Size-dependent properties of Zn(m)S(n) clusters: a density-functional tight-binding study

We present the results of our theoretical calculations on structural and electronic properties of ligand-free Zn(n)S(n) [with n ranging from 4 to 104 (0.8-2.0-nm diameter)] clusters as a function of size of the clusters. We have optimized the structure whereby our initial structures are spherical parts of either zinc-blende or wurtzite structure. We have also considered some hollow bubblelike structures. The calculations are performed by using a parametrized linear combination of atomic orbitals-density-functional theory-local-density approximation-tight-binding method. We have focused on the variation of radial distribution function, Mulliken populations, electronic energy levels, band gap, and stability as a function of size for both zinc-blende and wurtzite-derived ZnS clusters. We have also reported the results of some nonstoichiometric Zn(m)S(n) (with m+n=47, 99, 177) clusters of zinc-blende modification.     

Enantiospecific synthesis of methyl 11,12- and 14,15-epoxyeicosatrienoate

The methyl esters of the cytochrome P-450 epoxygenase metabolites 11(S),12(R)-and 14(R),15(S)-epoxyeicosatrienoic acid and some analogues were prepared enantiospecifically from 15(S)-HETE derived precursors.     

Dielectric properties of solutions of antimony (III) halides in methanol

The complex permittivities obtained by time domain reflectometry (TDR) measurements in the frequency range 0.1–15 GHz are reported for solutions of SbF₃, SbCl₃ and SbBr₃ in methanol at 25°C. By increasing the concentration of the solutes, a marked increase of the main relaxation time and of the static permittivity of methanol is observed. These effects have been attributed to the ability of antimony (III) halides to strengthen the hydrogen bonding in methanol and to promote the shift of the dynamic equilibria among the various populations of hydrogen bonded methanol aggregates toward those characterized by a parallel orientation of neighboring dipoles. A second relaxation, occurring at low frequencies, was attributed to the rotational dynamics of the solvated solute molecules.     

A quantum-classical approach to the photoabsorption spectrum of pyrazine

We have used the time-dependent discrete variable representation (TDDVR) method to simulate the photoabsorption spectrum of pyrazine. The time-dependent molecular dynamics of pyrazine after excitation to the S2 electronic state is considered as a benchmark to investigate the S2 absorption spectrum. We have carried out the dynamics on a basic four-mode model of pyrazine with the inclusion of five major modes as well as the rest of the vibrational modes as bath modes. Investigations reveal the effect of bath modes such as energy and population transfer from the subsystem to the bath. Calculated results demonstrate excellent agreement with traditional quantum-mechanical findings during the entire propagation and converge to the exact quantum results when enough gridpoints are used. It appears that TDDVR, as a numerical quantum dynamics methodology, is a good compromise between accuracy and speed.     

Electrostatic contribution to the interaction of α, β poly (N-hydroxyethyl)-DL-aspartamide with sodium dodecylsulfate micelles

The enthalpic effect due to the interaction between α, β poly(N-hydroxyethyl)-DL-aspartamide (PHEA) and sodium dodecylsulfate (SDS) in aqueous solutions as a function of the surfactant concentration was measured by the calorimetric technique at various NaCl concentrations. A marked influence of the added electrolyte on the PHEA-SDS interaction was observed. An analysis of the experimental enthalpies allows to estimate the electrostatic and the hydrophobic contributions to the enthalpy of interaction between PHEA and SDS micelles. The results were rationalized in terms of effects due to the screening of the charges residing on PHEA and SDS micelles.     

Preparation and Characterization of Thin Optically Transparent Alumina and Ce-doped Alumina

Transparent and porous boehmite, -Al₂O₃ (500°C) and -Al₂O₃ (900° and 1000°C) thin sheets (50–100 m) have been prepared from boehmite sols. -Al₂O₃ shows about 48% porosity and 292 m²/g surface area. On transformation from -Al₂O₃ (500°C) to -Al₂O₃ (900°C), the porosity still remains high, i.e. 45%; however, the surface area becomes 138 m²/g. The porosity and surface area of -Al₂O₃ become about 41% and 97 m²/g respectively on further heating to 1000°C. A gradual increase of average pore radius during this thermal treatment suggests that coarsening of the pore occurred during the densification process. Both -Al₂O₃ and -Al₂O₃ show high degree of transmission from UV to NIR wavelength region. Cerium exists in +4 oxidation state in the boehmite as well as in the - and -Al₂O₃. The ultraviolet absorption edge of the alumina was tailored by varying the concentration of cerium.     

Ab initio MO study of dipole—dipole interactions in acetonitrile dimers

MO STO-3G ab initio calculations have been carried out for the antiparallel dipole and the head-to-tail dipole model of acetonitrile dimers. The optimized interaction enthalpy is about half of the lowest experimental estimate. The calculated interaction distance for the antiparallel dipole model is very close to the sum of intermolecular radii of N and C; the distance for the head-to-tail model is about 20% higher than the sum of N and H intermolecular radii. The discussion of the interaction in terms of the supermolecule MO's suggests for both models a bonding of mainly electrostatic character. The shortcomings of the STO-3G basis set in dealing with this problem are compared with those reported in the literature. The influence of the basis set on the calculated electron distribution in acetonitrile monomer was examined as a preliminary part of the present study, and is also reported in the paper.     

Structural corroboration of two important building blocks of the anticancer drug eribulin mesylate through two‐dimensional NMR and single‐crystal X‐ray diffraction studies

Eribulin mesylate, one of the most synthetically challenging drugs to date, possesses 19 stereocentres in its structure and ascertaining the absolute stereochemistry at every stage of the 64‐stage synthesis is crucial. In our quest to synthesize eribulin, we identified two critical building blocks of this molecule, namely 3,4:6,7‐di‐O‐cyclohexylidene‐D‐glycero‐α‐L‐talo‐heptopyranose methanol monosolvate, C₁₉H₃₀O₇·CH₃OH, and (2R,3R,4R,5S)‐5‐allyl‐2‐[(S)‐2,3‐dihydroxypropyl]‐4‐[(phenylsulfonyl)methyl]tetrahydrofuran‐3‐ol, C₁₇H₂₄O₆S, for which two‐dimensional NMR (2D‐NMR) data were not sufficient to prove the absolute configuration. To ensure structural integrity, single‐crystal X‐ray diffraction data were obtained to confirm the structures. This information provides useful insights into the structural framework of the large eribulin mesylate molecule.     

Copper-Catalyzed Regiodivergent Internal Allylic Alkylations

We report a copper-catalyzed, regioselective, and stereospecific alkylation of unbiased internal allylic carbonates with functionalized alkyl and aryl Grignard reagents. The reactions exhibit high stereospecificity and regioselectivity for either S_N2 or S_N2′ products under two sets of copper-catalyzed conditions, which enables the preparation of a broad range of products with E-alkene selectivity. Density functional theory calculations reveal the origins of the regioselectivity based on the different behaviors of homo- and heterocuprates.