An empirical force field. II. Crystalline alkanes

The EFF'93 force field is used to study crystalline alkanes. In addition to detailed molecular geometries, the sublimation enthalpy, heat capacity, and lattice vibrations for a number of saturated hydrocarbons are reported and compared with experiment. © 1995 by John Wiley & Sons, Inc.     

Studies of the soluble part of oxidised coals

Soluble products obtained from the oxidation of four types of coal, each characterised by different degree of coalification and different degree of sulphur content, are studied. The coals are oxidised with peracetic acid (PAA) and nitric acid. Analyses are performed by Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR) and Fourier Transform Infrared Spectroscopy (FTIR). The soluble products contain much more sulphur than the insoluble products of oxidation. The products obtained from the reaction with HNO₃ contain higher amounts of inorganic sulphur compounds, while those obtained from the reaction with PAA are characterised by an increased content of organic sulphur species.     

An experimental and theoretical study of the Cp2VO2CO: use of 13CO3(2-) for structure investigation of d1-metallocene complexes

EPR study has shown that the anticancer agent vanadocene dichloride (Cp2VCl2) interacts with carbonate contained in physiological solutions. Chelate complex Cp2VO2CO (|A(iso)(51V)| = 175.1 MHz, g(iso) = 1.9861) is the only paramagnetic species formed in the range about the physiological pH (5.5-11.0). The super-hyperfine coupling (|a(iso)(13C)| = 24.1 MHz) was evidenced at measurements using 13C labelled carbonate. The structure of carbonate complex was validated by comparison of observed and theoretical calculated HFC tensors (at the density functional level of theory).     

Temperature quenched DODAB dispersions: fluid and solid state coexistence and complex formation with oppositely charged surfactant

Dilute dispersions of the synthetic bilayer forming double-chained cationic lipid dioctadecyldimethylammonium bromide (DODAB) were investigated. In dispersions sonicated above the chain melting temperature Tm (approximately 45 degrees C) it was found by H NMR that about 50% of the surfactant chains remained fluid when the samples were cooled to room temperature, which is 20 degrees C below Tm. In contrast, there was no sign of a fluid fraction in unsonicated samples at room temperature. The addition of the anionic surfactant sodium dodecyl sulfate (SDS) to DODAB dispersions at room temperature resulted in the formation of an essentially stoichiometric DODA-DS complex with frozen chains, as seen by titration calorimetry and H NMR experiments. For sonicated samples, turbidity experiments demonstrated that, after a fast complexation reaction, the system remains colloidally stable unless the SDS-to-DODAB mixing ratio is too close to unity. H NMR experiments also showed that in the unreacted DODAB the fraction of fluid chains remained close to 50%, indicating either that SDS reacts equally fast with fluid and frozen DODAB or that there is a relaxation of the fluid fraction after the complexation. The melting enthalpy and the melting temperature of the alkyl chains rise gradually as the mixing ratio increases. We observed with cryo-TEM that the fraction of large unilamellar vesicles was significantly larger after addition of SDS. This indicates vesicle fusion. Based on both wide- and small-angle X-ray scattering patterns, the structure of the equimolar SDS-DODAB complex at 25 degress C was proposed to be lamellar.     

Determination of calcium in silicate rocks by potentiometric titration with ethyleneglycol-bis-(2-Aminoethylether)tetraacetic acid and a calcium-selective electrode

The method reported for the determination of calcium in silicate rocks involves titration with ethyleneglycol-bis(2-aminoethylether)tetraacetic acid (EGTA) to a potentiometric end-point. A Crytur calcium-selective electrode is used; the selectivity constants (K_ca,x) are less than 10^-3 for Mg, Ba, Sr, K and Na. The method can be used for calcium contents of 2% or higher (and exceptionally for lower contents), in the presence of up to 60% Mg, 33% AI, 17% Fe, 5% Ba, 5% Ti, and 2.5% Mn. Al, Fe and Ti are masked with sulphosalicylic acid, barium is precipitated as its sulphate, and manganese is bound as its cyanide complex. If the magnesium content is less than that of calcium, EDTA can be used as titrant, magnesium being masked with acetylacetone.     

Gel-phase 19F NMR spectral quality for resins commonly used in solid-phase organic synthesis; a study of peptide solid-phase glycosylations

The spectroscopic properties for seven different commercial resins used in solid-phase synthesis were investigated with (19)F NMR spectroscopy. A fluorine-labeled dipeptide was synthesized on each resin, and the resolution of the (19)F resonances in CDCl(3), DMSO-d(6), benzene-d(6) and CD(3)OD were measured with a conventional NMR spectrometer, i.e. without using magic angle spinning. In general, resins containing poly(ethylene glycol) chains (ArgoGel, TentaGel and PEGA) were found to be favorable for the (19)F NMR spectral quality. Three serine containing tri-, penta-, and heptapeptides were then prepared on an ArgoGel resin functionalized with a fluorine-labeled linker. The resin bound peptides were glycosylated utilizing a thiogalactoside glycosyl donor carrying fluorine-labeled protective groups. Monitoring of the glycosylations with gel-phase (19)F NMR spectroscopy allowed each glycopeptide to be formed in similar 80% yield, using a minimal amount of glycosyl donor (3 x 2 equivalents). In addition, it was found that the glycosylation yields were independent of peptide length.     

Gold catalysis contra platinum catalysis in hydroarylation contra phenol synthesis

Three silylated γ-alkynylfurans were prepared and subjected to both gold and platinum catalysts. The TMS- and the TBDMS-substituted furans reacted. With AuCl₃ and the binuclear [(Ph₃PAu)₂Cl][BF₄] catalyst a hydroarylation of the alkyne was observed. Na[AuCl₄] gave phenols as the product, but these were formed only after in situ desilylation of the starting material by the gold catalyst and thus the wrong isomer dominated. Only with PtCl₂(MeCN)₂ phenols with a silyl group were formed. The TBDPS-substituted furan failed to react. Two alkynylsilanes were synthesized, but they also failed to react.     

Fractionation of starch hydrolysates into dextrins with narrow molecular mass distribution and their detection by high-performance anion-exchange chromatography with pulsed amperometric detection

Low levels of high-molecular-mass dextrins in starch hydrolysates can be detected by high-performance anion-exchange chromatography with pulsed amperometric detection in spite of their low responses by dialysis of the starch hydrolysate and fractionation of the resulting adialysate with ethanol (final concentration 30-80% at 6 degrees C). In doing so, dextrin fractions with a relatively narrow molecular mass distribution were obtained.     

Exploiting non-abelian point group symmetry in direct two-electron integral transformations

Summary A simple and general scheme to exploit any discrete point group symmetry in two-electron integral transformations is introduced. It has been implemented together with integral pre-screening techniques in direct two-electron integral transformations. Its application has also been extended to subsequent MO integral processing steps like MP2 or solution of the coupled-perturbed Hartree-Fock equations (CPHF). Timings for representative applications are presented.     

Hydrogen bond studies: 80. A deuteron magnetic resonance and infrared spectroscopic study of the water molecule in lithium formate monohydrate,LiHCOO · H2O

A deuteron magnetic resonance and infrared study of the water molecules in lithium formate monohydrate, LiHCOO · H₂O, has been made. The quadrupole coupling constants (e²qQ/h) and asymmetry parameters (η) were found to be 198.7±0.4 and 231.3±0.6 kHz, and 0.060±0.005 and 0.097±0.003, respectively, at 25 ° C.An interpretation is given of the infrared spectra in the OH-stretching region in terms of intra- and intermolecular couplings of the water molecules. It is found that the water molecules are vibrationally distorted by their environments such that the OH-stretching modes consist of independent stretchings of the two O-H bonds.