Necessity of aromatic carboxylate anions to be planar to induce growth of cationic micelles

It is well-known that some aromatic anions have the ability to induce viscoelastic transformation in aqueous solutions of cationic surfactants even at added salt concentrations as low as 10-20 mM. This behavior is associated with the formation of an entangled network of elongated micelles. However, the effect of aromatic ring substituents on the anion's ability to promote rapid micelle growth is not well-understood. We have performed ab initio calculations of the carbonyl group rotation barriers in a series of substituted benzoate and naphthoate anions at the MP2/STO-3G level of theory. It was found that aromatic carboxylates, known to be particularly effective in causing sphere-rod transition in cationic micelles, preferably adopt conformations with the COO(-) group in the same plane as the ring(s). This structural preference can be attributed to either intramolecular hydrogen bonding (o-hydroxyl derivatives) or pi-conjugation effects (m- and p-halogenated derivatives). In the former case the barrier to rotation is 40-50 kcal/mol, whereas in the latter case the threshold value is around 3.0 kcal/mol. Propensity for the planar conformation correlates with a greater depth of counterion penetration into the micelle surface, as inferred from NMR experiments, compared to the anions with less hindered carbonyl rotation. This points to favorable hydrophobic interactions between the surfactant methylene groups and the aromatic ring(s) of the anion as a possible explanation for the rapid growth of cationic micelles observed upon addition of certain aromatic carboxylates.     

Some symmetry-induced isotope effects in the kinetics of recombination reactions

Symmetry-induced isotope effects in recombination and collision-induced dissociation reactions are discussed. Progress on understanding the anomalous isotope effects in ozone is reviewed. Then, calculations are performed for the simpler reaction xNe+yNe+H<-->xNeyNe+H, where x and y label either identical or different isotopes. The atomic masses in the model are chosen so that symmetry is the only difference between the systems. Starting from a single potential energy surface, the properties of the bound, quasibound, and continuum states of the neon dimer are calculated. Then, the vibration rotation infinite order sudden approximation is used to calculate cross sections for all possible inelastic and dissociative processes. A rate constant matrix that exactly satisfies detailed balance is constructed. It allows recombination to occur both via direct three-body collisions and via tunneling into the quasibound states of the energy transfer mechanism. The eigenvalue rate coefficients are determined. Significant isotope effects are clearly found, and their behavior depends on the pressure, temperature, and mechanism of the reaction. Both spin statistics and symmetry breaking produce isotope effects. Under most conditions the breaking of symmetry enhances the rates, but a wide spectrum of effects is observed; they range from isotope effects with a normal mass dependence to huge, mass-independent isotope effects to cancellation and even to reversal of the isotope effects. This is the first calculation of symmetry-induced isotope effects in recombination rates from first principles. The relevance of the present effects to ozone recombination is discussed.     

An improved microwave plasma torch for atomic spectrometry

The analytical performance of a microwave plasma torch was improved through mechanical alterations. Several problems reported in earlier designs were addressed: the ignition and stabilization of a helium plasma in the MPT was difficult; high powers were required to both ignite and operate the plasma; otherwise, the plasma would erratically change from an annular to a filament type discharge. In the new torch, the helium discharge was stabilized by replacing the copper central tube with one made of quartz. In addition, air entrainment was alleviated through use of a sheathing gas. This modification simplified the background mass spectrum and raised the effective ionization temperature of the discharge. A detailed schematic diagram of the new microwave plasma torch is presented.     

A diastereoselective intramolecular hydroamination approach to the syntheses of (+)-, (+/-)-, and (-)-pinidinol

A diastereoselective, lanthanocene-catalyzed, intramolecular hydroamination reaction was applied to the preparation of 2,6-disubstituted piperidines. Various metal/ligand arrays in the catalysts were examined using a model substrate to allow optimization of the diastereoselectivity. It was determined that the relationship between metal size and ligand bulk plays an integral role in the transformation. The complex Cp2NdCH(TMS)2 converted 2-substituted 8-nonen-4-amines to 2,6-disubsituted piperidines with greater than 100:1 selectivity for the formation of the cis isomer. A short synthesis of pinidinol, an alkaloid isolated from various pine and spruce species, was then carried out to exploit this stereoselective reaction.