Fourier transform infrared study of low-temperature CO adsorption on CuMgAl-hydrotalcite

Single-phase CuMgAl ternary hydrotalcite with (Cu+Mg)/Al atomic ratio of 3.0 and Cu/Mg atomic ratio of 1.0 was synthesized by coprecipitation. Thermoanalytical studies of this sample showed transformations in three stages in the temperature range up to ca. 900 K yielding mainly CuO phase. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements showed the presence of carbonates even after calcination of the sample at 973 K. The genesis of Cu+ sites during thermal treatment in vacuo at different temperatures for this sample was followed by IR spectroscopy of CO adsorbed at low temperature. Essentially no Cu+ sites are present on a sample calcined at 723 K, consistent with X-ray photoelectron spectroscopic (XPS) data. However, sample subjected to activation (1 h of O2 treatment at 723 K followed by 1 h of evacuation at the same temperature) upon CO adsorption at 85 K unambiguously showed the presence of Cu+ sites. 12CO-13CO coadsorption studies confirmed the presence of dicarbonyls, which are converted to linear Cu+-CO species during evacuation at 85 K. Concentration of the accessible Cu+ sites increased with the increase in activation temperature up to 873 K and decreased with a further temperature rise. The copper sites on the sample are heterogeneously distributed and their distribution depends on the activation temperature. Two routes of reduction of Cu2+ to Cu+ are proposed: (i) autoreduction during evacuation and (ii) reduction by CO.     

Structure of docosahexaenoic acid-containing phospholipid bilayers as studied by (2)H NMR and molecular dynamics simulations.

Polyunsaturated phospholipids are known to be important with regard to the biological functions of essential fatty acids, for example, involving neural tissues such as the brain and retina. Here we have employed two complementary structural methods for the study of polyunsaturated bilayer lipids, viz. deuterium ((2)H) NMR spectroscopy and molecular dynamics (MD) computer simulations. Our research constitutes one of the first applications of all-atom MD simulations to polyunsaturated lipids containing docosahexaenoic acid (DHA; 22:6 cis-Delta(4,7,10,13,16,19)). Structural features of the highly unsaturated, mixed-chain phospholipid, 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC), have been studied in the liquid-crystalline (L(alpha)) state and compared to the less unsaturated homolog, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The (2)H NMR spectra of polyunsaturated bilayers are dramatically different from those of less unsaturated phospholipid bilayers. We show how use of MD simulations can aid in interpreting the complex (2)H NMR spectra of polyunsaturated bilayers, in conjunction with electron density profiles determined from small-angle X-ray diffraction studies. This work clearly demonstrates preferred helical and angle-iron conformations of the polyunsaturated chains in liquid-crystalline bilayers, which favor chain extension while maintaining bilayer flexibility. The presence of relatively long, extended fatty acyl chains may be important for solvating the hydrophobic surfaces of integral membrane proteins, such as rhodopsin. In addition, the polyallylic DHA chains have a tendency to adopt back-bended (hairpin-like) structures, which increase the interfacial area per lipid. Finally, the material properties have been analyzed in terms of the response of the bilayer to mechanical stress. Simulated bilayers of phospholipids containing docosahexaenoic acid were less sensitive to the applied surface tension than were saturated phospholipids, possibly implying a decrease in membrane elasticity (area elastic modulus, bending rigidity). The above features distinguish DHA-containing lipids from saturated or monounsaturated lipids and may be important for their biological modes of action.     

Excited-state energy-transfer dynamics in self-assembled triads composed of two porphyrins and an intervening Bis(dipyrrinato)metal complex

The synthesis and characterization of various triads composed of a linear array of two zinc porphyrins joined via an intervening bis(dipyrrinato)metal(II) complex are reported. The preparation exploits the facile complexation of dipyrrins with divalent metal ions to give bis(dipyrrinato)metal(II) complexes [abbreviated (dp)(2)M]. Copper(II) and palladium(II) chelates of dipyrrins (available by oxidation of dipyrromethanes) were prepared in 50-80% yield. A one-flask synthesis of bis(dipyrrinato)zinc(II) complexes was developed by oxidation of a dipyrromethane with DDQ or p-chloranil in the presence of Zn(OAc)(2).2H(2)O in THF ( approximately 80% yield). Three routes were developed for preparing porphyrin-dipyrrins: (1). Suzuki coupling of a boronate-substituted zinc porphyrin (ZnP) and bis[5-(4-iodophenyl)dipyrrinato]Pd(II) to give the (ZnP-dp)(2)Pd triad (50% yield), followed by selective demetalation of the (dp)(2)Pd unit by treatment with 1,4-dithiothreitol under neutral conditions (71% yield); (2). oxidation of a porphyrin-dipyrromethane with p-chloranil in the presence of Zn(OAc)(2).2H(2)O followed by chromatography on silica gel (71% yield); and (3). condensation of a dipyrrin-dipyrromethane and a dipyrromethane-dicarbinol under InCl(3) catalysis followed by oxidation with DDQ (10-16% yield). Four triads of form (ZnP-dp)(2)Zn were prepared in 83-97% yield by treatment of a porphyrin-dipyrrin with Zn(OAc)(2).2H(2)O at room temperature. Free base dipyrrins typically absorb at 430-440 nm, while the bis(dipyrrinato)metal complexes absorb at 460-490 nm. The fluorescence spectra/yields and excited-state lifetimes of the (ZnP-dp)(2)Zn triad in toluene show (1). efficient energy transfer from the bis(dipyrrinato)zinc(II) chromophore to the zinc porphyrins (98.5% yield), and (2). little or no quenching of the resulting excited zinc porphyrin relative to the isolated chromophore. Taken together, these results indicate that bis(dipyrrinato)zinc(II) complexes can serve as self-assembling linkers that further function as secondary light-collection elements in porphyrin-based light-harvesting arrays.     

Vibrational spectroscopic investigation of stereoregularity effects on syndiotactic polypropylene structure and morphology

Vibrational spectroscopy is used to sensitively detect specific morphologies and microstructures present in metallocene-catalyzed syndiotactic polypropylenes (sPP). Six materials, ranging in racemic triad content from 26 to 96% rr, are studied. Changes in high-resolution infrared (IR) and Raman spectra of melt-slow-cooled films are observed as the degree of syndiotacticity varies. Three different types of peak behavior are observed: splitting, wavenumber shift, and change in peak intensity. An overall trend toward greater molecular order (e.g. ordered chain conformations, increased crystallinity) is observed as syndiotacticity increases. By combining results with supporting evidence from X-ray diffraction and IR linear dichroism experiments of highly syndiotactic sPP, new peak assignments are proposed for tacticity-sensitive vibrational bands. Some very interesting spectral behavior is observed for material of intermediate stereoregularity (49% rr). Previously unobserved peaks appear in X-ray diffractograms and IR absorbance spectra, suggesting the presence of an as-yet unidentified “transitional” structure—perhaps a disordered modification of crystalline Form I. This moderate level of syndiotacticty appears to be a critical point or threshold below which sPP chains are unable to adopt characteristic helical or planar zigzag conformations. Results from this work provide a more thorough understanding of stereochemical effects on vibrational spectra, which will be very useful in the interpretation of ongoing IR linear dichroism studies of newly available semi-syndiotactic (semi-sPP) materials.     

Quick Approximation to Matrices and Applications

m ×n matrix A with entries between say −1 and 1, and an error parameter ε between 0 and 1, we find a matrix D (implicitly) which is the sum of simple rank 1 matrices so that the sum of entries of any submatrix (among the ) of (A−D) is at most εmn in absolute value. Our algorithm takes time dependent only on ε and the allowed probability of failure (not on m, n). We draw on two lines of research to develop the algorithms: one is built around the fundamental Regularity Lemma of Szemerédi in Graph Theory and the constructive version of Alon, Duke, Leffman, R?dl and Yuster. The second one is from the papers of Arora, Karger and Karpinski, Fernandez de la Vega and most directly Goldwasser, Goldreich and Ron who develop approximation algorithms for a set of graph problems, typical of which is the maximum cut problem. From our matrix approximation, the above graph algorithms and the Regularity Lemma and several other results follow in a simple way. We generalize our approximations to multi-dimensional arrays and from that derive approximation algorithms for all dense Max-SNP problems. Received: July 25, 1997     

An analytical method for the determination of perfluorinated compounds in whole blood using acetonitrile and solid phase extraction methods

A method for the analysis of perfluorinated compounds (perfluoroalkyl sulfonates: C4, C6, C8, C10; perfluoroalkyl sulfinates: C6, C8, C10; perfluorooctanesulfonamide, N-ethyl perfluorooctanesulfonamide, N-ethyl perfluorooctanesulfonamidoacetate, perfluorocarboxylates: C4–C14; fluorotelomer carboxylate (7:3, 8:2) in whole blood using acetonitrile and OASIS WAX^® solid phase extraction (SPE) cartridge was developed. Separation of target compounds in two HPLC columns (ion exchange JJ50-2D and C18 Betasil columns) was examined. Matrix recoveries of the developed methods ranged from 70% to 120%. Separation of possible inferences such as taurodeoxycholic acid (TDC) was accomplished using an ion exchange JJ50-2D column, and this separation was validated using whole blood of different animals.     

Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene) cycloalkanone units

Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene)cycloalkanone moieties were synthesized and characterized by structural, thermal, mesomorphic, and optical measurements. The bis(benzylidene) cycloalkanone chromophores in the main chain can constitute both as a mesogen and photoreactive center, whereas 1,3,4‐oxadiazole is a well‐known fluorophore. The thermal properties of polymers were found to be inversely proportional not only to the spacer length but also to ring‐size of cycloalkanones. Hot stage polarized optical microscopic investigations displayed enantiotropic nematic liquid crystalline phases and development of grainy to schlieren textures depends on the length of flexible spacer in the polymer backbone which was in accordance with DSC analysis. Both photoisomerization and photodimerization are observed from the absorption spectra and discussed. The fluorescence spectra in solution state at various concentrations showed that the polymers show blue‐emission maxima and the Stokes shifts being 48–49 nm. The energy transfer occurred when increasing the concentration of the solution. The band gap energies calculated from the absorption spectra are in the range of 3.17–3.41 eV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5760–5775, 2008     

Novel photo-crosslinkable liquid crystalline polymers containing vanillylidene cycloalkanones and azobenzene units

Two series of novel liquid crystalline photo-crosslinkable bis(vanillylidene-azobenzene) cycloalkanone containing polymers, namely poly(vanillylidene alkyloxy-4,4'-azobenzenedicarboxylic ester)s, have been synthesised from bis[m-hydroxyalkyloxy(vanillylidene)cycloalkanone] (m = 6, 8, 10) with azobenzene dicarbonylchloride by solution polycondensation method at ambient temperature. Polymers with varying spacer lengths have been synthesised and characterised by spectroscopic techniques. These variations have been correlated with the thermal properties and transition temperatures. Thermal transitions were analysed by differential scanning calorimetry (DSC) and the mesophases were identified by hot stage optical polarised microscopy (HOPM). All of the polymers were found to exhibit liquid crystalline properties. Transition temperatures were observed to decrease with increasing spacer length. The thermogravimetric analysis reveals that all of the polymers were stable up to 280°C undergo two-stage decomposition. Using the UV-visible photolysis studies we investigated the simultaneous behaviour of reactivity rates of crosslinking in the vanillylidene unit and isomerisation caused by the azobenzene unit in the photo-crosslinkable main chain liquid crystalline polymers. The photolysis of liquid crystalline bis(vanillylidene)cycloalkanone-based polymers reveals that there are two kinds of photoreactions in these systems: the EZ photoisomerisation of azobenzene unit and 2p+2p addition by vanillylidene units. The EZ photoisomerisation in the liquid crystal phase disrupts the parallel stacking of the mesogens, resulting in the transition from the liquid crystal phase to isotropic phase. The photoreaction involving 2p+2p addition of the bis(vanillylidene)cycloalkanone units in the polymers results in the conjoining of the chains. The cyclopentanone polymers exhibited a faster rate of photolysis than the cyclohexanone polymers.     

X-ray determination of charge transfer in solar grade GaAs

The magnitude and direction of charge transfer in GaAs at various temperatures have been analyzed using our precise and extensive single-crystal X-ray diffraction data. The charge transfer parameters were obtained by employing a quadratic equation method and precise X-ray structure factors collected at 170, 200, 250 and 300 K. A transfer of charge from gallium atom to arsenide atom is evidenced at all the above temperatures.