Steric Stabilization of Colloids by Poly(dimethylsiloxane) in Carbon Dioxide: Effect of Cosolvents

Steric stabilization and flocculation of colloids with surface-grafted poly(dimethylsiloxane) (PDMS) chains are examined in liquid and supercritical carbon dioxide with and without hexane as a cosolvent. Neither poly(methyl methacrylate) (PMMA) nor silica particles with grafted 10,000 g/mol PDMS could be stabilized in pure CO(2) at pressures up to 345 bar at 25 degrees C and 517 bar at 65 degrees C without stirring. The addition of 15 wt% hexane to CO(2) led to stable dispersions with sedimentation velocities of 0.2 mm/min for 1-2 μm PMMA particles. The critical flocculation pressure of the colloids in the hexane/CO(2) mixture, determined from turbidity versus time measurements, was found to be the same for silica and PMMA particles and was well above the upper critical solution pressure for the PDMS-CO(2) system. The addition of a nonreactive cosolvent, hexane, eliminates flocculation of PMMA particles synthesized through dispersion polymerization in CO(2) with PDMS-based surfactants. Copyright 2000 Academic Press.     

Thianaphthenoyltrifluoroacetone as an analytical reagent: The keto-enol tautomerism of the 2- and 3-isomers of thianaphthenoyltrifluoroacetone

Summary The keto-enol tautomerism of the 2- and 3-isomers of thianaphthenoyltrifluoroacetone has been studied and the results explained by considering the relative negativities of the 2- and 3-position of thianaphthene. The results of the comparison of the keto-enol tautomerism of 3-thianaphthenoyltrifluoroacetone to that of 2-thenoyltrifluoroacetone in 80% benzene-20% ethanol may explain in part the greater sensitivity of the former in trace metal analysis.

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Zusammenfassung Die Keto-Enol-Tautomerie der 2- und 3-Isomeren des Thianaphthenoyltrifluoracetons wurde untersucht und mit der relativen Negativität der 2- und 3-Stellung des Thianaphtens zu erklären versucht. Der Vergleich der Keto-Enol-Tautomerie von II mit der des 2-Thenoyltrifluoracetons in BenzolÄthanol (80+20) könnte eine Erklärung für die größere Sensitivität von II bei der Spurenmetallanalyse bieten.

     

Free radical-mediated vinyl amination: access to N,N-dialkyl enamines and their beta-stannyl and beta-thio derivatives

[reaction: see text] The first examples of free radical-mediated vinyl amination are described by nonconventional vinyl radical addition to azomethine nitrogen. This new vinyl amination protocol is mild and provides convenient synthetic access to nonstabilized N,N-dialkyl enamines and tandem bond-forming processes.     

Development of delta(15)N stratification of NO(3)(-) in soil profiles

New evidence, obtained using a robust method for measuring the delta(15)N of NO(3)(-)-N in soil, is consistent with denitrification being the major determinant in the vertical distribution of NO(3)(-)-delta(15)N in soil profiles. These data also suggest that varying moisture regimes result in different effects of soil NO(3)(-)-N leaching on residual whole soil delta(15)N.     

A systematic study of magnetic order in divalent europium perovskites

The magnetic properties of a series of cubic perovskite materials, Eu(Al_0.5Ta_0.5)O₃, Eu(Mg_0.5W_0.5)O₃, and Eu(Lu_0.5Ta_0.5)O₃ have been investigated using bulk magnetic and Mössbauer techniques. Eu(Al_0.5Ta_0.5)O₃ is a ferromagnet (T_c ～ 5°K, θ_c = + 8°K), Eu(Lu_0.5Ta_0.5)O₃ is an antiferromagnet (T_N = 4.0, θ_c = ?8°K), while Eu(Mg_0.5W_0.5)O₃ is probably antiferromagnetic (T_N = 2.8°K, θ_c = ? 1°K). These data are compared with the known properties of EuLiH₃ and EuTiO₃ and a sharp drop in θ_c as a function of increasing lattice constant is noted. A molecular field theory analysis of the data yields two possible sets of nn (J₁) and nnn (J₂) exchange constants for each compound which are compared with existing theories.     

Reactions and X-ray crystal structure of (3-cyano-2,4-pentanedionato-N)pentaamminecobalt(III) perchlorate chloride dihydrate

The pentaamminecobalt(III) complex with the 3-cyano-2,4-pentanedionate anion coordinated through the nitrile nitrogen has been characterized by X-ray crystallography. The crystals of [(NH₃)₅CoNCacac](Cl)(ClO₄)·2H₂O are triclinic, space group P , a = 10.245(2) Å, b = 14.071(4) Å, c = 6.971(2) Å, = 90.03(3)°, β = 109.86(2)°, γ = 108.91(2)°, V= 887.1 ų, Z = 2, D_c = 1.64 g cm⁻³, F(000) = 456, Mo-K radiation, λ = 0.71069 Å, μ(Mo-K) = 12.7 cm⁻¹. The structure was determined by the heavy-atom method, and refined by block-diagonal least-squares calculations, R = 0.0537, R_w = 0.0607, for 2499 observed reflections. Principal dimensions are: Co---N(NH₃) trans to NCacac⁻ 1.940(5), other Co---N(NH₃) 1.967(2), Co---N(NCacac⁻) 1.911(5) Å. The pendant acac moiety is best described in terms of a delocalized bond network with, for example, C---C distances in the range 1.44–1.52(1) Å. Several reactions involving this free acac group are also described including the preparation and characterization of the dimeric species pentaamminecobalt(III) - μ - (3 - cyano - 2,4 - pentanedionato) - bis(propylenediamine) cobalt(III) perchlorate.     

Bronsted acid-promoted olefin aziridination and formal anti-aminohydroxylation

A straightforward synthesis of aziridines is reported from an electron-rich azide (alkyl or aryl azide), electron-deficient olefin, and triflic acid in cold acetonitrile. The only coproduct of the reaction is dinitrogen (N2). Active ester substrates bearing a nucleophilic carbonyl engage the putative protonated aziridine intermediate to produce the product of olefin aminohydroxylation in which the nitrogen is benzyl protected and the oxygen is acylated. The possibility that a triazoline need not be an intermediate in aziridine production is advanced.