Origin of the relaxor state in Pb(B{x}B{1-x}{'})O(3) perovskites

Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.     

Theory and application of dissociative electron capture in molecular identification

The coupling of an electron monochromator (EM) to a mass spectrometer (MS) has created a new analytical technique, EM-MS, for the investigation of electrophilic compounds. This method provides a powerful tool for molecular identification of compounds contained in complex matrices, such as environmental samples. In particular, EM-MS has been applied to the detection of nitrated aromatic compounds, many of which are potent mutagens and/or carcinogens and are considered environmental hazards. EM-MS expands the application and selectivity of traditional MS through the inclusion of a new dimension in the space of molecular characteristics-the electron resonance energy spectrum. EM-MS also enhances detection sensitivity as well because the entire electron flux of the proper energy can be delivered into the negative ion resonance that is analytically most useful to solving the problem at hand. However, before this tool can realize its full potential, it will be necessary to create a library of resonance energy scans from standards of the molecules for which EM-MS offers a practical means of detection. Unfortunately, the number of such standards is very large and not all of the compounds are commercially available, making this library difficult to construct. Here, an approach supplementing direct measurement with chemical inference and quantum scattering theory is presented to demonstrate the feasibility of directly calculating resonance energy spectra. This approach makes use of the symmetry of the transition-matrix element of the captured electron to discriminate between the spectra of isomers. As a way of validating this approach, the resonance values for 25 nitrated aromatic compounds were measured along with their relative abundance. Subsequently, the spectra for the isomers of nitrotoluene were shown to be consistent with the symmetry-based model. The initial success of this treatment suggests that it might be possible to predict negative ion resonances and thus create a library of EM-MS standards.     

Copper(I)-only catalyzed reactions of (E)-2,3-difluoro-3-stannylacrylic ester with acid chlorides and mechanistic studies of the "copper effect" in stille coupling reactions

[reaction: see text] The reactions of (E)-2,3-difluoro-3-stannylacrylic ester with acid chlorides under catalysis of CuI stereospecifically afforded ethyl (2Z)-2,3-difluoro-4-oxo-substituted 2-butenoates in good yields. Moreover, direct spectroscopic observation of the corresponding organocopper reagent was observed for the first time in Stille coupling reactions of organostannanes.     

Heterogeneously catalyzed asymmetric C=C hydrogenation: origin of enantioselectivity in the proline-directed Pd/isophorone system

We have studied the proline-directed, Pd-catalyzed enantioselective hydrogenation of isophorone in the liquid state using a variety of methods. Our results unambiguously reveal the true reaction pathway and demonstrate that all earlier mechanistic hypotheses are wrong: although a proline/isophorone condensation product is formed, it is merely a spectator and not a key reaction intermediate in subsequent heterogeneous hydrogenation. Enantioselectivity is the result of kinetic resolution-a process that occurs homogeneously in solution and not at the metal surface. Racemic 3,3,5-trimethylcyclohexanone (TMCH) is produced by initial heterogeneous hydrogenation of isophorone; proline then reacts homogeneously, preferentially with one enantiomer of TMCH, leaving an excess of the other. Thus in complete contrast to the case of ketoester asymmetric hydrogenation, the metal surface is not involved in the crucial enantio-differentiation step. The mechanism we propose also explains why the maximum attainable yield of enantiopure TMCH cannot exceed 50%.     

A simple, short, and flexible synthesis of viridiofungin derivatives

Described herein is a simple, flexible, and efficient synthesis of the skeleton of the viridiofungins, a family of microbial secondary metabolites. The synthesis utilizes an asymmetric aldol reaction of a chiral oxazolidinone, a diastereoselective alkylation of a chiral 1,3-dioxolan-2-one, and a geometrically selective alkene cross-metathesis reaction as the key C-C bond-forming steps.     

Strangeness enhancement in Cu-Cu and Au-Au collisions at √S(NN)=200 GeV

We report new STAR measurements of midrapidity yields for the Λ, Λ[over ˉ], K(S)(0), Ξ(-), Ξ[over ˉ](+), Ω(-), Ω[over ˉ](+) particles in Cu+Cu collisions at √S(NN)==200 GeV, and midrapidity yields for the Λ, Λ[over ˉ], K(S)(0) particles in Au+Au at √S(NN)==200 GeV. We show that, at a given number of participating nucleons, the production of strange hadrons is higher in Cu+Cu collisions than in Au+Au collisions at the same center-of-mass energy. We find that aspects of the enhancement factors for all particles can be described by a parametrization based on the fraction of participants that undergo multiple collisions.     

The preparation and thermal dimerization study of a trifluorovinyl-containing imide. Application to the synthesis of a trifluorovinyl-containing polyimide precursor

Elaboration of 2-fluoro-α,β,β-trifluorostyrene 7, prepared by the palladium catalyzed coupling reaction of l-fluoro-2-iodobenzene with a trifluorovinylzinc reagent, yielded the trifluorovinyl-containing imide 1. Thermal dimerization of 1 at 140-160°C leads predominately to the corresponding E-cyclobutane 12, whose structure was established by x-ray crystallography. Similar methodology was utilized to prepare 2,2-bis{4-[4-amino-2-(l,2,2-trifluoroethenyl)phenoxy]phenyl)hexafluoropropane 19 , a precursor to the trifluorovinyl-containing polyimide 18 .