Synthetic metals based on tetramethyltetraselenafulvalene(TMTSF): Varied anisotropy in the low temperature (125K) structures of (TMTSF)2X,X=C104-, PF6-, and AsF6-.

The low temperature (125K) X-ray crystal structures of (TMTSF)₂X, X=C10₄^-, PF₆^-, and AsF₆^- reveal decreases in the intermolecular interand intrastack Se-Se contact distances upon cooling (298K to 125K) which are highly anisotropic and different from one salt to another. The changes in the interstack distances, which are normal to the stacking direction, are approximately twice those involving intrastack Se-Se interactions. These observations establish that the anisotropic structural changes which accompany decreased temperature are common to numerous (TMTSF)₂X radical cation conducting salts.     

A multi-objective optimization model for aquifer management under transient and steady-state conditions

A multiple objective waste-disposal model is developed and analysed. The model is a modification of the single objective waste-disposal model of Alley, Aguado and Remson. The solution structure is obtained using the method of constraints so that dual variables (shadow prices) are available with the solutions.     

The photochemistry of sulfur dioxide excited within its first allowed band (3130 Å) and the “forbidden” band (3700–;4000 Å)

The quantum yields of SO₃ formation have been determined in pure SO₂ and in SO₂ mixtures with NO, CO₂, and O₂ using both flow and static systems. In separate series of experiments excitation of SO₂ was effected within the forbidden band, SO₂(³B₁) ← \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SO}_2 (\tilde X,^1 A_1 ) $$\end{document}, and within the first allowed singlet band at 3130 Å. The values of Φ were found to be sensitive to the flow rate of the reactants. These results and the apparently divergent quantum yield results of Cox [10], Allen and coworkers [24, 26, 29], and Okuda and coworkers [11] were rationalized quantitatively in terms of the significant occurrence of the reactions SO + SO₃ → 2SO₂ (2), and 2SO → SO₂ + S [or (SO)₂] (3), in experiments of long residence time. From the present rate data, values of the rate constants were estimated, k₂=(1.2±0.7) × 10⁶; k₃=(5±4) × 10⁵ l˙/mole · sec. Φ values from triplet excitation experiments at high flow rates of NO? SO₂ and CO₂? SO₂ mixtures showed the sole reactant with SO₂ leading to SO₃ formation in this system to be SO₂(³B₁); SO₂(³B₁) + SO₂ → SO₃ + SO(³Σ^?) (la); k=(4.2±0.4) × 10⁷ l./mole · sec. With excitation of SO₂ at 3130 Å both singlet and triplet excited states play a role in SO₃ formation. If the reactive singlet state is ¹B₁, the long-lived fluorescent state, SO₂(¹B₁) + SO₂ → SO₃ + SO (¹ Δ or ³Σ^?) (lb), then k=(2.2±0.5) × 10⁹ l./mole · sec. From the observed inhibition of SO formation by added nitric oxide, it was found that the SO₃-forming triplet state, generated in this singlet excited SO₂ system, had a relative reactivity toward SO₂ and NO which was equal within the experimental error to that observed here for the SO₂(³B₁) species. Either SO₂(³B₁) molecules were created with an unexpectedly high efficiency in 3130 Å excited SO₂(¹B₁) quenching collisions, or another reactive triplet (presumably ³A₂ or ³B₂) of almost identical reactivity to SO₂(³B₁) was important here.     

A novel oxidative rearrangement of 6-methoxypyran-2-ones

As part of our continuing studies of pyrone-containing natural products, a series 6-methoxypyran-2-ones were synthesized. These were found to react with molecular oxygen at 20 degrees C, and this novel reaction yielded a series of highly functionalized alpha,beta-butenolides. [reaction: see text]     

Generalized anisotropic scaling theory and the transverse meissner transition

We consider a depinning transition in vortex systems with columnar disorder and tilted applied magnetic fields. From scaling arguments and Monte Carlo simulations, we find that this transverse Meissner transition is governed by a fixed point which is anisotropic in all three directions. This generalization of conventional anisotropic scaling means that the correlation length in different directions diverges with different rates, and we derive exact results for the anisotropy exponents. We make predictions which can be tested in experiments on superconductors with columnar disorder.     

Functionalized macrocycles from functionalized tetra-amines: pendent-arm macrocycles derived from dichloropivalic acid

Reaction between ethane-1,2-diamine and 3,3'-dichloropivalic acid results in different, isomeric tetra-amine derivatives, one a tetraamino carboxylic acid and the other a carboxamidotriamino alcohol, depending upon reaction conditions. Intended conversion of the Cu(II) complex of the former to a cyclam-like macrocycle through reaction with nitroethane and formaldehyde results in isolation of derivatives of both the former and the latter. This can be rationalized by assuming the intermediacy of an azetidinone, a species similar to that seen in simpler reactions of dichloropivalates. A single reaction thereby provides pendent-arm macrocycles where one has an electrophilic and the other a nucleophilic substituent. Parallel chemistry is not seen in the reaction between propane-1,3-diamine and 3,3'-dichloropivalate.     

Biomimetic synthesis of the pyrrolobenzoxazine core of paeciloxazine

Starting from a protected L-tryptophan derivative the pyrrolobenzoxazine core unit of paeciloxazine can be synthesized in two oxidation steps.     

Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-beta-diketonato ligands

The interaction of six aryl-linked bis-beta-diketones, including a new naphthylene linked species, with copper(II), iron(III) and, in one instance gallium(III), has been investigated with the aim of obtaining metallo-supramolecular assemblies exhibiting different geometries. New examples of two assembly types incorporating the above bis-beta-diketones (L) were generated. The first type is represented by a range of molecular triangles of formula [Cu(3)(L-H(2))(3)](solvent)(n) while the second is given by a corresponding selection of less-common neutral molecular tetrahedra of formula [Fe(4)(L-H(2))(6)](solvent)(n) as well as [Ga(4)(L-H(2))(6)].8.5THF.0.5H(2)O; an example of each type has been characterised by X-ray crystallography. A magnetochemical investigation of [Fe(4)(-H(2))(6)].6THF is reported. The susceptibility is Curie like and consistent with very weak coupling occurring between the iron(III) d(5)(high spin) centres. The X-ray structures of two trinuclear copper(II) as well as a tetranuclear iron(III) and a tetranuclear gallium(III) assembly confirm their discrete triangular and tetrahedral geometries, respectively. The structure of the gallium(III) species is closely related to that of the corresponding iron(III) species. The tetrahedral structures provide rare examples of such assemblies encapsulating guest solvent molecules--in each case tetrahydrofuran is incorporated in the central cavity.     

Uniqueness of travelling waves for nonlocal monostable equations

We consider a nonlocal analogue of the Fisher-KPP equation

0 ~{\rm on}~(0,1),\end{displaymath}">

and its discrete counterpart , , and show that travelling wave solutions of these equations that are bounded between and are unique up to translation. Our proof requires finding exact a priori asymptotics of a travelling wave. This we accomplish with the help of Ikehara's Theorem (which is a Tauberian theorem for Laplace transforms).

     

Molality as a unit of measure for expressing 1H MRS brain metabolite concentrations in vivo

Absolute concentrations of cerebral metabolite in in vivo 1H magnetic resonance spectroscopy studies (1H-MRS) are widely reported in molar units as moles per liter of tissue, or in molal units as moles per kilogram of tissue. Such measurements require external referencing or assumptions as to local water content. To reduce the scan time, avoid assumptions that may be invalid under specific pathologies, and provide a universally accessible referencing procedure, we suggest that metabolite concentrations from 1H-MRS measurements in vivo be reported in molal units as moles per kilogram of tissue water. Using internal water referencing, a two-compartment water model, a simulated brain spectrum for peak identification, and a spectroscopic bi-exponential spin-spin relaxation segmentation technique, we measured the absolute concentrations for the four common 1H brain metabolites: choline (Cho), myo-inositol (mIno), phosphocreatine + creatine (Cr), and N-acetyl-aspartate (NAA), in the hippocampal region (n = 26) and along the Sylvian fissure (n = 61) of 35 healthy adults. A stimulated echo localization method (20 ms echo time, 10 ms mixing time, 4 s repetition time) yielded metabolite concentrations, uncorrected for metabolite relaxation or contributions from macromolecule resonances, that were expectantly higher than with molar literature values. Along the Sylvian fissure the average concentrations (coefficient of variation (CV)) in mmoles/kg of tissue water were 17.6 (12%) for NAA, 14.2 (9%) for Cr, 3.6 (13%) for Cho, and 13.2 (15%) for mIno. Respective values for the hippocampal region were 15.7 (20%), 14.7 (16%), 4.6 (19%), and 17.7 (26%). The concentrations of the two regions were significantly different (p 相似文献