Enantiospecific total synthesis of (-)-polyoxamic acid using 2,3-aziridino-gamma-lactone methodology

The non-natural enantiomer of polyoxamic acid was synthesized in six steps from 2,3-aziridino-gamma-lactone 7 with an overall yield of 10%. The key step of the strategy is a deprotection-protection sequence on the nitrogen atom of the aziridine ring required for aziridine activation toward nucleophilic ring opening.     

A schematic overview of the historical evolution of Analytical Chemistry

The historical evolution of Analytical Chemistry is briefly discussed as related to the progress of Chemistry within the 16–19th centuries under the leadership of Paracelsus, Boyle, Lavoisier and Dalton. A clear distinction is made between chemical analysis (up to the end of the 19th c.) and today’s Analytical Chemistry, paying close attention to a number of aspects and consequences related to the chemical revolution which took place at the overlap of the 18–19th c. which resulted in the quantification of Chemistry, causing increasing development and improvement of the chemical metrology which was an essential factor for Chemistry to acquire a scientific dimension and to become more specialised during the 19th century. A panoramic view of the whole development is presented by resorting to the inclusion of a number of synoptical tables outlining the stepwise progress of Chemistry, chemical analysis and Analytical Chemistry within the five last centuries taking into consideration the main protagonists involved as well as the experimental means, techniques and methodologies used and/or developed. Received: 20 February 1996 / Accepted: 21 May 1996     

Strained aromatic oligoamide macrocycles as new molecular clips

Can one join both ends of a helix? A helical aromatic oligoamide was macrocyclized into a saddle-shaped bifunctional clip molecule that self-assembles into discrete circular dodecamers in the solid state and shows great potential for binding aromatic acid guests in solution. The cyclization step requires that the helix is only partly denatured in the reaction medium.     

Calculation of current densities using gauge-including atomic orbitals

A method for calculating the various components of the magnetically induced current-density tensor using gauge-including atomic orbitals is described. The method is formulated in the framework of analytical derivative theory, thus enabling implementation at the Hartree-Fock self-consistent-field (HF-SCF) as well as at electron-correlated levels. First-order induced current densities have been computed up to the coupled-cluster singles and doubles level (CCSD) augmented by a perturbative treatment of triple excitations [CCSD(T)] for carbon dioxide and benzene and up to the full coupled-cluster singles, doubles, and triples (CCSDT) level in the case of ozone. The applicability of the gauge including magnetically induced current method to larger molecules is demonstrated by computing first-order current densities for porphin and hexabenzocoronene at the HF-SCF and density-functional theory level. Furthermore, a scheme for obtaining quantitative values for the induced currents in a molecule via numerical integration over the current flow is presented. For benzene, a perpendicular magnetic field induces a (field dependent) ring current of 12.8 nA T(-1) at the HF-SCF level using a triple-zeta basis set augmented with polarization functions (TZP). At the CCSD(T)/TZP level the induced current was found to be 11.4 nA T(-1). Gauge invariance and its relation to charge-current conservation is discussed.     

Induced Absorption of C60 and a Water-Soluble C60-Derivative in SiO2 Sol-Gel Matrices

Porous sol-gel glasses, either impregnated with pure C₆₀ or doped with a methanofullerene derivative, have been studied and induced absorption or reverse saturable absorption (RSA) has been observed in both types of solid materials. The samples impregnated by pure C₆₀ mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C₆₀, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C₆₀-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics that are influenced by solid state effects. We observe an important decrease of transmission at high fluences for both kinds of samples, a shortened singlet-state lifetime to that observed in solution, but nonetheless, a triplet yield, that cannot be considered as negligible. In the case of pure C₆₀ in a sol-gel matrix, we can explain the faster de-excitation dynamics, relative to behavior in solution, mainly by the absence of stabilizing aromatic solvents and also by the interaction of the amorphous environment with the molecules. Concerning the methanofullerene samples, the acceleration of the de-excitation dynamics can be principally attributed to solid-state effects due to the micellar aggregation.     

Magnetic and Mössbauer Studies of Magnetite-Loaded Polyvinyl Alcohol Hydrogels

Materials producing strain in a magnetic field are known as magnetoelastic or magnetostrictive materials. A new type of material that is able to produce giant strain in a nonhomogeneous magnetic field has been developed. In these magnetic-field-sensitive gels (ferrogels) fine colloidal particles having superparamagnetic behavior are incorporated into a highly swollen elastic polymer network. Magnetic properties of ferrogels have been investigated using electron microscopy, static magnetization measurements, and M?ssbauer spectroscopy. Analysis of the data yielded information on the superparamagnetic behavior of ferrogels and made it possible to estimate the size distribution of the magnetic cores of magnetite particles made by chemical precipitation and built into a chemically cross-linked polyvinyl alcohol matrix. The results are interpreted on the basis of a core-shell model. Copyright 2000 Academic Press.     

First-principles calculations of K-edge X-ray absorption near edge spectra of some transition metal dimers

Theoretical X-ray absorption near-edge spectra have been evaluated for different bond lengths of Mn₂, Co₂ and Ni₂ using a modified Xα scattered-wave procedure.     

Solid-state NMR study of intercalated species in poly(epsilon-caprolactone)/clay nanocomposites

The structure and dynamics of surfactant and polymer chains in intercalated poly(epsilon-caprolactone)/clay nanocomposites are characterized by (31)P magic-angle spinning (MAS) and (13)C cross-polarization MAS NMR techniques. To obtain hybrid materials with the low polymer content required for this study, in situ intercalative polymerization was performed by adapting a published procedure. After nanocomposite formation, the chain motion of the surfactant is enhanced in the saponite-based materials but reduced in the Laponite ones. Compared to the starting clay, the trans conformer population of the surfactant hydrocarbon chain in the nanocomposite decreases for the saponite systems. Mobility of the polymer chain is higher in the nanocomposites than in the bulk phase. The charge of the modified saponite does not significantly influence chain mobility in the nanocomposites.