 have been carried out. The structure of compound 4 consists of monomeric distorted square pyramidal copper(II) species. The copper(II) ions are coordinated to the NNS atoms from the tridentate thiosemicarbazone ligand and one oxygen atom of a nitrate group in the equatorial position. The oxygen atom of the water molecule occupies the apical position. The structure of compound 3 consists of non-centrosymmetric {Cu₂(μ-SR)₂} entities in which the copper(II) ions exhibit five-coordinate square–pyramidal geometry. The thiosemicarbazone ligand and one nitrogen atom from the thiocyanate ion are in a basal position. The sulfur atom of the tridentate ligand acts as a bridge occupying the apical position. Structural and spectroscopic results suggest the presence of relevant σ ligand-to-metal charge transfer and metal-to-ligand π-backdonation character in these compounds. The ESR spectra of compounds 3 and 4 show rhombic symmetry. For complexes 1 and 2 the ESR spectra exhibit axial signals. Magnetic measurements on compounds 1, 2 and 3 show antiferromagnetic couplings. The susceptibility data were fitted by the Bleaney–Bowers’ equation for copper(II) dimers. The obtained J/k values are −4.22, −6.10 and −7.33 K for compounds 1, 2 and 3, respectively.     

Synthesis, crystal structures and magnetic properties of Cu(II) compounds bridged by the terephthalate ligand and hydrogen bonds

A 3D network [Cu(tmen)(tp)(H₂O)₂]_n (1) (tmen = N,N,N′,N′-tetramethylethylenediamine; tp = terephthalate) and a 2D sheet [Cu(pyrazole)₂(tp)]_n (2), featuring 1D chains interwoven by hydrogen bonds, have been prepared and characterized by means of X-ray analyses and magnetic measurements. For 1, coordinative zigzag chains contain Cu(II) centers capped by the chelate ligand tmen, in which the tetragonal structure is elongated due to Jahn–Teller distortion. Coordinated water molecules are hydrogen-bonded to two free carboxylate oxygens of tp bridges, leading to the observed 3D structure. The use of the non-chelating capping ligand pyrazole produced the covalent-bonded 1D linear compound 2 with hydrogen bonds. A severe octahedral distortion of the Cu(II) center arises from a small bite angle (52.3(1)°) of two carboxylate oxygen atoms of tp, which are in turn hydrogen-bonded to the N–H groups of pyrazole ligands coordinated to Cu(II) atoms in neighboring chains. Magnetic data were fitted with the high-temperature series expansion for the Heisenberg chain spin Hamiltonian H = −J∑_iS_i · S_i + 1 together with consideration of the molecular field approximation (zJ′). Both compounds interestingly exhibit ferromagnetic interactions with g = 2.17, J = 4.08 cm⁻¹, zJ′ = −0.28 cm⁻¹ for 1 and g = 2.09, J = 1.47 cm⁻¹, zJ′ = −0.04 cm⁻¹ for 2. By taking into account structural parameters of distances between Cu atoms, it is reasonably assigned that the ferromagnetic couplings (J > 0) in these systems originate from the hydrogen bonds. The spin density of the d_x²-y² orbital on a Cu(II) atom in a chain is propagated and induced over the d_z² orbital of another Cu(II) atom in an adjacent chain. This orbital orthogonality gives rise to such interactions. The negative zJ′ term suggests that the tp bridges communicate only tiny antiferromagnetic interactions.     

Magnetic investigation of Cu(NH3)4(NO3)2

The magnetic properties of Cu(NH₃)₄(NO₃)₂ have been measured at low temperatures. Broad maxima in both the susceptibility and specific heat are observed and are consistent with linear chain behavior of a Heisenberg antiferromagnet, with J/k = 3.9 ± 0.1 K. Long-range order sets in at T_c = 0.15 ± 0.01 K, and the ratio kT_c/|J| = 0.038 is the lowest observed as yet for a one-dimensional, S = 1/2 antiferromagnet.     

EPR spectra of Cu2+ and VO2+ ions in ammonium hydrogen oxalate hemihydrate [(NH4)HC2O4 . (1/2)H2O] single crystals

Cu(2+) and VO(2+) doped ammonium hydrogen oxalate hemihydrate, [(NH(4))HC(2)O(4) . (1/2)H(2)O], single crystals have been studied at room temperature and at 113K in three mutually perpendicular planes. Both ions yield unexpectedly large number of lines. The calculated results of the Cu(2+) and VO(2+) doped in [(NH(4))HC(2)O(4) . (1/2)H(2)O] indicate that both ions substitute with the NH(4)(+) ion in the structure. The EPR spectra of Cu(2+) ions are characteristic of tetragonally elongated octahedral site and the spectra of VO(2+) are characteristic of tetragonally compressed complex. The angular variation of the EPR spectra has shown that two different Cu(2+) and VO(2+) complexes are located in different chemical environments, and each environment contains two magnetically inequivalent Cu(2+) and VO(2+) sites in distinct orientations occupying substitutional positions in the lattice and show very high angular dependence. The principal g and the hyperfine (A) values of both ions are determined.     

Effects of cation siting and spin–spin interactions on the electron paramagnetic resonance (EPR) of Cu exchanged X Faujasite zeolite

Copper(II) exchanged Na X Faujasite zeolite was cation exchanged at levels from one Cu(II) in 30 unit cells (0.033 Cu(II)/UC) to 38 Cu(II) per unit cell (38 Cu/UC) and was examined by continuous wave and two-pulse and three-pulse electron paramagnetic resonance (EPR) at temperatures from 10 K to 300 K. In this work exchange of Cu²⁺ into X Faujasite zeolite is shown by EPR spectral and pulsed EPR relaxation measurements to begin into site I′, where it lies coordinated to a hexagonal prism face with Si:Al ratios of predominantly 4:2 and 5:1. Spin–spin interactions influence EPR g-value averaging, spin–spin relaxation, and spin spectral diffusion in a manner highly dependent on Cu exchange. Spin–lattice relaxation is relatively independent of exchange. The marked increase observed in spin–spin relaxation and g-value averaging at 8 Cu/UC and an effective Cu–Cu distance of 1.2 nm can be understood in terms of filling sodalite cages with an average of 1 Cu²⁺ each.     

Survey of ligand field parameters of strong field d complexes obtained from the g matrix

Using a newly proposed approach involving an internally consistent set of equations, the ligand field parameters Δ/ξ, V/ξ and k are obtained from literature values of the g matrix for strong field d⁵ systems of various conformations in which |Δ/ξ|≤10. Qualitative analysis of the observed results is done using the Angular Overlap Model, AOM.     

Predissociation of acetylene in à Au state

The fluorescence excitation spectrum, the MPI spectrum, and the absorption spectrum of acetylene due to the à ¹A_u← transition were observed in a gas and in a supersonic jet. A sudden decrease in the fluorescence quantum yield Φ_f was found above the V⁴ K² (46339 cm⁻¹) vibronic sublevel. The decrease is due to predissociation into C₂H + H. AK and J dependence on Φ_f was also found.     

Excess electrons as a probe for polar “fluid structure”

The transition of excess electrons from the quasi-free to the localized state in subcritical and supercritical ammonia vapour was observed in the density range from about 5.6 × 10⁻² to 1 × 10⁻¹ g/cm³. In the same density range, the Kirkwood correlation factory g as a measure of orientational correlation of ammonia molecules, has its strongest density dependence. From this fact and other experimental data on localized (solvated) electrons in sub- and super-critical ammonia, it is concluded that the excess electron is a proper probe to study polar fluid structure.     

Synthesis and spectroscopic, magnetic and EPR studies of di-μ-hydroxo-bis[(NN)(N-phenyl-2-pyridinamine)copper(II)]hexaflourophosphate, with NN=2,2′-bipyridine or 1,10-phenanthroline.: X-ray crystal structure of the 2,2′-bipyridine complex

Binuclear complexes [{Cu(NN)(PhNHpy)}₂(μ-OH)₂](PF₆)₂, where NN=2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen), have been synthesized and characterized by chemical analysis, conductance measurements and IR and electronic spectroscopy. The X-ray crystal structure of [{Cu(bipy)(PhNHpy)}₂(μ-OH)₂](PF₆)₂ shows a distorted square-planar pyramidal coordination for Cu(II), defined by two nitrogen atoms of bipy, two bridging oxygen atoms and the pyridinic nitrogen atom of the ligand. Magnetic susceptibility measurements (in the 4.8–290 K range) reveal coupling which is antiferromagnetic for the bipy complex (2J=−24.2 cm⁻¹) and slightly ferromagnetic for the phen complex (2J=3.3 cm⁻¹). The EPR spectra show the expected triplet signals.