Preparation of a mercaptopropyl bonded silica intermediate in supercritical carbon dioxide: synthesis, characterisation and chromatography of a quinine based chiral stationary phase

This research examines the preparation of a mercaptopropyl bonded silica intermediate in supercritical carbon dioxide (sc-CO(2)) and the subsequent conversion in sc-CO(2) to a quinine derived chiral stationary phase (CSP). The effects of reaction temperature, pressure and time on the surface coverage of the silica intermediate were investigated when porous silica particles (Exsil-Avanti, 3microm) were reacted with 3-trimethoxymercaptopropylsilane in sc-CO(2). We present results which demonstrate that a stable mercaptopropyl bonded silica intermediate can be successfully prepared under supercritical conditions of 40 degrees C, 483bar, in a substantially reduced reaction time of 1h with superior surface coverages compared to organic solvent based methods. The further utility of this supercritical fluid technology was demonstrated by the free radical addition of a quinine derived chiral selector onto a mercaptopropyl bonded silica intermediate in sc-CO(2). This supercritical fluid generated chiral stationary phase (CSP) was utilised for the direct LC enantioseparation of a series of 3,5-dinitrobenzoyl (DNB) amino acids. Bonded silica samples were characterised using elemental analysis, diffuse reflectance infrared fourier transform (DRIFT) spectroscopy, solid state (13)C and (29)Si CP-MAS NMR spectroscopy, and thermogravimetric analysis (TGA). This supercritical fluid functionalisation approach offers an efficient and cleaner alternative to existing organic solvent based approaches for the preparation of bonded silica phases.     

Foldamer‐Mediated Remote Stereocontrol: >1,60 Asymmetric Induction

An N‐terminal L ‐α‐methylvaline dimer induces complete conformational control over the screw sense of an otherwise achiral helical peptide foldamer formed from the achiral quaternary amino acids Aib and Ac₆c. The persistent right‐handed screw‐sense preference of the helix enables remote reactive sites to fall under the influence of the terminal chiral residues, and permits diastereoselective reactions such as alkene hydrogenation or iminium ion addition to take place with 1,16‐, 1,31‐, 1,46‐ and even 1,61‐asymmetric induction. Stereochemical information may be communicated in this way over distances of up to 4 nm.     

Americium and plutonium separation by extraction chromatography for determination by accelerator mass spectrometry

A simple method was developed to separate Pu and Am using single column extraction chromatography employing N,N,N′,N′-tetra-n-octyldiglycolamide (DGA) resin. Isotope dilution measurements of Am and Pu were performed using accelerator mass spectrometry (AMS) and alpha spectrometry. For maximum adsorption Pu was stabilized in the tetra valent oxidation state in 8 M HNO₃ with 0.05 M NaNO₂ before loading the sample onto the resin. Am(III) was adsorbed also onto the resin from concentrated HNO₃, and desorbed with 0.1 M HCl while keeping the Pu adsorbed. The on-column reduction of Pu(IV) to Pu(III) with 0.02 M TiCl₃ facilitated the complete desorption of Pu. Interferences (e.g. Ca²⁺, Fe³⁺) were washed off from the resin bed with excess HNO₃. Using NdF_3, micro-precipitates of the separated isotopes were prepared for analysis by both AMS and alpha spectrometry. The recovery was 97.7 ± 5.3% and 95.5 ± 4.6% for ²⁴¹Am and ²⁴²Pu respectively in reagents without a matrix. The recoveries of the same isotopes were 99.1 ± 6.0 and 96.8 ± 5.3% respectively in garden soil. The robustness of the method was validated using certified reference materials (IAEA 384 and IAEA 385). The measurements agree with the certified values over a range of about 1–100 Bq kg⁻¹. The single column separation of Pu and Am saves reagents, separation time, and cost.     

Hydrocarbon binding inside a hexameric pyrogallol[4]arene capsule

Pyrogallol[4]arene forms discrete hexameric capsules in neat normal, branched, and cyclic alkanes. These hydrocarbon guests fill the available space and show chemical exchange that is slow on the NMR time scale. [reaction: see text]     

Solid-supported cyclohexane-1,3-dione (CHD): a "capture and release" reagent for the synthesis of amides and novel scavenger resin

[reaction: see text] A three-step synthesis of cyclohexane-1,3-dione (CHD) resin 6 on polystyrene resin is described. Resin 6 was used to prepare an amide library of high purity by microwave-assisted serial "capture and release" and can be recycled for this purpose. High-loading CHD resin 10 was also shown to scavenge allyl cations in solution.     

Fast Kalman Filtering and Forward–Backward Smoothing via a Low-Rank Perturbative Approach

Kalman filtering-smoothing is a fundamental tool in statistical time-series analysis. However, standard implementations of the Kalman filter-smoother require O(d³) time and O(d²) space per time step, where d is the dimension of the state variable, and are therefore impractical in high-dimensional problems. In this article we note that if a relatively small number of observations are available per time step, the Kalman equations may be approximated in terms of a low-rank perturbation of the prior state covariance matrix in the absence of any observations. In many cases this approximation may be computed and updated very efficiently (often in just O(k²d) or O(k²d + kdlog?d) time and space per time step, where k is the rank of the perturbation and in general k ? d), using fast methods from numerical linear algebra. We justify our approach and give bounds on the rank of the perturbation as a function of the desired accuracy. For the case of smoothing, we also quantify the error of our algorithm because of the low-rank approximation and show that it can be made arbitrarily low at the expense of a moderate computational cost. We describe applications involving smoothing of spatiotemporal neuroscience data. This article has online supplementary material.     

Solving real-world sized container pre-marshalling problems with an iterative deepening branch-and-bound algorithm

Container terminals around the world regularly re-sort the containers they store according to their retrieval times in a process called pre-marshalling, thus ensuring containers are efficiently transferred through the terminal. State-of-the-art algorithms struggle to find optimal solutions for real-world sized pre-marshalling problems. To this end, we introduce an improved exact algorithm using an iterative deepening branch and bound search, including a novel lower bound computation, a new branching heuristic, new dominance rule and a new greedy partial solution completion heuristic. Our approach finds optimal solutions for 161 more instances than the state-of-the-art algorithm on two well known, difficult pre-marshalling datasets, and solves all instances in three other datasets in just several seconds. Furthermore, we find optimal solutions for a majority of real-world sized instances, and feasible solutions with very low relaxation gaps on those instances where no optimal could be found.     

On the symmetry of Welch- and Golomb-constructed Costas arrays

We prove that Welch Costas arrays are in general not symmetric and that there exist two special families of symmetric Golomb Costas arrays: one is the well-known Lempel family, while the other, although less well known, leads actually to the construction of dense Golomb rulers.