Transformations of chromanol and tocopherol and synthesis of ascorbate conjugates

α-Tocopherol and as a model compound pentamethylchromanol could be transferred into simple and more complex 5a-ether derivatives including galactopyranose as well as ascorbic acid conjugates. Following elaboration of a glucopyranoside spacer element this could be used for tethering the vitamin E and the vitamin C components to give novel conjugates for subsequent biostudies concerning their proposed synergism of antioxidant properties in tissues.     

Cation-anion interactions involving hydrogen bonds: Syntheses and crystal structures study of hexafluorotitanate(IV) salts with pyridine and methyl substituted pyridines

Fluorotitanates (LH)₂[TiF₆]·nH₂O (1: R = pyridine, n = 1, 2: R = 2-picoline, n = 2, 3: R = 2,6-lutidine, n = 0, 4: R = 2,4,6-collidine, n = 0) and (LH)[TiF₅(H₂O)] (3a: L = 2,6-lutidine) have been synthesized by the reaction of pyridine or corresponding methyl substituted pyridines and titanium dioxide dissolved in hydrofluoric acid. The crystal structures of ionic compounds 1, 2, 3, 3a and 4 have been determined by single-crystal X-ray diffraction analysis. The hydrogen bonding led to the formation of discrete (LH)₂[TiF₆] units (4), chains (1-3), and layers (3a). The additional π-π interactions present in 1, 2, and 4 results in chain structures of 1 and 4 and in a layer structure of 2. The [TiF₆]²⁻ and [TiF₅(H₂O)]⁻ anions were observed by ¹⁹F NMR spectroscopy in aqueous solutions of 1, 2, 3, 3a and 4.     

Nitrogen heteroaromatic cations by [2+2+2] cycloaddition

A modular approach to the construction of monocationic quaternary N-heteroaromatic frameworks was developed capitalizing on a direct pyridine-type nitrogen quaternization followed by metal-catalyzed [2+2+2] cycloaddition with gaseous acetylene. The flexibility of the route is demonstrated on 12 diverse scaffolds based on pyridinium, quinolinium, thiazolium, benzothiazolium, imidazolium, and pyrimidinium. Electrochemical study revealed a quinolinium redox system with two electrochemically distinct forms that are interconverted by a homogeneous chemical reaction triggered by fast electron transfers (reduction at -0.7 V and oxidation at -0.05 V).     

Polarization induced water molecule dissociation below the first-order electronic-phase transition temperature

Hydrogen produced from the photocatalytic splitting of water is one of the reliable alternatives to replace the polluting fossil and the radioactive nuclear fuels. Here, we provide unequivocal evidence for the existence of blue- and red-shifting O-H covalent bonds within a single water molecule adsorbed on the MgO surface as a result of asymmetric displacement polarizabilities. The adsorbed H-O-H on MgO gives rise to one weaker H-O bond, while the other O-H covalent bond from the same adsorbed water molecule compensates this effect with a stronger bond. The weaker bond (nearest to the surface), the interlayer tunneling electrons and the silver substrate are shown to be the causes for the smallest dissociative activation energy on the MgO monolayer. The origin that is responsible to initiate the splitting mechanism is proven to be due to the changes in the polarizability of an adsorbed water molecule, which are further supported by the temperature-dependent static dielectric constant measurements for water below the first-order electronic-phase transition temperature.     

Comparative molecular dynamics study of vapor-exposed basal, prismatic, and pyramidal surfaces of ice

We present the results of molecular dynamics simulations in which ice I(h) slabs with free basal, prismatic, 28° pyramidal, and 14° pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within ～10 ns and QLL stratification into outer (ε(1)) and inner (ε(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14° pyramidal and basal facets (～230 Pa), while significantly lower values are obtained for the prismatic and 28° pyramidal facets (～200 Pa). The geometric thickness of the QLL also varies between facets, with the 14° pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28° pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.     

Use of 9-methylfluorene as an indicator in the titration of common group IA and group IIA organometallic reagents

9-Methylfluorene was tested as an indicator in the titration of commonly used organometallic reagents. This indicator is readily prepared in three steps from fluorenone. In THF solution the deprotonated indicator is red and exhibits a sharp endpoint. The highly basic reagents sec-butyllithium and tert-butyllithium can be titrated in ether solution, where the color of the deprotonated indicator is yellow.     

Analysis of potential toxic elements in soil of the Czech dump-side

Journal of Radioanalytical and Nuclear Chemistry - The mass fractions of V, Cr, Co, Ni, Cu, Zn, As, Br, Mo, Cd, W, Hg, and Pb in soil of the Czech dump-side were measured by instrumental neutron...     

Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by 27Al{1H} REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics

Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework Al_FR Lewis acid sites are often found in their H‐forms. The formation of Al_FR Lewis sites in zeolites is a key issue regarding their selectivity in acid‐catalyzed reactions. The local structures of Al_FR Lewis sites in dehydrated zeolites and their precursors—“perturbed” Al_FR atoms in hydrated zeolites—were studied by high‐resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)₃AlOH groups and are characterized by a broad ²⁷Al NMR resonance (δ_i=59–62 ppm, C_Q=5 MHz, and η=0.3–0.4) with a shoulder at 40 ppm in the ²⁷Al MAS NMR spectrum. Dehydroxylation of (SiO)₃AlOH occurs at mild temperatures and leads to the formation of Al_FR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)₃Al Lewis sites exhibit an extremely broad ²⁷Al NMR resonance (δ_i≈67 ppm, C_Q≈20 MHz, and η≈0.1).