Re-entrant phase transitions of the Blume-Emery-Griffiths model

We have investigated the spin-1 Ising model on the simple cubic lattice with bilinear, biquadratic interaction and anisotropic energy (BEG model). We have been specially interested in the case of antiferro biquadratic interaction, because the interaction will cause the competition with bilinear interaction and anisotropy. A two-sublattice ordering, so called the staggered quadrupole (SQ) phase, occurs as long as biquadratic interaction is negative large enough. We have obtained a full phase diagram in the whole interaction parameter space (for the positive bilinear interaction) by the Bethe approximation, and found several kinds of phase transitions, such as successive, re-entrant and double re-entrant transitions. These transitions are also confirmed by Monte Carlo simulations on simple cubic lattices.     

Simulation of electron bunch generation by an ultrashort-pulse high-intensity laser-driven wakefield

Electron acceleration due to a wakefield excited by a ultrashort-pulse intense laser propagating through a finite-length underdense plasma layer is studied by two-dimensional particle-in-cell simulation. The electron energy distribution is analyzed for moderate to high intensity. For the electron density, where the pulse length is almost half of the plasma wavelength, dramatic changes of the density structure occur with cavity and bunch formation with an increase in the laser intensity, also leading to the appearance of a fast electron component well confined in phase space. The analytical form of the fast electron energy spectrum is also presented.     

A cutting plane algorithm for a clustering problem

In this paper we consider a clustering problem that arises in qualitative data analysis. This problem can be transformed to a combinatorial optimization problem, the clique partitioning problem. We have studied the latter problem from a polyhedral point of view and determined large classes of facets of the associated polytope. These theoretical results are utilized in this paper. We describe a cutting plane algorithm that is based on the simplex method and uses exact and heuristic separation routines for some of the classes of facets mentioned before. We discuss some details of the implementation of our code and present our computational results. We mention applications from, e.g., zoology, economics, and the political sciences.     

Enantioselective spiro ene-carbocyclization of 1,6-enynes catalyzed by tropos rhodium(I) complex with skewphos ligand

A tropos rhodium(I) complex having skewphos ligand is shown to be a highly enantioselective catalyst for asymmetric ene-type carbocyclization of 1,6-enynes with tri-substituted olefins to control quaternary stereogenic centers or spiro-rings.     

In Situ Immobilization of Ultrafine Particles Synthesized in a Water/Oil Microemulsion

We investigated the in situ immobilization of ultrafine particles synthesized in a water/oil (w/o) microemulsion to silica for its possible application to supported metal catalysts. ZnS particles immobilized to silica by the ME method were consistent with those synthesized in a w/o microemulsion. Therefore, ZnS particles in a w/o microemulsion could be immobilized to silica without aggregation by this method. The relationship between the method of synthesizing Rh ultrafine particles in a w/o microemulsion and the diameter and diameter distribution of Rh particles immobilized to silica was studied. Rh-SiO(2) catalysts with a sharp diameter distribution could be prepared by immobilizing Rh-hydrazine complex particles because these complex particles would be very stable in a w/o microemulsion. The Rh particle diameters of Rh-SiO(2) catalysts prepared by changing the amount of silica produced were almost identical. Accordingly, the Rh particle diameter of Rh-SiO(2) catalysts could be controlled independently of Rh content by the ME method. Copyright 2001 Academic Press.     

2-alkoxy and 2,2-dialkoxy nitriles from acetals and orthoesters — exchange of alkoxy into cyano group by means of cyanotrimethylsilane

Title transformation is accomplished by the catalytic action of SnCl₂ or BF₃·OEt₂ Lithio derivative of 2,2-dimethoxyacetonitrile is used as synthetic equivalent of methyl lithioformate.     

Vibrational energies for the X1A1, A1B1, and B1A1 states of SiH2/SiD2 and related transition probabilities based on global potential energy surfaces

Transition probabilities were evaluated for the X(1)A(1)-A(1)B(1) and A(1)B(1)-B(1)A(1) systems of SiH(2) and SiD(2) to analyze the X-->A-->B photoexcitation. The Franck-Condon factors (FCFs) and Einstein's B coefficients were computed by quantum vibrational calculations using the three-dimensional potential energy surfaces (PESs) of the SiH(2)(X(1)A(1),A(1)B(1),B(1)A(1)) electronic states and the electronic transition moments for the X-A, X-B, and A-B system. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least-squares method combined with the Shepard interpolation. The obtained FCFs for the X-A and A-B systems exhibit that the bending mode is strongly enhanced in the excitation since the equilibrium bond angle greatly varies with the three states; the barrier to linearity is evaluated to be 21,900 cm(-1) for the X state, 6400 cm(-1) for the A state, and 230-240 cm(-1) for the B state. The theoretical lifetimes for the pure bending levels of the A and B states were calculated from the fluorescence decay rates for the A-X, B-A, and B-X emissions.     

Towards the selective formation of specific isomers of fullerenes: T - and p-dependence in the yield of various isomers of fullerenes C60–C84

Systematic fractional change in the yield of various isomers of fullerenes was revealed to strongly depend on temperature of a buffer gas. A new kinetic consideration is proposed for understanding the observed temperature- and pressure-dependence of yield of fullerenes. The model consists of three competitive reactions in consideration of plausible behaviors of a precursor, (1) decomposition into smaller fragments, (2) isomerization leading to formation of a stable fullerene cage, and (3) growth into a larger carbon cluster. Arrhenius activation energy of formation of stable fullerenes was determined to be 0.8 eV for both C₆₀ and C₇₀, while a higher energy of 2.0?3.3 eV for seven different isomers of higher fullerenes ranging from C₇₆ to C₈₄. Correlation in the activation energy is noted for a series of higher fullerenes with different sizes, suggesting the existence of a specific precursor in their formation processes.     

A free energy calculation study of the effect of H-->F substitution on binding affinity in ligand-antibody interactions

Changes in binding affinity to catalytic antibody 6D9 of chloramphenicol phosphonate derivatives (CPDs) containing H or F were investigated by performing free energy calculations based on molecular dynamics simulations. We calculated the binding free energy, enthalpy, and entropy changes (DeltaDeltaG, DeltaDeltaH, and -TDeltaDeltaS) attributable to H-->F substitution by comparing results for CPDs containing a trifluoroacetylamino group (CPD-F) or an acetylamino group (CPD-H). The calculated DeltaDeltaG, DeltaDeltaH, and -TDeltaDeltaS values were -2.9, -6.3, and 3.5 kcal mol(-1) and close to experimental values observed for a series of similar ligands, chloramphenicol phosphonates with F and H (-1.4, -3.5, and 2.1 kcal mol(-1)). Therefore, CPD-F binds more strongly to 6D9 than does CPD-H. To clarify the origin of the large difference in DeltaDeltaG, we apportioned the calculated values of DeltaDeltaG and DeltaG for the associated and dissociated states into contributions from various atomic interactions. We found that the H-->F substitution increased the binding affinity mainly by decreasing the hydration free energy and not by increasing favorable interactions with the antibody. The decreased hydration free energy of the ligand was mainly due to unfavorable coulombic interactions between the trifluoroacetylamino group and solvent waters, which increased the free energy of the dissociated state (by about 3.7 kcal mol(-1)). Also, the trifluoroacetylamino group slightly increased the free energy level of the associated state (about 0.8 kcal mol(-1)) because favorable van der Waals interactions compensated for unfavorable coulombic interactions with antibody atoms. In addition, the enthalpy and entropy changes, DeltaDeltaH and -TDeltaDeltaS (computationally -6.3 and 3.5 kcal mol(-1)), originated mainly from a decrease in hydration free energy in the dissociated state. The CPD-F and CPD-H ligands had substantially different structures in the dissociated and complexed states.