Aqueous speciation studies of europium(III) phosphotungstate

The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent, magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW9O34 9-, is a function of pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW11O39)7- predominates and the 1:1 Eu(PW11O39)4-, 2, forms. As the pH is increased, the 1:2 complex, Eu(PW11O39)2 11- species, 3, and (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O, a Eu8 hydroxo/oxo cluster, 1, form. Countercations modulate this effect; large countercations, such as K+ and Cs+, promote the formation of species 3 and 1. Addition of Al(III) as a counterion results in low pH and formation of [Eu(H2O)3(alpha-2-P2W17O61)]2, 4, with Al(III) counterions bound to terminal W-O bonds. The four species observed in these speciation studies have been isolated, crystallized, and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate tech-niques. These species are 1, (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O (P; a=20.2000(0), b=22.6951(6), c=25.3200(7) A; alpha=65.6760(10), beta=88.5240(10), gamma=86.0369(10) degrees; V=10550.0(5) A3; Z=2), 2, Al(H3O)[Eu(H2O)2PW11O34].20H2O (P, a=11.4280(23), b=11.5930(23), c=19.754(4) A; alpha=103.66(3), beta=95.29(3), gamma=102.31(3) degrees; V =2456.4(9) A3; Z=2), 3, Cs11Eu(PW11O34)2.28H2O (P; a=12.8663(14), b=19.8235(22), c=21.7060(23) A; alpha=114.57(0), beta=91.86(0), gamma=102.91(0) degrees ; V=4858.3(9) A3; Z=2), 4, Al2(H3O)8[Eu(H2O)3(alpha-2-P2W17O61)]2.29H2O (P; a=12.649(6), b=16.230(8), c=21.518(9) A; alpha=111.223(16), beta=94.182(18), gamma=107.581(17) degrees ; V=3842(3) A3; Z=1).     

Raman spectroscopic study of a post-medieval wall painting in need of conservation

Raman spectroscopic studies of four specimens from an important angel wall painting in need of conservation work in a medieval church have provided some information about the pigments and pigment compositions which will influence possible future preservation and restoration strategies. Excitation of the Raman spectra at 1,064 nm in macroscopic mode and at 785 nm in microscopic mode revealed that the white pigment on the angel's wings was a mixture of barytes with calcite and lead white in minor composition. Although the specimens provided were not directly associated with coloured regions of the painting, yellow and blue microcrystals were found and they were identified as chrome yellow and lazurite, respectively. Red and brown particles were identified as cinnabar/vermilion and haematite. Several green particles were also found but could not be identified. The green and blue crystals could be related to neighbouring coloured regions of the artwork and the yellow colour could be identified as a background to the angel figure. Particles of carbon were found to be dispersed throughout the specimens and can be ascribed to soot from candles, heating stoves or oil lamps providing lighting in the church. No evidence for biological deterioration was found from the spectra. The unusual pigment palette is strongly suggestive of a later date of painting than was originally believed but there is a possibility that an earlier rendition exists underneath. Following a review of the spectroscopic data, a more extensive sampling protocol is recommended, from which some stratigraphic evidence could identify the underlying plaster and possible artwork.     

Complexes of functionalized dipyrido[3,2-a:2',3'-c]-phenazine: a synthetic, spectroscopic, structural, and density functional theory study

The ligands 11-bromodipyrido[3,2-a:2',3'-c]phenazine and ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate have been prepared and coordinated to ruthenium(II), rhenium(I), and copper(I) metal centers. The electronic effects of substitution of dipyrido[2,3-a:3',2'-c]phenazine (dppz) have been investigated by spectroscopy and electrochemistry, and some photophysical properties have been studied. The crystal structures of [Re(L)(CO)(3)Cl] (L = ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate or 11-bromodipyrido[3,2-a:2',3'-c]phenazine) are presented. Density functional theory calculations on the complexes show only small deviations in bond lengths and angles (most bonds within 0.02 Angstroms, most angles within 2 degrees) from the crystallographic data. Furthermore, the vibrational spectra of the strongest Raman and IR bands are predicted to within an average 6 cm(-1) for the complexes [Re(L)(CO)(3)Cl] and [Cu(L)(triphenylphosphine)(2)]BF(4) (in the 1000-1700 cm(-1) region). Spectroscopic and electrochemical evidence suggest that reduction of the complex causes structural changes across the entire dppz ligand. This is unusual as dppz-based ligands typically have electrochemical properties that suggest charge localization with reduction on the phenazine portion of the ligand. The excited-state lifetimes of the complexes have been measured, and they range from ca. 200 ns for the [Ru(L)(2,2'-bipyridine)(2)](PF(6))(2) complexes to over 2 mus for [Cu(11-bromodipyrido[3,2-a:2',3'-c]phenazine)(PPh(3))(2)](BF(4)) at room temperature. The emission spectra suggest that the unusually long-lived excited states of the copper complexes result from metal-to-ligand charge transfer (MLCT) transitions as they are completely quenched in methanol. Electroluminescent films may be fabricated from these compounds; they show MLCT state emission even at low doping levels [<0.1% by weight in poly(vinylcarbazole) polymer matrix].     

Nondestructive analysis of ancient Egyptian funerary relics by Raman spectroscopic techniques

The non-destructive Raman spectroscopic analysis of a range of ancient Egyptian funerary artefacts has been undertaken with a view to establish the identification of the pigment composition and nature of the interaction with the substrate. The specimens date from the New Kingdom to the Graeco-Roman period, ca. 3900-1800 years b.p. In this study a comparison has also been effected between the near-infrared and red excitation wavelengths for the characterisation of these materials. A particular problem for Raman analysis of these artefacts was found in the application of resin or organic compounds to the surface or substrata, as colourants or to aid adhesion of the pigments, and degradation of the resins with burial environment and exposure has resulted in large fluorescent backgrounds. Nevertheless, useful information has been obtained which assists in the interpretation of the construction of some of the artefacts.     

Comparison of isothermal and dynamic methods for the determinationof activation energy by thermogravimetry

Several dynamic methods for estimating activation energies have been developed. This development has arisen largely as a matter of convenience and the desire to minimize analysis time. While these methods generally afford values which are somewhat similar, the agreement among values from various methods is never outstanding. Further, the values obtained are often, at best, only approximations of the values obtained by the traditional isothermal approach. To better ascertain the utility of dynamic methods for the determination of activation energies, the activation energy for the thermal degradation of a standard vinylidene chloride/methyl acrylate (five-mole percent) copolymer has been generated by a variety of methods. The degradation of this polymer is an ideal reaction for evaluation of the various methods. At modest temperatures (<200°C), the only reaction that contributes to mass loss is the first order evolution of hydrogen chloride, i.e., there is only one significant reaction occurring and it is not impacted by competing processes. The best values (most reproducible; best correspondence to values obtained by titrimetry and other methods) are those obtained by plotting the natural logarithm of rate constants obtained at various temperatures vs. the reciprocal of the Kelvin temperature. Various dynamic methods yield values which are less reproducible and which approximate these values to a greater or lesser degree. In no case is the agreement good.     

Directed ring-opening of 1,5-dioxaspiro[3.2]hexanes: selective formation of 2,2-disubstituted oxetanes

1,5-Dioxaspiro[3.2]hexanes undergo ring-opening reactions with many heteroatom nucleophiles to provide alpha-substituted-beta'-hydroxy ketones. However, certain Lewis acidic nucleophiles provide 2,2-disubstituted oxetanes. Herein, the results of reactions of 3-phenyl-1,5-dioxaspiro[3.2]hexane with a variety of nitrogen-containing heteroaromatic bases are reported. There appears to be a correlation between the pK(a) of the nucleophile and the reaction outcome with more acidic nucleophiles providing 2,2-disubstituted oxetanes. Moreover, the mode of ring opening can be directed toward the substituted oxetane by the addition of a Lewis acid. These results are rationalized by calculation of stationary points on the potential energy surfaces for the various possible reaction pathways using ab initio molecular orbital methods.     

Stress communication and filtering of viscoelastic layers in oscillatory shear

We revisit a classical topic: response functions of viscoelastic layers in large amplitude oscillatory shear. Motivated by questions concerning protective biological layers, we focus on boundary stresses in a parallel plate geometry with imposed oscillatory strain or stress. These features are gleaned from resolution and analysis of coupled standing waves of deformation and stress. We identify a robust non-monotone variation in boundary stress signals with respect to all experimental controls: viscoelastic moduli of the layer, layer thickness, and driving frequency. This structure of peaks and valleys in boundary values of shear and normal stress indicates redundant mechanisms for stress communication (by tuning to the peaks) and stress filtering (by tuning to the valleys). In this paper, we first restrict to a single-mode non-linear Maxwell model for the viscoelastic layer, where analysis renders a transparent explanation of the phenomena. We then consider a Giesekus constitutive model of the layer, where analysis is supplanted by numerical simulations of coupled non-linear partial differential equations. Parametric studies of wall stress values from standing waves confirm persistence of the Maxwell model phenomena. The analysis and simulations rely on and extend our recent studies of shear waves in a micro parallel plate rheometer [S.M. Mitran, M.G. Forest, L. Yao, B. Lindley, D. Hill, Extenstions of the Ferry shear wave model for active linear and nonlinear microrheology, J. Non-Newtonian Fluid Mech. 154 (2008) 120–135; D.B. Hill, B. Lindley, M.G. Forest, S.M. Mitran, R. Superfine, Experimental and modeling protocols from a micro-parallel plate rheometer, UNC Preprint, 2008].     

Understanding the influence of hydrocarbon insulators in fluorinated amines: Reactivity of poly(hexafluoropropylene oxide) amine containing methylene spacers

Although reactions involving hydrocarbon amines have been thoroughly investigated, very little is currently known about reactions of corresponding fluorinated amines containing a methylene spacer group. Furthermore, such reactions involving the poly(hexafluoropropylene oxide) (herein, polyHFPO) amine have been completely unexplored. The addition of acyl, sulfonyl and alkyl halides, isocyanates, aldehydes, anhydrides and esters to polyHFPO amine has been accomplished. The results of these reactions, including reaction mechanisms, yields, byproducts, etc. are discussed.