Calculation of proton N.M.R. chemical shifts by means of a finite perturbation method

The magnetic shielding effect due to neighbouring carbon atoms through the diamagnetic and paramagnetic terms is calculated by means of a finite perturbation method in cooperation with a semi-empirical INDO molecular orbital method. The behaviour of the components of the shielding tensor arising from 2s and 2p orbitals centred on a neighbouring carbon is shown as a function of the distance, R, between the proton considered and the carbon. It is pointed out that the treatment by Pople and McConnell in the calculation of the proton chemical shift must be used carefully in unsaturated hydrocarbons, because some of the contributions from a neighbouring carbon do not show an R ^-3 dependence but an R ^-2 dependence. It is also shown that the gross trend of the observed proton chemical shifts in hydrocarbons is reproduced fairly well in the present calculation.     

The Adaptive Projected Subgradient Method over the Fixed Point Set of Strongly Attracting Nonexpansive Mappings

This paper presents an algorithmic solution, the adaptive projected subgradient method, to the problem of asymptotically minimizing a certain sequence of non-negative continuous convex functions over the fixed point set of a strongly attracting nonexpansive mapping in a real Hilbert space. The method generalizes Polyak's subgradient algorithm for the convexly constrained minimization of a fixed nonsmooth function. By generating a strongly convergent and asymptotically optimal point sequence, the proposed method not only offers unifying principles for many projection-based adaptive filtering algorithms but also enhances the adaptive filtering methods with the set theoretic estimation's armory by allowing a variety of a priori information on the estimandum in the form, for example, of multiple intersecting closed convex sets.     

MAS, STMAS and DQMAS NMR Studies of the Thermal Transformation of Kaolinite

Thermal transformations of kaolinite at different temperatures were monitored using X-ray diffraction (XRD), high-resolution solid-state nuclear magnetic resonance (800 MHz for ¹H Larmor frequency) with single-pulse magic-angle spinning, double-quantum filter satellite-transition magic-angle spinning, and double-quantum homo-nuclear correlation under magic-angle spinning experiments. Results show that combined experiments clearly manifest the transitions of silicon and aluminum structures at different thermal treatment stages; and moreover, high magnetic field offers higher sensitivity and resolution, hereby the slim resonances are obtained successfully at less stringent conditions. The dehydroxylation process of kaolinite causes the presence of short-range order in metakaolinite, which is absence of XRD reflections. Particularly, the features of metakaolinite with high concentration of defects are found with dispersive aluminum coordinations; and further, the distorted tetrahedral aluminum is detected in kaolinite-derived mullite because of the locally disorganized structure. The framework structure of kaolinite-derived mullite is considered to be primarily formed by the tetrahedral aluminum bonding with octahedral aluminum. In addition, except for the primary limitation caused by potentially long relaxation time of sample, such approaches are applicable to obtain detailedly structural characteristics of aluminosilicate.     

Bis(allyl)zinc revisited: sigma versus pi bonding of allyl coordination

The reinvestigation of two allyl zinc compounds, parent bis(allyl)zinc [Zn(C(3)H(5))(2)] (1) and 2-methallyl chloro zinc [Zn(C(4)H(7))Cl] (2), revealed two new coordination modes in the solid state for the allyl ligand, viz cis- and trans-μ(2)-η(1):η(1). These results call for modification of the conventional interpretation of zinc-allyl interactions. Computational results indicate that the classical η(3)-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc-olefin interaction in the dimer of [Zn(η(1)-C(3)H(5))(OC(C(3)H(5))Ph(2))] was found in the monoinsertion product of 1 with benzophenone.     

A convergent synthesis of the right-hand fragment of ciguatoxin CTX3C

A convergent synthesis of the right-hand fragment of ciguatoxin CTX3C was investigated. The first and second generation stereocontrolled syntheses of the LM ring fragment were achieved via spiroacetalization as a key step, respectively. The polyether framework of the HIJKLM ring fragment was constructed in a convergent manner by using intramolecular allylation, ring-closing metathesis, and stereoselective hydrogenation to form the 36-methyl substituent as the key transformations.     

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide which is a polyol marine natural product with a molecular weight of 2860 has been accomplished. The synthetic route features Achmatowicz rearrangement and RuO₄-catalyzed dihydroxylation for the construction of the tetrahydropyran moiety and the dithiane addition to the aldehyde for the introduction of the side chain.     

A new approach to characterization of insulin derived from different species using 1H-NMR coupled with multivariate analysis

Most of the active components of polypeptides have a complex molecular structure, large molecular size. Such components may also be structurally heterogeneous. Therefore, development of a method that can confirm the consistency of polypeptides amino-acid sequences for product characterization is desirable. In general, it is extremely difficult to distinguish differences of a few amino acid residues in the 1H-NMR spectrum of polypeptides with molecular weights greater than several thousand. However, we have been able to distinguish between three insulin species differing in one to three amino acid residues using a combination of multivariate statistics and 1H-NMR spectra. These results demonstrate that this methodology could be useful for characterization of polypeptides.     

Allyl strontium compounds: synthesis, molecular structure and properties

The synthesis and attempted isolation of neutral bis(allyl)strontium [Sr(C(3)H(5))(2)] (1) resulted in the isolation of potassium tris(allyl)strontiate K[Sr(C(3)H(5))(3)] (2). In situ generated 1 shows a pronounced Br?nsted basicity, inducing polymerisation of THF. Ate complex 2 crystallises as [K(THF)(2){Sr(C(3)H(5))(3)}(THF)](∞) (2·(THF)(3)). The salt-like solid state structure of 2·(THF)(3) comprises a two-dimensional network of (μ(2)-η(3):η(3)-C(3)H(5))(-) bridged potassium and strontium centres. Synthesis of allyl complexes 1 and 2 utilised SrI(2), [Sr(TMDS)(2)] (3) (TMDS = tetramethyldisilazanide), and [Sr(HMDS)(2)] (HMDS = hexamethyldisilazanide) as strontium precursors. The solid state structure of previously reported [Sr(TMDS)(2)] (3) was established by X-ray single crystal analysis as a dissymmetric dimer of [Sr(2)(TMDS)(4)(THF)(3)] (3·(THF)(3)) with multiple Si-HSr agostic interactions. The presence of ether ligands (THF, 18-crown-6) influenced the Si-HSr resonances in the NMR spectra of the amido complex 3.