Spin projected EHF method: Calculations for a four-electron model system

The spin projected extended Hartree-Fock equations for successive optimization of the orbitals are derived for the four-electron case and applied to butadiene in the PPP approximation. The method shows an excellent convergence and gives 91.4% of the correlation energy for the given model-Hamiltonian.     

Long-range electron transfer across Peptide bridges: the transition from electron superexchange to hopping

Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL-Pron-apyRuIII)(NH3)5]5++ proline residues (n) varying from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless kmax values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant beta = 1.4 A-1 to a hopping mechanism with attenuation constant beta = 0.17 A-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value.     

Theory of quantum energy transfer in spin chains: superexchange and ballistic motion

Quantum energy transfer in a chain of two-level (spin) units, connected at its ends to two thermal reservoirs, is analyzed in two limits: (i) in the off-resonance regime, when the characteristic subsystem excitation energy gaps are larger than the reservoirs frequencies, or the baths temperatures are low and (ii) in the resonance regime, when the chain excitation gaps match populated bath modes. In the latter case, the model is studied using a master equation approach, showing that the dynamics is ballistic for the particular chain model explored. In the former case, we analytically study the system dynamics utilizing the recently developed Energy-Transfer Born-Oppenheimer formalism [L.-A. Wu and D. Segal, Phys. Rev. E 83, 051114 (2011)], demonstrating that energy transfers across the chain in a superexchange (bridge assisted tunneling) mechanism, with the energy current decreasing exponentially with distance. This behavior is insensitive to the chain details. Since at low temperatures the excitation spectrum of molecular systems can be truncated to resemble a spin chain model, we argue that the superexchange behavior obtained here should be observed in widespread systems satisfying the off-resonance condition.     

Angular overlap model parameterisation of Anderson''s superexchange theory. I. A quantification of Goodenough-Kanamori rules

A parameterisation of Anderson's exchange formulas on the basis of an extension of the angular overlap model (AOM) is proposed. Transfer integrals are expressed in terms of the metal-metal and metal-ligand bonding parameters, which can be estimated from independent spectroscopic studies, or calculated using solid state expressions. Analytical expressions for the transfer integrals between various d-orbitals, appropriate for cubic crystal lattices, comprising octahedra sharing common vertices and common edges serve as a quantification of the Goodenough-Kanamori rules. On the basis of the present parameterisation we give an explanation of the “exchange integral versus bond-distance” dependence. Some potential applications of the model are briefly discussed.     

Metal-metal coupling elements of mixed-valence pentaammineruthenium dimers: The hole-transfer superexchange case

The extent of metal-metal coupling in the mixed-valence complexes [Ru(NH₃)₅₂(μ-L)]³⁺], where L is 2,5-dimethyl-(Me₂dicyd²⁻), 2,5-dichloro- (Cl₂dicyd²⁻), 2,3,5,6-tetrachloro- (Cl₄dicyd²⁻) or unsubstituted (dicyd²⁻) 1,4-dicyanamidobenzene dianion, was evaluated by comparing theoretical values of metal-metal coupling elements with estimates of the free energy of resonance exchange which were derived from the free energies of comproportionation. Poor agreement was found with the Hush model; however, an excellent correlation was seen with the model of Creutz, Newton and Sutin (CNS). It would appear that the CNS model is remarkably successful in describing the extent of metal-metal coupling for the strongly coupled valence trapped complexes of this study.     

Effective spin Hamiltonian model for superexchange interaction between rare earth ions in rare earth elpasolite crystals

The fine structure of zero phonon lines in the electronic absorption spectra of lanthanide ion systems is interpreted by an effective spin Hamiltonian model. The splittings of several cm-1 observed for lanthanide ions with interionic separations near 1 nm in elpasolite lattices are attributed to superexchange interactions.     

小儿流行性出血热与5例误诊分析

对小儿流行性出血热5例误诊病例进行了分析,提出要全面分析病史,早查EHF－IgM等对策。     

Description of metallic and covalent clusters with icosahedral symmetry: the polytope model

Compact and tetravalent clusters with icosahedral local or global symmetries are generated by mapping from ideal structures in curved space onto a tangent euclidean 3D space. The observed elastic energy of the clusters can thus be interpreted as an intrinsic curvature associated to a frustrated local order. It is then proposed a kind of classification of the very rich family of possible clusters using a limited set of parameters.     

Description of the kinetics of a model tribopolymerization process

A three‐dimensional system of ordinary differential equations of first order has been proposed to describe the kinetics of a model tribopolymerization process for which a linear coupling of the material surface dislocation density and polyreaction fields is assumed. Another important physical motivation on which the model is based stresses the predominant role of a highly rough surface under friction as a catalyst in the polyreaction process. In consequence, the fractal‐like chemical reaction concept has been applied. It is demonstrated that the system proposed is suitable for describing a variety of tribopolymerization processes and possesses non‐trivial time‐dependent behaviour even in some limiting analytical cases studied. As a specific result, a nonlinear equation generalizing the Vogel–Fulcher temperature‐dependence of the maximum time scale for the polyreaction caused by friction, characteristic of the selective transfer during friction process, has been recovered. Some comparison with an approach made earlier (G.P. Shpenkov, Friction Surface Phenomena (Elsevier, Amsterdam, 1995) pp. 165–172 and Ref. [239] therein) has been presented as well. Certain basic trends of modelling and experiment are in good agreement.     

On the efficiency of different EHF wavefunctions

A new computational scheme for the spin-extended Hartree-Fock (EHF) equations is shown to be suitable for large Π-electron systems. It is demostrated that the triplet EHF wavefunction with equal numbers of orbitals of different spins gives a much larger energy decrease than that with the maximum total-spin projection value.     

Crossover from superexchange to hopping as the mechanism for photoinduced charge transfer in DNA hairpin conjugates

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.     

Hydration of DNA-binding biological active compounds: EHF dielectrometry and molecular modeling results

We report hydration properties of several DNA-binding ligands (pharmaceutical drug caffeine; mutagens proflavine, ethidium bromide, propidium iodide; polyamines putrescine, spermine), which are able to interact with DNA in different modes via external binding, intercalation or minor groove binding. We show that the detection of the bound water molecules and the estimation of their amount in aqueous solutions of ligands can be efficiently carried out using the measurements of complex dielectric permittivity of the solutions in the millimeter range of radio waves. Our dielectrometric data are combined with the results of the molecular modeling including quantum chemical calculations and Monte Carlo simulations. We show that number of water molecules able to form hydrogen bonds with donor–acceptor groups of ligands correlates with hydration numbers taken from the literature or obtained in EHF dielectrometry experiment. The latter indicates that the EHF dielectrometry method is sensitive for the tightly bound water molecules.     

Extended Hartree-Fock (EHF) theory of chemical reactions

The triplet-instability analysis of the closed-shell RHF solutions has been carried out in relation to the orbital and spin correlation effects for various transition structures (TS) and reaction intermediates. It is found that the RHF solutions even for cyclic transition states of the Woodward-Hoffmann symmetry-allowed reactions often involve the triplet instability, indicating the crucial role of correlation corrections. The di- and tetra-radical characters for the transition structures are calculated by the projected UHF (PUHF) solutions resulting from the instability. The spin projection is also crucial for the UHF Møller-Plesset (UMP) correlated wavefunctions obtained for the transition structures of 1,3-dipolar, Diels-Alder, ene and related reactions. The relative stability between cyclic and acyclic TS for these reactions is examined at the approximately projected UHF MP2 (APU MP2) level. The former is found to be more favorable than the latter if the correlation correction is taken into account for TS in a well-balanced manner.Dedicated to Professor J. Koutecký on the occassion of his 65th birthday     

Spin-projected EHF method. III. Applications to π-electron systems

The surely convergent procedure to obtain extended Hartree–Fock (EHF ) solutions of the spin-projected scheme, for which the equations are given in Part II, is applied to eleven π-electronic systems with 2–10 electrons at the PPP level of integral approximations. The method takes into account a considerable part of the correlation energy. The symmetry properties of the EHF wave functions obtained are discussed together with some computational details. A simplified algorithm is also described.     

A multilayer model for the study of space distributed redox modified electrodes: Part I. Description and discussion of the model

A multilayer model for the study of space distributed redox modified electrodes (redox polymer electrodes, or adsorption of an electroactive substance in several layers) is described and discussed in detail. It is shown in particular that it is equivalent to a system in which the electrons diffuse in the coating.     

Copper clusters built on bulky amidinate ligands: spin delocalization via superexchange rather than direct metal-metal bonding

Entry into a new class of tetra- and dicopper clusters was assisted by a fine steric tuning of bulky amidinate ligands that provide spin-delocalizing superexchange pathways in class III mixed-valence clusters, the properties of which are best understood without invoking metal-metal bonding.