A study of the copolymerization of hydroxyethyl methacrylate and tetrahydrofurfuryl methacrylate

The free radical copolymerizations of hydroxyethyl methacrylate and tetrahydrofurfuryl methacrylate have been investigated at 50°C. The compositions of polymers prepared at low conversions have been determined using ¹³C-NMR, and the glass transition temperatures determined by DSC. The copolymerizations were found to be best described by a terminal model with reactivity ratios of r_H = 1.79 and r_T = 0.76. The triad fraction sequence distributions have been calculated based on the terminal model and the calculated reactivity ratios. The glass transitions have been fitted to the Gordon–Taylor equation. The best value of the Gordon–Taylor constant was found to be k_H = 1.42 ± 0.2, indicating nonideal mixing of the two monomer components in the copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3730–3737, 1999     

The multiple roles of computational chemistry in fragment-based drug design

Fragment-based drug discovery (FBDD) represents a change in strategy from the screening of molecules with higher molecular weights and physical properties more akin to fully drug-like compounds, to the screening of smaller, less complex molecules. This is because it has been recognised that fragment hit molecules can be efficiently grown and optimised into leads, particularly after the binding mode to the target protein has been first determined by 3D structural elucidation, e.g. by NMR or X-ray crystallography. Several studies have shown that medicinal chemistry optimisation of an already drug-like hit or lead compound can result in a final compound with too high molecular weight and lipophilicity. The evolution of a lower molecular weight fragment hit therefore represents an attractive alternative approach to optimisation as it allows better control of compound properties. Computational chemistry can play an important role both prior to a fragment screen, in producing a target focussed fragment library, and post-screening in the evolution of a drug-like molecule from a fragment hit, both with and without the available fragment-target co-complex structure. We will review many of the current developments in the area and illustrate with some recent examples from successful FBDD discovery projects that we have conducted.     

A cube in a tetrahedron: microwave-assisted synthesis of an octametallic Fe(III) cluster

The microwave-assisted reaction of Fe(O2CMe)2 with salicylaldoxime (saoH2) in pyridine produces an octametallic cluster in crystalline form in 2 min. The core describes a cube encapsulated in a tetrahedron, while sao2- exhibits a novel coordination mode.     

Enhanced all-optical tuning of leaky eigenmodes in photonic crystal waveguides

We demonstrate all-optical switching in an active two-dimensional photonic crystal waveguide, observing as large as 16 nm blueshifts of a leaky eigenmode at 839 nm and switching ratios of almost 70%. These results are larger than those previously observed in similar experiments performed on passive photonic crystal waveguides; the enhancement is due to resonant photogeneration of carriers by In(0.12)Al(0.2)Ga(0.68)As quantum wells in the core of the waveguide. The effective change in the refractive index of the structure is approximately10(-2), with a rise time of approximately1 ps and a decay time of approximately10 ps, potentially allowing high-speed switching and fast modulation rates.     

Effect of bucket-wheel scale on excavation forces and soil motion

Bucket-wheels enable planetary rovers to perform lightweight digging operations in support of sustained space exploration. Using an excavation tool whose performance scales well for robots of varying sizes builds confidence in a wide range of future digging missions, much as scaled versions of the rocker-bogie suspension have enabled mobility for Mars rovers of vastly different sizes. Bucket-wheel excavation force increases approximately with the cube of excavation dimensions. The excavation forces were measured for bucket-wheels of different scales at proportionate depths and advance speeds, and these results were compared to predictions by excavation models. Analytical and empirical investigated models exhibit force scaling tendency similar to experiment despite their independent backgrounds. Soil particle motion imaging shows that a curved shear interface is prevalent for the conditions tested. This agrees with literature and allows the application of analytical models.     

Using a DS-DBR laser for widely tunable near-infrared cavity ring-down spectroscopy

Studies into the suitability of a novel, widely tunable telecom L-band (1,563–1,613 nm) digital supermode distributed Bragg reflector (DS-DBR) laser for cavity ring-down spectroscopy (CRDS) are presented. The spectrometer comprised of a 36.6?cm long linear cavity with ring-down times varying between 19–26 μs across the 50 nm DS-DBR wavelength range due to changes in the cavity mirror reflectivities with wavelength. The potential of such a broadband, high-resolution CRD spectrometer was illustrated by investigating several transitions of CO₂ in air, a 5 % calibrated mixture and breath samples. Allan variance measurements at a single wavelength indicated an optimal minimum detectable absorption coefficient (α _min) of 3 × 10^?10 cm^?1 over 20 s.     

Synthesis of tertiary alkyl amines from terminal alkenes: copper-catalyzed amination of alkyl boranes

A method for highly selective anti-Markovnikov hydroamination of terminal alkenes is reported. The one-pot procedure involves hydroboration of the alkene followed by a novel electrophilic amination of the alkyl borane catalyzed by an NHC-Cu complex. Terminal alkenes are successfully transformed into tertiary alkyl amines in the presence of a variety of functional groups in yields ranging from 80 to 97% with excellent regioselectivity. Results of a preliminary study of the reaction mechanism are also described.     

Fano resonances in semiconductor superlattices

We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier–Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter – the coupling strength Γ between the discrete state and the degenerate continuum – by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances.     

Measurements of opacity and temperature of warm dense matter heated by focused soft X-ray laser irradiation

The transmission of plasma-based soft X-ray lasers through thin targets can be used to measure the target opacity. Measurements of warm dense matter transmission obtained using a focused 59 eV photon energy laser irradiation on thin targets of polyimide (C₂₂H₁₀N₂O₅) and aluminum are shown to produce simultaneous heating and probing enabling opacity and temperature measurements of warm dense matter. It is shown that the opacity of the warm dense matter considered in the experiments follows closely tabulated cold ‘room temperature’ opacities at temperatures below ～10 eV. Transmission measurements of thin iron targets which are highly opaque to the X-ray laser radiation are also presented.     

Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser

We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm . The noise equivalent absorbance for our measurement was 5 parts in 10(5), limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N(2)O in 10(9) parts N(2) in a 1-m path length.