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.     

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     

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.     

Magnetic properties of d–d–d MV trimers with partial electron delocalization in the generalized vibronic model

The effects of magnetic exchange, double exchange and vibronic coupling on magnetic properties of the d¹–d¹–d² trimer with C_2V symmetry are discussed in the generalized vibronic model. The magnetic moments are calculated with the semiclassical adiabatic approach. The results show that the PKS vibronic coupling and the P vibronic coupling favor the localization and the delocalization of the ‘extra’ electron, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange and vibronic coupling.     

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.     

Spectral-band multiplet structures in intervalency electron transfer in exchange-coupled mixed-valency dimers

A study is made on the effects of exchange interaction on the shapes of intervalency-transfer electron spectra in binuclear mixed-valency complexes. The spectrum is determined by means of calculated spin-vibronic cluster states. It is shown that it is a superposition of components from the different spin states. The temperature dependence of the maximal intensity is not uniquely determined by the sign of the exchange interaction and is dependent on the order of the spin-vibronic states.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 2, pp. 211–215, March–April, 1987.We are indebted to I. B. Bersuker and J. J. Girerd for their useful advice and discussions.     

Exchange interactions in oxovanadium phosphates: towards the understanding of the magnetic patterns

We report the calculations of exchange magnetic constants in a series of vanadium phosphorus oxides for which structural and magneto-chemical data are available. Four different types of dimers have been extracted from the crystal lattices in all solids studied. On the basis of a combined density functional theory broken symmetry approach, our calculations on molecular models allow us to define schemes of magnetic interactions. The largest absolute magnetic interaction is provided by the double O–P–O and di-μ-oxo bridges, suggestive of an alternating dimer chain model and an isolated dimer one for the VO(HPO₄)·0.5H₂O and α-VO(HPO₄)·2H₂O phases, respectively. Conversely, VO(HPO₄)·4H₂O is consistent with a bi-dimensional magnetic pattern whereas VO(H₂PO₄)₂ and α-VO(PO₃)₂ with three-dimensional magnetic schemes. The use of dimer models has been justified by the analysis of higher-nuclearity clusters.     

Structure of the 1:2 complex formed by the cis-anti-cis diastereomer of dicyclohexano-18-crown-6 with 2-methylbenzenesulfamide

Applied Physics Institute, Moldovian Academy of Sciences. Technical Physics Institute, Ukrainian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 2, pp. 138–144, March–April, 1992.     

Identification of additional components of the sex pheromone of Dendrolimus pini

Institute of Biological Plant Protection of the Moldovian Academy of Sciences, Kishinev. All-Union Scientific-Research Institute of Forestry and Forestry Mechanization, Pushkino. Moscow Institute of Wood Technology; Georgian Institute of Mountain Forestry, Tbilisi. Translated from Khimiya Prirodnykh Soedeninii, No. 1, pp. 159–160, January–February, 1993.     

Electronic and vibronic states of an asymmetric trimeric mixed-valence cluster

Electronic and adiabatic vibronic states of d¹-d¹-d² trimeric clusters are considered in terms of the many-electron theory of mixed-valence states. The initial model includes two d-orbitals on each center and allows for Heisenberg and double exchange, asymmetry of the system, and vibronic coupling with local shifts of ligands. Influence of distortions in the trimers on the structure of tunnel-exchange states, on ferro-or antiferromagnetism of the clusters, and on the form of the adiabatic potential is examined. State University, Moldova Republic. Valencia University, Spain. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 35–45, July–August, 1994. Translated by L. Smolina     

Hole-type mixed-valence tetrahedral cluster: Exchange-tunnel states and magnetic properties

The paper deals with the exchange-tunnel states of a tetrahedral tetrameric hole-type cluster d¹-d²-d²-d² used as a model of metal core of vrious protiens. A new method for group-theoretic classification ofthe quantum states of mixed-valence clusters is suggested. The nature ofthe ground spin state of the above systems has been investigated. The effect of partial spin alignment depending on the double exchange parameter value is discussed.Moldova State University. Translated fromZhumal Strukturnoi Khimii, Vol. 34, No. 3, pp. 14–25, May–June 1993.Translated by L. Smolina     

Diacetylene's weak bonding to acetylene clusters

An interaction potential previously developed for the acetylene–polyyne dimer was used to explore the interaction potential surfaces for clusters containing a diacetylene molecule and two or more acetylene molecules. Ab initio calculations were performed on the smallest clusters in order to assess the energetic and structural features predicted by the model potential. The preferred arrangements of the monomers in the clusters maximize the favorable quadrupole–quadrupole interactions between the monomers.     

Effect of magnetic exchange,double exchange,vibronic coupling,and asymmetry on magnetic properties in d2–d3 mixed‐valence dimers

The effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties of d²–d³ systems is discussed. The temperature‐dependent magnetic moment was calculated with the semiclassical adiabatic approach. The results show that the vibronic coupling from the out‐of‐phase breathing vibration on the metal sites (Piepho, Krausz, and Schatz [PKS] model) and the vibronic coupling from the stretching vibration between the metal sites (P model) favor the localization and delocalization of the “extra” electron in mixed‐valence dimers, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange, and vibronic coupling. The results obtained by analyzing d²–d³ systems can be generalized to other full delocalized dinuclear mixed valence systems with a unique transferable electron. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

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     

Inelastic scattering of unpolarized neutrons on dimer clusters

On the basis of a microscopic calculation of the inelastic neutron scattering spectra for scattering by exchange dimer clusters, we have determined the conditions for applicability of the semiphenomenological approach to determination of the scattering cross sections based on the spin model. The scattering cross sections obtained in the spin model are distinguished from the exact values by the fact that modified form factors of the SS transitions are used in the exact calculation instead of the magnetic form factors of the individual atoms. In the case of weak overlap of the one-electron wave functions for different centers, the exact scattering cross sections coincide with the cross sections calculated on the basis of the spin model.We have investigated the inelastic neutron spectrum for scattering by a dimer d²–d¹ of mixed valency. Tunneling transfer of the extra electron leads to splitting of the exchange line corresponding to the transition with S=1 in the system with localized electrons and the appearance of new lines. The spectrum consists of a series of forbidden transitions (with conservation of parity and S=1) and allowed transitions (with a change in parity and S=0, 1). The results obtained allow us to determine the parameters of Heisenberg and double exchange in mixed-valency dimers using the inelastic neutron scattering spectra.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 392–398, July–August, 1988.     

Superacid cyclization of esters of some bishomoisoprenoid acids

It is shown that on the superacid cyclization of esters of bishomobicyclogeranylgeranic and E,E-bishomofarnesic acids fully cyclized hydroxyesters are formed in good yield, while on the interaction of esters of 6-hydroxy- and 6-acyloxy-15-bishomobicyclogeranylgeranic acids with a superacid no carbocyclization takes place.Institute of Chemistry, Moldovian Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 645–653, November–December, 1992.     

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     

Geometrical configurations of Pu4 and the Jahn–Teller effect

The present work has found seven stable geometrical configurations of Pu₄ using the density functional method Becke3LYP with relativistic effective core potentials (RECP). Based on the Jahn–Teller effect, vibronic interaction and the resolution of group representations, the analysis of the relationships among these various geometrical configurations is in fair agreement with the calculated results. It is found that the spin–polarization effect is rather important for the plutonium clusters and, in general, actinide element molecules.     

Resonance-enhanced 2PI detection of ammonia clusters via a linear reflectron TOF mass spetrometer

Ammonia clusters are resonantly two-photon ionized using a tunable pulsed UV laser (208–215 nm) and are subjected to time-of-flight (TOF) mass analysis with a linear ion reflector. The small ammonia clusters (from dimer to heptamer) exhibit broad vibronic structures, almost the same over the wavelength region studied.