In situ disorder-order transformation in synthetic gallosilicate zeolites with the NAT topology

Here, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly (but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry. During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase segregation, or changes in the chemical composition are detected. Intermediate materials show variations in the degree of Si-Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which fits well within the experimental disorder-order series.     

Dipolar and nonpolar altitudinal molecular rotors mounted on an Au(111) surface

We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.     

Multinuclear magnetic resonance study of some mesoionic oxatriazoles containing nitrogenous exocyclic groups

¹H,¹³C,¹⁴N and¹⁵N NMR measurements are reported for four mesoionic 1-oxa-2, 3, 4-triazoles containing exocyclic nitrogenous groups. The NMR signal assignments are discussed and compared with those previously published for some corresponding oxatriazoles. The results obtained support the proposed cyclic mesoionic structures for the compounds studied. The questions of possible charge delocalization and valence tautomerism are addressed. Compound with N^– H as a exocyclic group (Fig. 1) is found to be relatively unstable, this is attributed to proton migration in the corresponding non-cyclic form of this molecule.Published in Khimiya Geterotsikiicheskikh Soedinenii, No. 9, pp. 1260–1263, September, 1995.     

3-[2-(Boronophenyl)benzoxazol-5-yl]alanine derivatives as fluorescent monosaccharide sensors

Two 3-[2-(boronophenyl)benzoxazol-5-yl]alanine derivatives were synthesized and their potential application as fluorescent monosaccharide sensors was studied. It was found that both non-proteinogenic amino acids bound glucose and fructose at physiological pH, however, the latter much stronger. As a result they are selective sensors for fructose. Moreover, one of them (3-[2-(3-boronophenyl)benzoxazol-5-yl]alanine methyl ester) can be used to quickly distinguish, which monosaccharide is present in the solution because of the different character of fluorescence intensity changes (increase in the presence of fructose and decrease in the presence of glucose).     

Adiabatic triplet state tautomerization of p-hydroxyacetophenone in aqueous solution

The primary photophysical processes of p-hydroxyacetophenone (HA) and the ensuing proton transfer reactions in aqueous solution were investigated by picosecond pump-probe spectroscopy and nanosecond laser flash photolysis. Previous studies have led to mutually inconsistent conclusions. The combined data allow us to rationalize the excited-state proton transfer processes of HA in terms of a comprehensive, well-established reaction scheme. Following fast and quantitative ISC to the triplet state, (3)HA*, adiabatic proton transfer through solvent water simultaneously forms both the anion, (3)A(-)*, and the quinoid triplet enol tautomer, (3)Q*. The latter subsequently equilibrates with its anion (3)A(-)*. Ionization and tautomerization are likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.     

Molecular simulation and macroscopic modeling of the diffusion of hydrogen, carbon monoxide and water in heavy n-alkane mixtures

The self-diffusion coefficient of hydrogen (H(2)), carbon monoxide (CO) and water (H(2)O) in n-alkanes was studied by molecular dynamics simulation. Diffusion in a few pure n-alkanes (namely n-C(8), n-C(20), n-C(64) and n-C(96)) was examined. In addition, binary n-C(12)-n-C(96) mixtures with various compositions as well as more realistic five- and six-n-alkane component mixtures were simulated. In all cases, the TraPPE united atom force field was used for the n-alkane molecules. The force field for the mixture of n-alkanes was initially validated against experimental density values and was shown to be accurate. Moreover, macroscopic correlations for predicting diffusion coefficient of H(2), CO and H(2)O in n-alkanes and mixtures of n-alkanes were developed. The functional form of the correlation was based on the rough hard sphere theory (RHS). The correlation was applied to simulation data and an absolute average deviation (AAD) of 5.8% for pure n-alkanes and 3.4% for n-alkane mixtures was obtained. Correlation parameters vary in a systematic way with carbon number and so they can be used to provide predictions in the absence of any experimental or molecular simulation data. Finally, in order to reduce the number of adjustable parameters, for the n-alkane mixtures the "pseudo-carbon number" approach was used. This approach resulted in relatively higher deviation from MD simulation data (AAD of 18.2%); however, it provides a convenient and fast method to predict diffusion coefficients. The correlations developed here are expected to be useful for engineering calculations related to the design of the Gas-to-Liquid process.     

Switching the Mechanism of NADH Photooxidation by Supramolecular Interactions

A series of three Ru(II) polypyridine complexes was investigated for the selective photocatalytic oxidation of NAD(P)H to NAD(P)⁺ in water. A combination of (time-resolved) spectroscopic studies and photocatalysis experiments revealed that ligand design can be used to control the mechanism of the photooxidation: For prototypical Ru(II) complexes a ¹O₂ pathway was found. Rudppz ([(tbbpy)₂Ru(dppz)]Cl₂, tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2′,3′-c]phenazine), instead, initiated the cofactor oxidation by electron transfer from NAD(P)H enabled by supramolecular binding between substrate and catalyst. Expulsion of the photoproduct NAD(P)⁺ from the supramolecular binding site in Rudppz allowed very efficient turnover. Therefore, Rudppz permits repetitive selective assembly and oxidative conversion of reduced naturally occurring nicotinamides by recognizing the redox state of the cofactor under formation of H₂O₂ as additional product. This photocatalytic process can fuel discontinuous photobiocatalysis.     

Ink‐Jet Microdispensing for the Formation of Gradients of Immobilised Enzyme Activity

Summary: The integration of gradients of enzyme activity in microstructured biosensor arrays enables intrinsic on‐line quality control of biosensor performance. Multiple sensor elements with different compositions and hence varying responses for the same analyte are evaluated as a basis for improving data reliability. The formation of glucose oxidase/polymer microstructures using a piezo microdispenser and their examination by scanning electrochemical microscopy (SECM) are used to demonstrate the feasibility of this approach.

Optical microscope image of grids obtained by dispensing of 1 mg/mL GOx and 2 mg/mL Vinnapas® mixture.     

Morphology and molecular organisation of liquid-crystalline perylene-3,4,9,10-tetra-(n-hexylester) deposited on a solid substrate

Liquid-crystalline perylene-3,4,9,10-tetra-(n-hexylester) forms characteristic dendritic or flower-like structures at room temperature when it is deposited on a hydrophilic glass substrate using the zone-casting technique. It was found that such unique structures were not possible to be created simply by recrystallisation of this dye from a liquid-crystalline columnar phase. On the basis of the observations using a confocal microscope and the study of wide angle X-ray scattering (WAXS) as well as the analysis of the absorption and fluorescence spectra, some conclusions, concerning the molecular organisation in the dendritic structure, are drawn. Based on the research, one can assume that the dendrites are formed by columnar molecular aggregates with the column axes parallel to the substrate. Such an organisation of the molecules can be interesting from the point of view of organic electronics.