Asymmetric synthesis, stereochemistry and rearrangement reactions of naturally occurring 7'-hydroxylignano-9,9'-lactones

The asymmetric synthesis of a series of (7'S,8R,8'R)-7'-hydroxylignano-9,9'-lactones is presented, among them the mammalian lignan (7'S)-hydroxyenterolactone and (7'S)-parabenzlactone, allowing the stereochemistry of natural occurring (-)-parabenzlactone to be re-assigned. A hydroxylactone rearrangement and its possible mechanisms are discussed. Finally a brief survey of the current naming and numbering variants of 7'-hydroxylignano-9,9'-lactones is presented, along with a suggestion for harmonization of the nomenclature.     

Encapsulation of metal cations and anions within the cavity of bis(1,4,7-triazacyclononane) receptors

The synthesis and characterisation of a new bis([9]aneN3) ligand (L4) containing two [9]aneN3 macrocyclic moieties separated by a 2,6-dimethylenepyridine unit is reported. A potentiometric and 1H NMR study in aqueous solution reveals that ligand protonation occurs on the secondary amine groups and does not involve the pyridine nitrogen. The coordination properties toward Cu(II), Zn(II), Cd(II) and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand can form mono- and binuclear complexes in aqueous solution. In the 1 : 1 complexes, the metal is sandwiched between the two [9]aneN3 moieties and the pyridine N-donor is coordinated to the metal, as actually shown by the crystal structure of the compound [ZnL4](NO3)2.CH3NO2. L4 shows a higher binding ability for Cd(II) with respect to Zn(II), probably due to a better fitting of Cd(II) ion inside the cavity generated by the two facing [9]aneN3 units. The formation of binuclear complexes is accompanied by the assembly of OH-bridged M2(OH)x (x = 1-3) clusters inside the cavity defined by the two facing [9]aneN3 units, and pyridine is not involved in metal coordination. A potentiometric and (1)H NMR study on the coordination of halogenide anions by L4 and its structural analogous L3 in which the two [9]aneN3 units are separated by a shorter quinoxaline linkage, shows that bromide is selectively recognised by L4, while chloride is selectively bound by L3. Such a behaviour is discussed in terms of dimensional matching between the spherical anions and the cavities generated by the two [9]aneN3 units of the receptors.     

Insights into the making of a stable silylene

Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2Bu(t)) with potassium is known to lead to the stable silylene Si[(NR)2C6H4-1,2] (1). However, silylene is now shown to react further with an alkali metal (Na or K) to yield the (1)(2)2-, c-(1)(3)-*, c-(1)(3)2- or c-(1)(4)2- derivatives. Reduction of Cl2Si[(NR)2C6H4-1,2] (R = CH2CH3 or CH2CHMe2) with potassium does not lead to an isolable silylene, but such a silylene is proposed to be an intermediate and, as for 1, reacts further to afford the potassium salts of c-[Si{(NR)2C6H4-1,2}]4-* and c-[Si{(NR)2C6H4-1,2}](4)2-. The pathways leading to the anionic cyclotri- and cyclotetrasilanes are discussed and supported experimentally; including by X-ray structures of relevant intermediates.     

Efficient quenching of photoluminescence from functionalized single-walled carbon nanotubes by nitroaromatic molecules

The photoluminescence from functionalized single-walled carbon nanotubes was found to be highly sensitive to the presence of nitroaromatic compounds such as nitrobenzene, 4-nitrotoluene, and 2,4-dinitrotoluene. The strong luminescence quenching in solution was at the upper limit of diffusion-control and also showed significant static quenching contributions. Mechanistic implication of the results and potential applications are discussed.     

Ornstein–Uhlenbeck processes in Hilbert space with non-Gaussian stochastic volatility

We propose a non-Gaussian operator-valued extension of the Barndorff-Nielsen and Shephard stochastic volatility dynamics, defined as the square-root of an operator-valued Ornstein–Uhlenbeck process with Lévy noise and bounded drift. We derive conditions for the positive definiteness of the Ornstein–Uhlenbeck process, where in particular we must restrict to operator-valued Lévy processes with “non-decreasing paths”. It turns out that the volatility model allows for an explicit calculation of its characteristic function, showing an affine structure. We introduce another Hilbert space-valued Ornstein–Uhlenbeck process with Wiener noise perturbed by this class of stochastic volatility dynamics. Under a strong commutativity condition between the covariance operator of the Wiener process and the stochastic volatility, we can derive an analytical expression for the characteristic functional of the Ornstein–Uhlenbeck process perturbed by stochastic volatility if the noises are independent. The case of operator-valued compound Poisson processes as driving noise in the volatility is discussed as a particular example of interest. We apply our results to futures prices in commodity markets, where we discuss our proposed stochastic volatility model in light of ambit fields.     

Finite computable dimension and degrees of categoricity

We first give an example of a rigid structure of computable dimension 2 such that the unique isomorphism between two non-computably isomorphic computable copies has Turing degree strictly below ${\mathbf{0}}^{″}$, and not above ${\mathbf{0}}^{\prime }$. This gives a first example of a computable structure with a degree of categoricity that does not belong to an interval of the form $[{\mathbf{0}}^{(\alpha )},{\mathbf{0}}^{(\alpha +1)}]$ for any computable ordinal α. We then extend the technique to produce a rigid structure of computable dimension 3 such that if ${\mathbf{d}}_0$, ${\mathbf{d}}_1$, and ${\mathbf{d}}_2$ are the degrees of isomorphisms between distinct representatives of the three computable equivalence classes, then each ${\mathbf{d}}_i<{\mathbf{d}}_0\oplus {\mathbf{d}}_1\oplus {\mathbf{d}}_2$. The resulting structure is an example of a structure that has a degree of categoricity, but not strongly.     

Density functional studies of selected metal dioxides

This study investigates the optical properties of selected metal oxides due to their high dielectric constants. The local-spin-density approximation plus Hubbard U (commonly called LDA+U) is used in a study of the structural, mechanical and optical properties of UO₂. The inclusion of a Hubbard U correction to 5f electrons of uranium changes UO₂ from a metal to an insulator and, therefore, has a dramatic effect on the localisation of the electron spin and charge density of uranium. However although the band gap can be reproduced using the effective U parameter, which is equal to 3.5 eV and optical properties were calculated in our previous work, it is difficult to calculate ionic contribution to the static dielectric constant within LDA+U formalism for this compound. It is shown in the present work that the electronic structures of both ceria and thoria exhibit similarities to urania within LDA or PBE functional implementations. Within this functional and linear response theory one can easily calculate static dielectric permittivity and it is shown that in agreement with experiment the predicted values are an order of magnitude larger than the dielectric constant of SiO₂. In this work, high accuracy, first-principles calculations are also used to compare properties of urania versus ceria and thoria and how these similarities can help in understanding these compounds. It is also shown that the B3LYP functional predicts slightly overestimated band gaps for ceria and thoria as well as smaller than experimentally observed electronic contribution to the static dielectric constant, while the index of refraction is well reproduced for thoria.     

Structural Modifications of cis‐Glycofused Benzopyran Compounds and Their Influence on the Binding to Amyloid‐β Peptide

A small library of glycofused tricyclic compounds with a central pyran ring chemically modified in the position para to the ring oxygen has been synthesised. The influence of the chemical modification on the structural conformation of the compounds and on their ability to bind Aβ peptide has been evaluated respectively using molecular mechanics (MM) and molecular dynamics (MD) simulations, and STD NMR spectroscopy. The introduction of particularly polar/charged groups leads to the loss of binding ability, without a significant change in the conformation, whilst other substitutions does not significantly affect either the structural conformation or the binding.     

Theory of retention in reversed phase liquid chromatography with ternary mobile phases

The partition model of retention is developed for reversed phase liquid chromatography with multicomponent mobile phases. Simple equations for the retention and selectivity in ternary mobile phases are derived. For the systems in which the ratio of volume fractions of organic modifiers remains fixed, new linear dependences for retention factor and selectivity are proposed. These equations are successfully used to describe experimental data found in the literature. An influence of the nature of organic solvents and proportion in which they are mixed on retention and selectivity is discussed.