Molecular structure of helical supramolecular dendrimers

The molecular structure of helical supramolecular dendrimers generated from self-assembling dendrons and dendrimers and from self-organizable dendronized polymers was elucidated for the first time by the simulation of the X-ray diffraction patterns of their oriented fibers. These simulations were based on helical diffraction theory applied to simplified atomic helical models, followed by Cerius2 calculations based on their complete molecular helical structures. Hundreds of samples were screened until a library containing 14 supramolecular dendrimers and dendronized polymers provided a sufficient number of helical features in the X-ray diffraction pattern of their oriented fibers. This combination of techniques provided examples of single-9(2) and -11(3) helices, triple-6(1), -8(1), -9(1), and -12(1) helices, and an octa-32(1) helix that were assembled from crownlike dendrimers, hollow and nonhollow supramolecular crownlike dendrimers, hollow and nonhollow supramolecular disklike dendrimers, and hollow and nonhollow supramolecular and macromolecular helicene-like architectures. The method elaborated here for the determination of the molecular helix structure was transplanted from the field of structural biology and will be applicable to other classes of synthetic helical assemblies. The determination of the molecular structure of helical supramolecular assemblies is expected to provide an additional level of precision in the design of helical functional assemblies resembling those from biological systems.     

Chemical modification of poly(n-hexylsilane) - Synthesis of functional polysilanes bearing thienyl groups on the appended sila-alkyl side chains and their application in the generation and stabilization of silver nanoparticles

The linear polysilanes [{RR′₂Si(CH₂)_ySi(n-hex)}_x{HSi(n-hex)}_1−x]_n (1-4; R = 2-thienyl, R′ = H; R = Me, R′ = 2-thienyl; y = 2, 3) have been synthesized by hydrosilylation reaction between preformed poly(n-hexylsilane) and (2-thienyl)vinyldichlorosilane/allyl(2-thienyl)dichlorosilane/bis(2-thienyl)methylvinylsilane/allyl-bis(2-thienyl)methylsilane using AIBN as the free radical initiator. GPC analysis reveals a monomodal molecular weight distribution in each case with M_w = 2492-3280 and PDI = 1.18-1.44. The polysilane 1 (R = 2-thienyl, R′ = H, y = 2) acts as reducing agent towards silver tetrafluoroborate under mild conditions (cyclohexane, rt, 5 h) to afford spherical silver nanoparticles of size 8.4 ± 0.7 nm, as evident from the TEM and dynamic light scattering (DLS) studies. The silver nanoparticles in the polymer matrix exhibit surface plasmon absorption at 420 nm suggesting the donor-acceptor interaction between the thienyl group and the metal nanocluster surface. This stabilization effect provides long shelf life stability to the nanoparticles in solution with no sign of agglomeration even after three months.     

Stereoelectronic effects on molecular geometries and state-energy splittings of ligated monocopper dioxygen complexes

The relative energies of side-on versus end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet versus triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands and, in particular, depend upon their geometric arrangement relative to the O2 binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for the side-on geometry, and this is promoted by ligands that raise the energy, thereby enhancing resonance, of the filled Cu dxz orbital that hybridizes with the in-plane pi* orbital of O2. Conversely, ligands that raise the energy of the filled Cu dz2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O2 pi*. Because the overlap of Cu dz2 with O2 pi* is reduced as compared to the overlap of Cu dxz with the same O2 orbital, the resonance is also reduced, leading to generally more stable triplet states relative to singlets in the end-on geometry as compared to the side-on geometry, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O2 superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multireference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another.     

Artificial β‐Double Helices from Achiral γ‐Peptides

Double helices are not common in polypeptides and proteins except in the peptide antibiotic gramicidin A and analogous l,d ‐peptides. In contrast to natural polypeptides, remarkable β‐double‐helical structures from achiral γ‐peptides built from α,β‐unsaturated γ‐amino acids have been observed. The crystal structures suggest that they adopted parallel β‐double helical structures and these structures are stabilized by the interstrand backbone amide H‐bonds. Furthermore, both NMR spectroscopy and fluorescence studies support the existence of double‐helical conformations in solution. Although a variety of folded architectures featuring distinct H‐bonds have been discovered from the β‐ and γ‐peptide foldamers, this is the first report to show that achiral γ‐peptides can spontaneously intertwine into β‐double helical structures.     

Preparation of organic-inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization

Organic-inorganic composite membranes based on poly(vinyl alcohol)/SiO(2) were prepared via an aqueous dispersion polymerization route and anion-exchange groups were introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired content. These membranes were extensively characterized for their surface morphology, thermal stability, water content, and surface-charge properties using SEM, TEM, FTIR, TGA, water uptake, and ion-exchange capacity measurements. Counterion transport numbers across these membranes were estimated from membrane potential data. Membrane conductance measurements were also performed and these data were used for the estimation of values of counterion diffusion coefficients in the membrane phase. Physicochemical and electrochemical properties of these membranes and equivalent pore radius (estimated from electroosmotic flux measurements) were found to be highly dependent on the 4-vinylpyridine (4-VP) content in the membrane phase. It was also observed that for better selectivity and membrane conductivity of anion-exchange membranes complete optimization of the loading of 4-VP in the membrane phase is necessary. Furthermore, among these, membrane with 25% loading with 4-VP exhibited very good selectivity, water content, and ion-exchange capacity along with moderate membrane conductivity, which may be used for their application in electro-driven separation at elevated temperatures or for other electrochemical processes.     

Choline Chloride Assisted Synthesis of N and Metal Codoped TiO2 and their Photocatalytic Activity under Visible Light

A few nanocrystalline N,metal codoped TiO₂ (metal = Cr, Mn, Fe, Co, Ni, Cu and Zn) have been synthesized by a simple sol–gel method using choline chloride which is biodegradable, low cost, nontoxic ionic salt both as a structure directing agent and source of nitrogen. The prepared samples were well characterized by XRD, HRTEM, FTIR, DRS, EDX, XPS and BET techniques. The photocatalytic activity of all synthesized N, metal codoped TiO₂ has been carried out for the degradation of Reactive Black 5 dye under visible light irradiation and among them, N, Fe codoped TiO₂ was found to be the best for the degradation of Reactive Black 5 dye. The effect of incorporated metals on the photocatalytic activity of the various modified TiO₂ has been discussed in detail based on the mechanism involved in the degradation of dye and their physico‐chemical properties which includes surface area, particle size, defect sites, phase, band gap and electron–hole recombination effect.     

Isolation and characterization of ingenol esters from Euphorbia cornigera

Methanolic extract of Euphorbia cornigera Boiss.on HPLC separation yielded three new compounds.Their structures and relative stereochemistry were established through 2D-NMR spectroscopic measurements.     

Local equivalence ratio measurements in turbulent partially premixed flames using laser-induced breakdown spectroscopy

One of the most recently applied laser-based techniques in combustion environment is the laser-induced breakdown spectroscopy (LIBS). The technique has been extensively and successfully applied to elemental concentration measurements in solids and liquids. The LIBS signal is much weaker in gases and hence more work is required for quantitative measurements in flames. In the present work we used two orthogonal Nd:YAG lasers that operate at the fundamental wavelength with laser pulse energy of about 100 mJ/pulse. A Princeton-Instruments IMAX ICCD camera attached to a PI-Echelle spectrometer was used for signal detection. The lasers are focused using two 5-cm lenses. Several calibration points have been collected in well defined and homogeneous mixtures of air and fuel in order to be used as references for the measurements in turbulent partially premixed flames. This work shows that the application of the LIBS technique in a turbulent combustion environment is feasible and signal is enhanced by applying an orthogonal dual-pulse arrangement for air–fuel.