Determination of inorganic chlorine stable isotopes by continuous flow isotope ratio mass spectrometry

Chlorine stable isotope analyses of inorganic samples were conducted using continuous flow isotope ratio mass spectrometry (CF-IRMS) coupled with gas chromatography (GC). Inorganic chloride was precipitated in the form of silver chloride (AgCl) by using silver nitrate in a standard methodology. Chlorine stable isotope analysis was carried out on methyl chloride (CH3Cl) after converting AgCl into CH3Cl by reacting it with methyl iodide (CH3I). The reaction between AgCl and CH3I took place in 20 mL size vials. Addition of CH3I was performed in a glove bag under helium flow. An Agilent 6890 gas chromatograph equipped with a CTC Analytics CombiPAL autosampler and a DB-5MS 60 m column was used to separate CH3Cl from CH3I. This new technique uses samples as small as 0.2 mg of AgCl (1.4 micromol of Cl-). The chlorine stable isotope analysis using continuous flow technology showed excellent precision and accuracy. The internal precision using pure CH3Cl gas is better than +/-0.04 per thousand (+/-STDV). The external precision using seawater standard is better than +/-0.07 per thousand (+/-STDV) for n=12. Moreover, the sample analysis time is much shorter and many more samples can be analyzed in one day than by using the conventional off-line techniques.     

Free-energy density functions for nematic elastomers

The recently proposed neo-classical theory for nematic elastomers generalizes standard molecular-statistical Gaussian network theory to allow for anisotropic distributions of polymer chains. The resulting free-energy density models several of the novel properties of nematic elastomers. In particular, it predicts the ability of nematic elastomers to undergo large deformations with exactly zero force and energy cost—so called soft elasticity. Although some nematic elastomers have been shown to undergo deformations with unusually small applied forces, not all do so, and none deform with zero force. Further, as a zero force corresponds to infinitely many possible deformations in the neo-classical theory, this non-uniqueness leads to serious indeterminacies in numerical schemes. Here we suggest that the neo-classical free-energy density is incomplete and propose an alternative derivation that resolves these difficulties. In our approach, we use the molecular-statistical theory to identify appropriate variables. This yields the choice for the microstructural degrees of freedom as well as two independent strain tensors (the overall macroscopic strain plus a relative strain that indicates how the deformation of the elastomeric microstructure deviates from the macroscopic deformation). We then propose expressions for the free-energy density as a function of the three quantities and show how the material parameters can be measured by two simple tests. The neo-classical free-energy density can be viewed as a special case of our expressions in which the free-energy density is independent of the overall macroscopic strain, thus supporting our view that the neo-classical theory is incomplete.     

A Dimeric Magnesium(I) Compound as a Facile Two‐Center/Two‐Electron Reductant

The odd couple : A dimeric magnesium(I) complex acts as a facile and selective two‐center/two‐electron reductant towards a series of unsaturated substrates (see scheme; Ar=2,6‐iPr₂C₆H₃, Ad=1‐adamantyl). The novel reduced or reductively coupled products obtained from these reductions suggest that magnesium(I) compounds may find wide use in organic and organometallic syntheses.

     

On Ramanujan’s function k(q)=r(q)r 2(q 2)

In both his second and lost notebooks, Ramanujan introduced a function, related to the Rogers–Ramanujan continued fraction and its quadratic transformation, and listed several of its properties. We extend these results and develop a systematic theory.     

Synthesis of the mycolactone core by ring-closing metathesis

The undecenolide core of mycolactone was synthesized by ring-closing metathesis and the structure confirmed using single-crystal X-ray diffraction techniques.     

Quantum chemical study of hydrogen abstraction reactions of the ethynyl radical with hydrogen compounds (C2H+HX)

We have theoretically investigated the hydrogen abstraction reactions of ethynyl radical with simple hydrogen compounds, C₂H+HX, using quantum chemical computations. Computations have been performed using the density functional theory with the recently proposed MPW1K functional and the 6-311++G(3df,2p) basis set. An analysis of the resulting energy barriers for hydrogen abstraction reactions has been carried out using the bond dissociation energy of the breaking X–H bond and DFT-based reactivity parameters to rationalize the reaction behavior.     

Spectroscopic and quantum chemical studies on low-spin FeIV=O complexes: Fe-O bonding and its contributions to reactivity

High-valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three FeIV=O (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(O)(N4Py)]2+. These complexes are characterized by their strong and covalent Fe-O pi-bonds. The MCD spectra show a vibronic progression in the nonbonding --> pi* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the pi-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [FeIV(O)(N4Py)]2+ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O pi-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O pi-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied beta-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant FeIV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (pi-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied alpha-spin d(z2) orbital (sigma-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of FeIV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.     

Trigonal (-3) symmetry octahedral lanthanide(III) complexes of zwitterionic tripodal ligands: luminescence and magnetism

Sandwich coordination complexes, [Ln^III(H₃L)₂]X₃?solvents, of Tb(III), Eu(III), Dy(III), Ho(III) and Er(III) were prepared with two new zwitterionic ester-substituted tripodal amine ligands, tris((2-hydroxy-5-n-butyl benzoate)aminoethyl)-amine (H₃L¹) and tris((2-hydroxy-5-methyl benzoate)aminoethyl)-amine (H₃L²). These ligands were synthesised by condensation of the appropriately substituted salicylaldehyde with tris(2-aminoethyl)amine (tren) followed by in situ reduction of the tris-imine to tris-amine. Subsequent 2:1 reaction with lanthanide(III) ions yields [Ln^III(H₃L)₂]X₃?solvents (L = L¹, L²; X = Cl^?, NO₃^?; solvents = MeOH or H₂O). All complexes were characterised by microanalysis, infrared spectroscopy, high resolution mass spectrometry and solid-state photoluminescence measurements. The crystal structures of [Tb^III(H₃L¹)₂]Cl₃·6MeOH, [Dy(H₃L¹)₂]Cl₃·6MeOH, [Eu^III(H₃L¹)₂]Cl₃·6MeOH and [Tb^III(H₃L¹)₂](NO₃)₃ reveal high-crystallographic ?3 symmetry at the O₆-coordinated octahedral lanthanide(III) ions and that the tripodal ligands are bound in zwitterionic form: the protons from the phenolic oxygens have migrated to the amino nitrogens. Photoluminescence measurements indicate various degrees of energy transfer of the ligand chromophore to the lanthanide ions, as both ligand and lanthanide emission features are observed. Despite the high-crystallographic symmetry and the likely small transverse magnetic anisotropy of the complexes, no evidence of slow relaxation of the magnetisation, characteristic of a single-molecule magnet, was observed for [Tb^III(H₃L¹)₂]Cl₃·MeOH·3H₂O, [Dy^III(H₃L¹)₂]Cl₃·6H₂O, [Ho^III(H₃L¹)₂](NO₃)₃·2H₂O, [Er^III(H₃L¹)₂]·H₂O and [Tb^III(H₃L¹)₂](NO₃)₃ down to 2.0 K.     

TLC Image Capture and Analysis by use of a Prototype Device for Visualizing Fluorescence

JPC – Journal of Planar Chromatography – Modern TLC - This paper reports the evaluation of a novel, low-cost imaging device for use in TLC — a commercial flat-bed scanner modified...