Heisenberg-Dirac-van Vleck model applied to describing the magnetic parameters of mixed-valency clusters

A study has been made on using a Heisenberg-Dirac-van Vleck model in the magnetochemistry of class II mixed-valency cluster compounds. That model may be modified with allowance for electron transfer but cannot be applied to mixed-valency dimers when there is a high degree of excess-electron localization but with small values of the exchange integral. For large values of the exchange integral, that model leads to an incorrect value for the ground-state spin even for localized systems. The correct approach here is based on calculating the spin-vibronic energy spectrum.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 28, No. 2, pp. 121–124, March–April, 1992.     

Certain aspects of heterophase ion exchange

Ion exchange between an aqueous solution of KCl and a solution of tetraoctylammonium bromide in toluene has been studied at 20–70°C. Equilibrium exchange constants were measured. The free energy of the ion exchange is determined to a large extent by Coulombic interaction, which increases with decrease in the radius of the ion. The strength of the Q⁺X^– bond is weakened on passing from the normal onium salt to the hydrate and the rate of ion exchange increases. The position of equilibrium during approach to it from different sides may be affected by different formation processes of complexes with low activity in respect to exchange.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1773–1776, August, 1991.     

Magnetic properties of Mn(III)−Mn(IV) mixed-valence dimers in a dynamic vibronic model

For Mn(III)−Mn(IV) mixed-valence dimers, a dynamic pseudo-Jahn-Teller vibronic problem is solved. Temperature dependences of the magnetic moment of the clusters are calculated with inclusion of Heisenberg and double exchange as well as vibronic coupling. Conditions for antiferromagnetic, intermediate, and ferromagnetic ground spin-vibronic states and the accompanying low-temperature limits of magnetic moments are determined. State University, Moldova Republic. Valencia University, Spain. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 28–34, July–August, 1994. Translated by L. Smolina     

Synthesis and magnetism of a binuclear copper(II)–manganese(II) complex with N,N-bis(N-hydroxyethylaminoethyl)oxamido as a bridging ligand

The heterobinuclear complex, [CuMn(L)(phen)2](ClO4)2· H2O, [L = N,N-bis(N-hydroxyethyleneamine)oxamido, phen=1,10-phenanthroline], has been synthesized with N,N-bis(N-hydroxyethylaminoethyl)oxamido as the bridging ligand. The electronic reflectance spectrum indicates the presence of exchange-coupling interaction between bridging MnII and CuII ions. The variable-temperature magnetic susceptibility of the complex was measured over the 4–300K range. The magnetic coupling parameter is consistent with an antiferromagnetic exchange between the MnII ion and CuII ion and fits the data for a heterobinuclear CuII–MnII magnetic exchange model based on the Hamiltonian operator (H= –2JS1S2, S1=1/2, S2=5/2), giving the antiferromagnetic coupling parameter of 2J=–74.0cm–1.     

The first oxalato-bridged dinuclear CoII–VIVO complexes: synthesis and magnetism

Two novel CoII–VIVO oxalato complexes have been synthesized and characterized, namely [CoL2VO(OX)2·3H2O [L = 1,10-phenanthroline (phen) or 2,2-bipyridyl (bipy), OX = oxalate dianion]. Based on i.r. and electronic spectra and elemental analyses, extended oxalato-bridged structures consisting of VO2+ and CoII species, in which each VIV has a distorted square pyramidal environment and each CoII has a distorted octahedral environment are proposed for the two complexes. The temperature dependence of the magnetic susceptibility for [CoL2VO(OX)2]·3H2O (L = phen, bipy) was measured over the 77–300 K range and the observed data were successfully simulated by an equation based on the spin Hamiltonian operator H^=–2JS^1S^2 giving the exchange integral J=–62.5 cm–1 and J=–40.3 cm–1 for the phen and bipy complexes, respectively. These indicate antiferromagnetic spin exchange interaction between the CoII and VO2+ centres.     

Binuclear Cu(II) and Ni(II) Complexes with Schiff Bases—the Products of Condensation of Amino Alcohols with 2-Phenylhydrazone 1-Phenylbutane-1,2,3-Trione: Synthesis and Magnetic Properties

The binuclear complexes of Cu²⁺ and Ni²⁺ with a new ligand system obtained through condensation of amino alcohols with 2-phenylhydrazone 1-phenylbutane-1,2,3-trione were synthesized. The compositions and structures of ligands and complexes were determined using elemental analysis, IR and NMR spectroscopy and magnetochemistry. The nickel complexes were found to be diamagnetic; in the copper complexes, substantial antiferromagnetic exchange interaction was observed (for Cu²⁺ complexes with n = 2 and 3, 2J is equal to –396 and –393 cm^–1, respectively). The structure of the exchange fragment is discussed.     

Vibronic effects and exchange interaction in dinuclear clusters

A study was made on exchange in dinuclear clusters, which contain centers having degenerate or pseudodegenerate states. The semiclassical approximation was used to examine the temperature dependence of the exchange parameters arising from the vibronic coupling.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 641–647, November–December, 1988.     

Synthesis and magnetic properties of nickel(II) complexes with the nitronyl nitroxides

Four novel complexes of nickel(II), Ni(tfac)2(NITPa)2 (1), Ni(tfac)2(NITPhNO2)2 (2), Ni(pfpr)2(NITPa)2 (3) and Ni(pfpr)2(NITPhNO2)2 (4), [tfac=trifluoro- acetato, NITPa=2-(3,4-methylenedioxyphenyl)-4,4,5, 5-tetramethylimidazoline-1-oxy-3-oxid, NITPhNO2= 2-(3-nitrophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxid, pfpr=pentafluoropropionato], have been prepared and characterized by elemental analysis, i.r., and electronic spectra, and molar conductances. The temperature dependence of the magnetic susceptibility for complexes (1) and (3) was measured (4–300K). The observed data were successfully simulated giving the exchange integral J=–1.48cm–1 for (1) and J=–1.25cm–1 for (3). These results indicate a weak antiferromagnetic spin exchange interaction between nickel(II) ion and the radicals.     

Intermolecular interaction in radical recombination in the system O2 + H2

The complex of O₂ + H₂ and the recombination of hydrogen atoms, which occurs in linear contact with oxygen, are calculated by the STO-6G SCF method in the complete active space of nine orbitals. The singlet and triplet (T) states of the H₂ system are mixed on account of configurational interaction with the T-state of oxygen (X^3– _g 8). The intermolecular exchange interaction shifts the potential of the atomic recombination and accelerates it.Cherkassy Institute of Industrial Engineering, 460 Shevchenko Boulevard, Cherkassy, Ukraine 257006. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 4, pp. 229–233, July–August, 1996. Original article submitted October 6, 1995.     

The structure and magnetic properties of the di-μ-pyrazolate complex [Cu2(PEAH2)2(NO3)(H2O)](NO3)[PEAH3 = 3,5-bis-(N-ethylamido) pyrazole]

Summary A complex of the formula[Cu₂(PEAH₂)₂(NO₃)(H₂O)]-(NO₃)[PEAH₃ = 3,5-bis(N-ethylamido) pyrazole] was synthesized and its crystal structure determined by XRD methods. It consists of a dimeric cation with two bridging pyrazolate groups and a NO ₃ ^– anion. The geometries of the coordination polyhedra around the copper(II) ions are distorted square pyramidal. The variable temperature (20–300 K) magnetic susceptibility data gave the antiferromagnetic interaction constant as J=-191cm^–1, showing that the pyrazole molecule is very effective in transmitting the exchange interaction.     

Static luminescence quenching by oxygen and nitric oxide for phenanthrene in glassy matrices

Steady-state and pulsed methods have been applied to static phosphorescence and fluorescence quenching for phenanthrene caused by oxygen and nitric oxide. The characteristic fluorescence quenching distances have been determined for oxygen (8.7±0.5 Å) and nitric oxide (8.9±0,5 Å); the phosphorescence quenching involves an exchange mechanism, and measurements have been made on the characteristic quenching distances R₀ and exchange-interaction attenuation parameters L. Estimates have been made on the rate constants for dynamic quenching of the triplet excited states of phenanthrene in low-viscosity liquids, which agree well with the measured values for hexane.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 667–672, November–December, 1988.     

PMR study of the degenerate electron transfer between 1,2,3-trimethyl-2-phenylbenzimidazoline and its cation radical

The degenerate electron transfer (electron exchange) between 1,2,3-trimethyl-2-phenylbenzimidazoline and the perchlorate of its radical cation has been studied by PMR in deuteriated aceonitrile. We have determined rate constants and activation parameters for this process, and also some hyperfine interaction (HFI) constants for the cation radical, inaccessible by ESR measurements.Translated from Teoreticheskie i Éksperimental'naya Khimiya, Vol. 26, No. 6, pp. 658–663, November–December, 1990.     

Synthesis and magnetism of tetrachlorophthalato-bridged cobalt(II) binuclear complexes

Three new cobalt(II) binuclear complexes have been prepared and characterized, namely [Co2(TCPHTA)(L)4](ClO4)2 [L=1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline(NO2-phen) and 2, 2-bipyridyl (bipy), respectively], where TCPHTA is the tetrachlorophthalate dianion. Based on i.r. spectra, elemental analyses and conductivity measurements, tetrachlorophthalato-bridged structures consisting of two cobalt(II) ions in which each cobalt(II) ion has a distorted octahedral environment are proposed for these complexes. The temperature dependence of the magnetic susceptibility for [Co2(TCPHTA)(L)4](ClO4)2·nH2O (L=phen, NO2-phen and bipy) has been measured over the 77–300 K range and the observed data successfully simulated by an equation based on the spin Hamiltonian operator (H=–2JS1S2), giving the exchange integral J=–2.92, –3.45, –4.03 cm–1, respectively. This result indicates the presence of a weak antiferromagnetic spin exchange interaction between the metal ions.     

Protein–protein interaction dynamics by amide H/H exchange mass spectrometry

Amide H/²H exchange detected by mass spectrometry provides a powerful tool for observing changes that occur upon protein–protein interactions. In general, it is possible to observe protection of surface amides, they become less solvent exposed when they are buried at the interface. The information thus obtained about the location of the protein–protein interface is useful for building a correct docked structure of the protein–protein complex. Examples of protein–protein interfaces that were correctly identified by such methods include the thrombin–thrombomodulin interaction and the interaction between the regulatory and catalytic subunit of protein kinase A. Amide exchange also affords a view into the subtle changes in the ensemble of states that occur upon protein modification or protein–protein binding. Examples of proteins in which amide exchange has been used to observe phosphorylation-induced changes include ERK2 and CheB. Amide exchange showed the pathway of communication between the cAMP-binding site and the catalytic subunit site within the regulatory subunit of protein kinase A. Clues as to how thrombomodulin regulates the catalytic activity of thrombin were also obtained.     

Structure distortion effects in the spectrum of tunnel-exchange states of tetrahedral mixed-valence tetramers

Tunnel-exchange states of a tetrameric mixed-valence (MV) d₁–d₁–d₁–d₂ cluster are considered. Energy levels of distorted pseudotetrahedral conjiprations with rhombic and trigonal symmetry are calculated. It is demonstrated that the correlation diagrams of planar systems are symmetric with respect to the sign of the double exchange parameter. A strong double exchange always results in the ferromagnetic ground state of planar systems. The following types of spectrum defamations of the d₁–d₁–d₁–d₂ tetramer from tetrahedral to planar systems have been found: a) with a weak positive double exchange, deformation of a tetrahedral system does not lead to any change of spin of the antiferromagnetic ground state of the cluster; b) with a strong positive double exchange, the defonnation alters the antiferromagnetic ground (S = 1/2) state of the tetrahedral system to the ferromagnetic (S = 5/2) state of the planar system, or to an intermediate (S = 3/2) state; c) with a negative double exchange, irrespective of its absolute value, the tetrahedral system deformation does not alter the spin of the ground state. With a weak double exchange, this ground state remains antiferromagnetic, and with a strong double exchange, it is ferromagnetic or intermediate.Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 33–46, September–October, 1993.Translated by L. Chernomorskaya     

Vibronic reduction of Heisenberg and double exchange in multielectron mixed-valency clusters

Vibronic reduction is considered for Coulomb interaction between centers for Heisenberg and double exchange in multielectron mixed-valency d²-d³ clusters. A simplified multimode vibration model indicates how effects arise via vibrations from the correlation interaction between electron shells. Vibronic reduction in double exchange can alter the magnetic parameters for the cluster's ground state.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 114–121, March–April, 1992.     

Electron paramagnetic resonance of the excited states of the three-spins-1/2 ZnTPP–3-NOPy complex

EPR spectra of the excited quartet and doublet molecular states of (tetraphenylporphinato)zinc(II)covalently bounded to 3-(N-nitronyl-notroxide) pyridine stable radical are modeled in terms of the spin-Hamiltonian given by the sum of the contributions from the radical and triplet moieties, and the interaction between them. The later is represented by anisotropic point dipolar and isotropic exchange electron spin–spin interactions. It is shown that the high field (W-band) EPR spectra depend on energy separation between the electronic doublet (D) and quartet (Q) states. This dependence was utilized to estimate the upper limit of the intensity of exchange interaction between the radical and porphyrin moieties.     

Magnetic properties of tetrameric μ3-methoxophenolates of nickel(II)

The magnetic properties of the tetranuclear nickel(II) complexes[Ni(OCH₃)(L) (CH₃OH)]₄ (L stands for a residue of 2,4,6-trichlorophenol or 2,4-dinitrophenol) have been investigated. The variation of the effective magnetic moment (per metal ion) with the temperature for both compounds points out the presence of a ferromagnetic exchange interaction. When the magnetic properties of these compounds are described, agreement with the experiment is achieved, if it is taken into account that there is a weak ferromagnetic intercluster interaction in addition to the Isotropic intracluster Heisenberg interaction. It has been established on the basis of a comparison of the data obtained and the literature data that changes in the Ni-O-Ni angle in the 90±14° range have little influence on the value of the parameter J, while a decrease in the Ni-O_br. distance causes an increase in J.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 3, 285–291, May–June, 1986.     

The syntheses,crystal structure and magnetic behavior of μ-imidazolato-dicopper(II) complexes with the dinucleating hexaazamacrocycles

The synthesis, crystal structure and magnetic properties of the imidazolate-bridged dinuclear copper(II) complex [LCu2(Im)](ClO4)3(H2O) ·1/2(MeCN), (ImH=imidazole, L=bis-p-xylylBISDIEN) have been studied. Single crystal X-ray diffraction determination reveals the distorted square planar geometries of the imidazolate bridged dicopper(II) center are incorporated within the dinucleating macrocycle. The Cu—Cu separation in the complex is 6.005Å. Magnetic measurements reveal an antiferromagnetic exchange interaction with a coupling constant of J=–26.52cm–1. The enzymatic activity of the title complex is 5.9 percent of that of the protein.     

Vibronic coupling effects in trimeric mixed-valence clusters

The spectrum of a trimeric mired-valence cluster d₁–d₁–d₂ is calculated in the Piepha-Krausz-Schatz vibronic model. Based on the vibronic spectra obtained magnetic properties of the d₁–d₁–d₂ cluster are investigated It is shown that spin of the system's ground state may change not only as a result of double exchange reduction by vibronic coupling, but also due to a diflerence in the eneqy dependence of the lowest vibronic states on the vibronic coupling constant for diflerent total spin values. Unlike previous studies of vibronic interactions in the d₁–d₁–d₂ cluster, in this study the diagonalizations of the vibronic-coupling and double-exchange matrices are fuljilled simultaneously, using Lanczos' method. Due to this, all types 4 vibronic state mixing by double exchange could be taken into account properly.Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 20–32, September–October, 1993.Translated by L. Smolina