Theoretical studies of the spin Hamiltonian parameters and local structures for Ag<Superscript>2+</Superscript> in AgCl and KCl crystals

The spin Hamiltonian parameters (g factors, hyperfine structure constants and superhyperfine parameters) and local structures for Ag²⁺ centers in AgCl and KCl crystals are theoretically studied using the high-order perturbation formulas for a tetragonally elongated 4d ⁹ cluster. The impurity centers undergo relative elongations (≈0.05 Å and 0.23 Å for Ag²⁺ in AgCl and KCl, respectively) along the C ₄ axis owing to the Jahn–Teller effect. All the calculated spin Hamiltonian parameters show good agreement with the experimental data, and the ligand contributions to the spin Hamiltonian parameters are important and should be taken into account. The unpaired spin densities in the superhyperfine parameters are determined from molecular orbital coefficients based on the cluster approach, instead of being taken as the adjustable parameters in the previous treatments. Increasing tetragonal elongation from AgCl to KCl is attributed to a decrease in chemical bonding (or lower force constant) with increasing Ag²⁺–Cl distance.     

Electron spin resonance spectroscopy of copper(II) complexes with schiff bases as ligands

The ESR spectra of a series of Cu^II Schiff base complexes have been investigated in liquid and frozen solutions. The structures of these complexes are estimated on the basis of superhyperfine interactions and molecular orbital coefficients α² and β²₁ calculated from copper hyperfine parameters.     

ESR study of weak exchange interactions in binuclear copper(ii) complexes with acyldihydrazones of fluorinated β-diketones

A series of binuclear copper(ii) complexes with acyldihydrazones of aliphatic dicarboxylic acids (from malonic to adipic) and fluorinated -diketones (trifluoro- and hexafluoroacetylacetone) of composition Cu₂L·2Py (H₄L is acyldihydrazone) were studied by ESR spectroscopy. The ESR spectra of solutions of complexes with trifluoroacetylacetone acyldihydrazones have an isotropic signal with a seven-line hyperfine structure from two equivalent copper nuclei (g = 2.112, a _Cu = (39—40)·10^–4 cm^–1), which is indicative of weak exchange interactions between the paramagnetic centers due to spin density delocalization through a chain of the -bonds of the polymethylene bridge. On going to hexafluoroacetylacetone derivatives, the coupling is suppressed and the ESR spectra of solutions of such complexes show a signal with a four-line hyperfine structure (g = 2.121—2.131, a _Cu = (55—63)·10^–4 cm^–1) typical of mononuclear copper complexes.     

Jahn-Teller Effects in a Vanadate-Stabilized Manganese-Oxo Cubane Water Oxidation Catalyst

Heuristic rules that allow identifying the preferred mixed-valence isomers and Jahn-Teller axis arrangements in the water oxidation catalyst [(Mn₄O₄)(V₄O₁₃)(OAc)₃]ⁿ⁻ and its activated form [(Mn₄O₄)(V₄O₁₃)(OAc)₂(H₂O)(OH)]ⁿ⁻ are derived. These rules are based on computing all combinatorially possible mixed-valence isomers and Jahn-Teller axis arrangements of the Mn^III atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn-Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn-Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn-Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane-vanadate bonds that are chemically inert, cubane-acetate/water bonds that can be activated through a Jahn-Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn-Teller axes plays an essential role in the reactivity of this and probably related compounds.     

Mass,size and shape of micelles formed in aqueous solutions of ethoxylated nonyl-phenols

The study was extended to analysis of mass, size and conformation of micelles formed in aqueous solutions of ethoxylated nonyl phenols. The results obtained by ultracentrifugal technique between 293 and 323 K have proved that the slightly ethoxylated nonyl phenols form micelles with high molecular mass and larger size at constant temperature, while the increasing length of the ethylene oxide chain favours formation of micelles of smaller molecular mass and size. The transformation of conformation from oblate to spherical shapes ensues with increasing temperature at constant ethoxy number or with ethoxylation at constant temperature. The second virial coefficient decreases with increasing temperature and decreasing ethoxy number. In accordance with the earlier conclucions, the change of the second virial coefficient relates to enhanced variation of monomer solubility, stabilization of micelle structure and increased deviation from ideal behaviour of a given micellar system.Symbols a major axis of micelle, Å - a _m attractivity factor, cm³ erg molecule² - b minor axis of micelle, Å - c concentration, g dm^–3 - c _b equilibrium concentration at the bottom of the cell, g dm^–3 - c _m equilibrium concentration at the meniscus of the cell, g dm^–3 - c _o initial concentration in the cell, g dm^–3 - c _M critical micellization concentration, mol dm^–3 - e eccentricity - f _IS Isihara-constant - f/f _o frictional ratio of micelle - amount of water in micelle per ethoxy group, mol H₂O/mol EO - n aggregation number, monomer micelle^–1 - n _EO number of ethoxy groups - r distance of Schlieren peak from the axis, cm - r _b distance of cell bottom from the axis, cm - r _m distance of cell meniscus from the axis, cm - R _h equivalent hydrodynamic radius of micelle, Å - s _t sedimentation coefficient, s - reduced sedimentation coefficient, s - reduced limiting sedimentation coefficient, s - ¯v _t volume of micelle, cm³ micelle^–1 - partial specific volume of solute, cm³g^–1 - partial specific volume of solute reduced to 293 K, cm³ g^–1 - B _a, B_e constants, cm³ mol g^–2 - B ₂ second virial coefficient, cm³ mol g^–2 - M _m ^a mass average apparent molecular mass of micelle, g mol^–1 - M _m mass average molecular mass of micelle corrected withB ₂, g mol^–1 - M _m ^cM mass average molecular mass of micelle belonging toc _M, g mol^–1 - M ¹ mass average molecular mass of monomer, gmol^–1 - N _A the Avogadro's number, molecule mol^–1 - R universal gas constant, erg mol^–1 K^–1 - T temperature, K - _t ^o dynamic viscosity of solvent atT temperature, g cm^–1 s^–1 - dynamic viscosity of solvent at 293 K, g cm^–1 s^–1 - _t density of solution atT temperature, g cm^–3 - _t ^o density of solvent atT temperature, g cm^–3 - density of solvent at 293 K, g cm^–3 - angular velocity, rad s^–1 - time, s     

Synthesis and study of binuclear vanadyl complexes with acyldihydrazones of salicylaldehyde and dicarboxylic acids

The binuclear vanadyl(ii) complexes [(VO)₂·2Py·2EtOH]·mH₂O with acyldihydrazones of salicylaldehyde (H₄L) and dicarboxylic acids were synthesized and studied. In these complexes, two chelate vanadyl(ii) complexes with the tridentate bicyclic ligands are linked to each other by the polymethylene bridges —(CH₂) _n — of different lengths varying from one to four units. The ESR spectra of solutions of these complexes, unlike those of analogous copper(ii) complexes, have an isotropic signal with an eight-line hyperfine structure (g = 1.972, a _V = 93·10^–4 cm^–1) typical of monomeric vanadyl complexes, which indicates that no exchange interactions occur between the paramagnetic centers through the polymethylene chain.     

Density-functional investigation of the excited state properties and the Jahn-Teller effect in [CrX6]3− (X=Cl−, Br−)

Summary The luminescence of [CrX₆]^3– X=Br^–, Cl^– has been studied through density functional theory (DFT) using both deMon and ADF codes. Multiplet energies⁴A₂,²E,⁴T₂, and⁴T₁ have been expressed as energies of non-redundant single determinants and calculated as in Ref. [1]. The influence of the metal ligand distance on the multiplet energies has been investigated. Of particular interest to this work is the Jahn-Teller effect distortion. We found that the system moves to a more stable geometry when the axial bond length is compressed and the equatorial one elongated in agreement with the experimental value.     

Synthesis,crystal structure,and properties of [Cu(LH)]2[M(mnt)2] [M = Ni or Cu; H2L = 3,3′-(1,3-propanediyldinitrilo)-bis(2-butanone oxime); mnt2− = maleonitriledithiolate] complexes containing bridging cyano groups

Interesting complexes containing a mnt^2– bridge, based on the reaction of [M(mnt)₂]^2– [M = Ni or Cu] with [Cu(LH)]⁺, have been prepared and characterized by e.s.m.s., i.r and u.v–vis. spectroscopic techniques and by electrochemistry. The complexes show weak antiferromagnetic interactions between magnetic centers. The X-ray analysis of the molecular structure of [Cu(LH)]₂[Ni(mnt)₂] has been completed. It structurally features the CN group of mnt^2– which can act as a bridging ligand between two transition metals.     

Formation Mechanism of O2– Radical Anions in the Adsorption of NO + O2 and NO2 + O2 Mixtures on ZrO2 According to EPR and TPD Data

The effects of various factors on the formation of O₂ ^– radical anions in the adsorption of an NO + O₂ or NO₂ + O₂ mixture on ZrO₂ were studied. It was found that the thermal stability of the O₂ ^– species depends on the composition of the adsorbed gas. It was suggested that nitrogen oxide complexes on ZrO₂ centers are responsible for the formation of O₂ ^–. These centers are formed upon the treatment of the oxide in a vacuum; however, they are different from both coordinatively unsaturated Zr⁴⁺ cations (NO adsorption centers at 77 K) and Zr⁴⁺–O^––O^––Zr⁴⁺ centers, at which O₂ ^– are formed because of the adsorption of H₂ + O₂. Based on the experimental data, the mechanism of O₂ ^– formation in the adsorption of an NO + O₂ mixture is discussed.     

DFT calculations of the defect structures,electronic structures,and EPR parameters for three Rh2+ centers in AgCl

The local structures for various Rh²⁺ centers in AgCl are theoretically studied using density functional theory (DFT) with periodic CP2K program. Through geometry optimizing, the stable ground states with minimal energies and electronic structures are obtained for the tetragonally elongated (T_E), orthorhombically elongated (O_E), and tetragonally compressed (T_C) centers, and the corresponding g and hyperfine coupling tensors are calculated in ORCA level. The calculations reveal obvious Jahn–Teller elongation distortions of about 0.109 and 0.110 Å along [001] axis for T_E and O_E centers without and with 1 next nearest neighbor (nnn) cation vacancy V_Ag in [100] axis, respectively. Whereas T_C center with 1 nnn V_Ag along [001] axis exhibits moderate axial compression of about 0.066 Å due to the Jahn–Teller effect. For O_E and T_C centers with 1 nnn V_Ag, the ligand intervening in the central Rh²⁺ and the V_Ag is found to displace away from the V_Ag by about 0.028 and 0.024 Å, respectively. The present results are discussed and compared with those of the previous calculations based on the perturbation formulas by using the improved ligand field theory.     

Electron spin resonance and electronic spectral investigation on square planar copper(II) complexes ofN-ethyl- andN-propylimidazole

Summary Electron spin resonance studies have been carried out onN-ethylimidazole andN-propylimidazole 4 : 1 complexes of CuX₂ salts (X = ClO ₄ ^– , NO ₃ ^– , , Br^– or Cl^–) in their polycrystalline and undiluted form at 295 K and 77 K. Cupric ion hyperfine structural resolution is observed for all the complexes at 295 K and the spectra are characteristic of a CuN₄ chromophore with axial symmetry. In complexes involving ClO , Br^– and Cl^– the anions are nonbonding, whereas those with the NO ₃ ^– anion are weakly bonding. The electronic and e.s.r. spectral data have been correlated. The resolution of Cu²⁺ ion hyperfine structure in these complexes is attributed to a decrease in the dipolar interaction and has been observed for the first time since the first resolution reported in 1954 for CuN₄ coordination with square planar symmetry for ,,,-tetraphenylporphyrincopper (CuTPP).     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Investigations on the local structure and spin Hamiltonian parameters for the orthorhombic Cu<Superscript>2+</Superscript> center in Ca(OD)<Subscript>2</Subscript>

The local structure and the spin Hamiltonian parameters (the g factors g _i (i = x, y, z) and the hyperfine structure constants A _i ) for Cu²⁺-doped Ca(OD)₂ are theoretically investigated from the perturbation formulas of these parameters for a 3d ⁹ ion in an orthorhombically elongated octahedron. From the studies, the planar Cu²⁺-OD⁻ bonds are found to experience the relative variation ΔR (≈0.014 ?) along the X and Y axes, while those parallel to the Z axis may undergo the relative elongation ΔZ (≈0.25 ?) due to the Jahn-Teller effect. The theoretical spin Hamiltonian parameters based on the above local lattice distortions agree well with the experimental data. As compared with the previous treatments, the improvements of the theoretical spin Hamiltonian parameters are achieved in this work by adopting the uniform calculation formulas and the tetragonal field parameters based on the superposition model.     

On the detection by EXAFS spectroscopy of multimeric species and of complex anions in diethylether and diethylcarbonate

EXAFS spectroscopy has been shown to be a reliable tool for detecting the formation in solutions of molecular associations (LiBr)_n where n is solvent dependent. The signal Br^*...Br observed at 3.8₇ Å confirms the formation of tetrameric species (n=4) in diethylether (Et₂O) but only of dimers (n=2) in diethylcarbonate. Another signal observed at 2.4₅ Å has·been attributed to the pair Br^*...Li. The study of Li₂MBr₄–Et₂O solutions (M=Co, Cu, Zn) also affords new evidence for the formation of complex anions MBr ₄ ^2– but the spectra obtained at the bromine or metalK-edges for LiZnBr₃–Et₂O solutions seems to indicate the formation of more complex aggregates (LiMBr₃)_q. While the signal of the shell Br^*...Br is found quite intense for the reference CBr₄–Et₂O system, it does not appear for the MBr ₄ ^2– complex except for CuBr ₄ ^2– where the Jahn-Teller effect might rigidify a distorted tetrahedral structure.Presented, in part, at the VIIth International Conference on Non-Aqueous Solutions, Regensburg, West Germany, Ausgust 1980.     

Stability and crystal field splitting of N inf2 sup− in Barium Azide

The crystal field splitting of the ground²_g state and the equilibrium orientation of the N _inf2 ^sup– radical in the monoclinic barium azide are calculated employing a point ion representation of the lattice potential and assuming three different plausible trapping sites for the radical. When the N _inf2 ^– is assumed to replace an azide ion which is parallel to thec axis of the unit cell, the previously suggested trapping site, the calculated crystal field splitting is in order of magnitude agreement with the value deduced from theg shift in the ESR spectrum. The calculated orientational angle of the molecular axis with thec axis of the unit cell is 9.0 degrees which compares favorably with the experimentally determined angle of 5.0 ± 1.5 degrees. Possible reasons why the N _inf2 ^– can not be trapped at the site of an azide ion which is perpendicular to the c axis are suggested from the calculation.     

Investigations of TsVM zeolite acidities with different degrees of exchange of Na+ by H+

The method of programmed thermal desorption of ammonia has been used to establish that on the surface of the sodium form of type TsVM zeolite having a modulus SiO₂/Al₂O₃= 34.5 there are two types of acidic centers having ammonia desorption activation energies E_d equal to 40–45 and 80–100 kJ/mole. Replacement of Na⁺ by H⁺ leads to the appearance of a third type of center with a value E_D=130–150 kJ/mole. In this case in the IR spectra of specimens saturated with ammonia a band appears at 1410 cm_–1, which is evidence for the presence of NH ₄ ⁺ ions. Increasing the degree of Na⁺ replacement by H⁺ leads to a nonproportional rapid increase in the concentration of highly energetic centers, which may be explained by the partial transfer of aluminum into cationic positions with the formation of additional acidic centers.Translated from Teoreticheskie i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 752–755, November–December, 1986.     

Steps toward a high precision evaluation of the hyperfine interactions in neutral muonic helium

The solution of the Schrödinger equation of the neutral muonic helium is sketched by an eigenfunction expansion method: the eigenfunctions of the two Coulombic centres problem (of chargesZ ₁=2,Z ₂=–1) are used to expand the wave function. Our Born expansion method is a generalization of the Born-Oppenheimer approximation to a system in which the two centres (He, ^–) do not have a stable equilibrium distance. The adiabatic approximation is solved, upper-lower bounds on the eigenvalue are given for a number of states. The hyperfine energy corrections are calculated in general terms and are given numerically for the ground state and for the first muonically and electronically excited states in the frames of the adiabatic approximation. Our best value fails to give the observed hyperfine splitting of the ground state by some 5 × 10^–4 (2 MHz).     

Formation of linkage isomers via the substitution of halides by selenocyanate in ruthenium(III) complexes

Summary Substitution of the halide ion of [Ru(NH₃)₅X]²⁺ (X=Cl^– or Br^–) by SeCN^– was monitored spectrophotometrically between 45 and 60° C in aqueous medium. The pseudo-first order rate constants were evaluated by Guggenheim's procedure, which shows linearity for 2–3 half-lives. The rate increases linearly with [SeCN^–] and all the plots have a positive intercept on the rate axis. The SeCN^–-independent path represents aquation and the SeCN^–-dependent path represents anation of the aqua complex with simultaneous formation of two isomeric (the N bonded and the Se bonded) products. The formation of the isomeric products is entropy controlled.     

Anomalies of the partition coefficient and properties of crystals in the region of low concentrations of an isomorphous admixture. 1. Unusual decomposition of solid solutions of the NaCl—CdCl2 system

Solid solutions NaCl—CdCl₂ were studied in an interval of CdCl₂ concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na₂CdCl₄ decomposes: Na₂CdCl₄ 2Na⁺ + Cd²⁺ + 2Cl₂ ^– + 2e^–; 2Na + 2e^– 2Na⁰. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl₂ in NaCl (K jump).     

Investigation of electron distribution and hyperfine properties of hemin by first-principles Hartree-Fock self-consistent field procedure

Summary As part of a program for first-principles quantitative study of hyperfine properties of large biological molecules, we have applied the Self-Consistent Field Unrestricted Hartree-Fock procedure to investigate the electronic structure of hemin, the associated charge and unpaired spin population distributions and the hyperfine constants of⁵⁷Fe,¹⁴N,¹³C,¹H and³⁵Cl nuclei. The results of the present work indicate significantly more localized charge and spin distributions than were found earlier by the semi-empirical Self-Consistent Charge Extended Hückel procedure. The theoretical values of the⁵⁷Fe,¹⁴N and methene proton hyperfine constants, the only nuclei for which experimental data are available, are found to be –34.99, 11.89 and –1.50 MHz in satisfactory agreement with the experimental values of –26.47, 7.55 and –1.014 MHz, respectively. The contact and dipolar contributions to the hyperfine constants and their breakdowns into direct and exchange polarization contributions are analyzed. Possible sources that could bridge the remaining gap between experiment and theory will be analyzed.