Vapor pressure studies of hydrophobic association. Thermodynamics of fluorobenzene in dilute aqueous solution

An automated vapor pressure apparatus has been used to obtain measurements of the vapor pressure of aqueous solutions of fluorobenzene at temperatures of 15, 25, 35, and 45°C, and in the concentration range 0 to 0.011M. The results have been interpreted to infer the dimerization constant of fluorobenzene in very dilute aqueous solutions, equivalent to the second virial coefficient of interaction between fluorobenzene molecules. The hydrophobic association of fluorobenzene molecules is thermodynamically quite similar to that of benzene at comparable temperatures and concentrations. A dimerization constant of fluorobenzene of 0.56 M^–1 at 30°C and an endothermic enthalpy of association equal to 3.9 kcal-mol^–1 are calculated from the measurements.     

Soft X-ray emission from a CO2 laser-heated Z-pinch plasma

We report results of soft X-ray measurements in which a high-power (10¹⁰–10¹¹ W/cm²) CO₂ laser was used to heat a near critical density (<10¹⁹ cm^–3) helium Z-pinch plasma. Frequency-integrated X-ray data show that the unheated Z-pinch plasma is Maxwellian with a temperature of about 30 eV. During laser heating, the X-ray emissions were enhanced over the unheated emissions. Analysis of the experimental X-ray spectra indicate that the low-energy portion of the X-ray emission spectrum (up to 600 eV) is enhanced over the baseline 30 eV Maxwellian emissions. This result is consistent with an inverse bremsstrahlung-modified distribution which results when the plasma heating rate is more rapid than the collisional thermalization rate. These results suggest that it may be possible to enhance the soft X-ray yield of a plasma lithographic source with laser heating.     

Synthesis and X-ray structure of the Mo(VI) species Mo4O12(C12H30N4S2)2(C3H7ON)n (n ≈ 2)

Dissolution of Mo₂O₅((CH₃)₂NCH₂CH₂NHCH₂CH₂S)₂ in dimethylformamide results in the formation of a species without coordinated sulphur, as indicated by ⁹⁵Mo NMR spectroscopy. Subsequent crystallization of this solution yielded the compound Mo₄O₁₂(C₁₂H₃₀N₄S₂)₂(C₃H₇ON)₂ which X-ray crystallography shows to consist of a novel Mo₄O₁₂ core, containing an eight-membered Mo₄O₄ ring with the two pairs of diagonal molybdenum atoms linked by disulphido-containing groups.     

Investigating the deformation of nanocrystalline copper with microscale kinematic metrics and molecular dynamics

Atomistic simulations are employed to investigate the deformation of nanocrystalline copper and the associated strain accommodation mechanisms at 10 K as a function of grain size. Volume-averaged kinematic metrics based on continuum mechanics theory are formulated to analyze the results of molecular dynamics simulations. The metrics rely on both reference and current configurations, along with nearest neighbor lists to estimate nanoscale behavior of atomic deformation fields in nanocrystalline copper. Various deformation mechanisms are activated in the structures, and shown to depend on average grain size of the nanocrystalline structure. Furthermore, grain boundaries, along with dislocation glide, become an important source of strain accommodation as grain size is reduced. It is demonstrated that the metrics capture the contributions of various mechanisms, and provide a sense of the history of atomic regions undergoing both elastic and plastic deformation. The significance of this research is that unique kinematic signatures of the mechanisms are uncovered using certain metrics, and we are able to resolve the contributions of the deformation mechanisms to the overall strain of the structure using Green strain.     

A nested molecule-independent neural network approach for high-quality potential fits

It is shown that neural networks (NNs) are efficient and effective tools for fitting potential energy surfaces. For H2O, a simple NN approach works very well. To fit surfaces for HOOH and H2CO, we develop a nested neural network technique in which we first fit an approximate NN potential and then use another NN to fit the difference of the true potential and the approximate potential. The root-mean-square error (RMSE) of the H2O surface is 1 cm(-1). For the 6-D HOOH and H2CO surfaces, the nested approach does almost as well attaining a RMSE of 2 cm(-1). The quality of the NN surfaces is verified by calculating vibrational spectra. For all three molecules, most of the low-lying levels are within 1 cm(-1) of the exact results. On the basis of these results, we propose that the nested NN approach be considered a method of choice for both simple potentials, for which it is relatively easy to guess a good fitting function, and complicated (e.g., double well) potentials for which it is much harder to deduce an appropriate fitting function. The number of fitting parameters is only moderately larger for the 6-D than for the 3-D potentials, and for all three molecules, decreasing the desired RMSE increases only slightly the number of required fitting parameters (nodes). NN methods, and in particular the nested approach we propose, should be good universal potential fitting tools.     

Detection of a single DNA base-pair mismatch using an anthracene-tagged fluorescent probe

A novel anthracene-tagged oligonucleotide can discriminate between a fully-matched DNA target sequence and one with a single mismatching base-pair through a remarkable difference in fluorescence emission intensity upon duplex formation.     

Contrasting solvent and capping ligand effects directing the photochemistry of uranyl(VI) Schiff base complexes

Photolysis of the uranyl(VI) Schiff base complex UO2(tBu4-salphen)(THF) (1a) with cobaltocene in THF affords [Cp2Co][UO2(tBu4-salphen)(OH)] (2) in high yield while irradiation in toluene yields no reaction. Electronic emission spectra of 1a reveal a large Stokes' shift in toluene similar to that observed in the free ligand, while in THF the structural rearrangement responsible for this shift is blocked. Instead, the ligand-centered excited state is redirected to the uranyl(VI) center by way of energy transfer, thus generating 2 from the intramolecular activation of a coordinated THF molecule.     

Patterning of robust self-assembled n-type hexaazatrinaphthylene-based nanorods and nanowires by microcontact printing

The one-dimensional (1-D) self-assembly property of an n-type hexaazatrinaphthylene (HATNA) discotic pi-conjugated molecule was studied. Structurally robust unimolecular columnar stacks of HATNA with tunable length have been fabricated through a combination of supramolecular self-assembly and post-polymerization approach. Moreover, microcontact printing can be utilized to transfer the self-assembled nanostructures to the surface to create desired functional patterns.     

Rapid analysis of tetracaine for a tape stripping pharmacokinetic study using short‐end capillary electrophoresis

A rapid and simple short‐end (reverse) capillary zone electrophoresis method was developed and validated for the separation and quantification of tetracaine in skin using tape samples. The separation was performed in a 485 mm (400 mm to window) × 50 µm internal diameter fused silica capillary using a background electrolyte of phosphoric acid–Tris pH2.5 at –25 kV. The extraction of tetracaine from tape samples was achieved using methanol diluted to 50% with water before injection. Procaine was the internal standard. The migration times for procaine and tetracaine were 1.25 and 1.36 min, respectively. The limit of quantification for tetracaine was 50 µg, with a signal‐to‐noise ratio greater than 10. The calibration curve was linear from 50 to 1200 µg with r² greater than 0.99. The CV for both within‐ and between‐assay imprecision and the percentage inaccuracy for the quality control samples including lower and upper limits of quantitation were <12.1% and <11%, respectively. The absolute mean recovery of tetracaine was >97%. The accuracy and selectivity of this method allowed the rapid measurement of tetracaine in tape samples obtained from a skin tape stripping study of local anaesthetics in healthy subjects. Copyright © 2008 John Wiley & Sons, Ltd.