Selective crystallization of di-sec-butylmagnesium. N,N,N′,N′-tetramethylethylenediamine from solutions containing mixtures of normal-butyl and secondary-butyl isomers

Found to crystallise selectively from treatment of commercial ^sBu(ⁿBu)Mg solutions with N,N,N′,N′-tetramethylethylenediamine (TMEDA), the complex (^sBu₂Mg·TMEDA) is mononuclear in the crystal state and undergoes a solvation-desolvation equilibrium in arene solution. The crystal structure is noteworthy for the large C---Mg---C bond angle of 133.6(2)°, attributable to the branched nature of the alkyl substituents. This angular distortion is thought to encourage the partial loss of TMEDA from Mg centres in solution.     

The nuclear magnetic resonance spectra of porphyrins—XI : Self-aggregation of zinc(II) meso-trifluoroacetoxyoctaethylporphyrin

Complex formation in zinc(II) meso-trifluoroacetoxyoctacthylporphyrin (1) and the corresponding acetate (3) has been studied by ¹H and ¹³C NMR spectroscopy. The large concentration dependence of the ¹H chemical shifts of 1 has been analysed in terms of a monomer-dimer equilibrium to give K ～ 3.01 mole^?1 and monomer-dimer shifts of up to 4.2 ppm (for the γ-meso proton). The dimers are immediately dissociated upon addition of methanol. In complete contrast, 3 shows no concentration dependence nor any change upon addition of methanol.The conformations of the model compounds phenyl acetate (4) and phenyl trifluoroacetate (5) were studied by CNDO and solvation calculations and, for 4, by a LIS experiment. Compound 4 exists as the endo conformer in non-polar media, but the exo conformer is preferentially solvated and is also formed when 4 complexes with Eu(fod)₃. In compound 5, the exo-endo energy difference is smaller and is not so affected by solvation; 5 shows no LIS.The large complex shifts found for compound 1 are best explained on the basis of a dimer structure in which the O·CO·CF₃ groups play no part in the association, which is presumably due to a novel metal to porphyrin interaction.     

The proton magnetic resonance spectra of porphyrins—VII: Synthesis and NMR studies of thallium(III) coproporphyrin chelates

The four coproporphyrin “type-isomers” have been synthesized as their tetramethyl esters (1) through modifications of existing procedures. When treated with thallium(III) trifluoroacetate, these porphyrins furnish the corresponding aquo porphinatothallium(III) hydroxides (2) after ligand exchange induced by chromatography on deactivated alumina.The proton NMR spectra of the chloroform solutions of thallium(III) coproporphyrins show a pronounced concentration dependence, all resonances moving to low field upon dilution; the spectra of the type-III and -IV isomers show additional fine structure in solution. Both the meso- and β-Me protons show thallium-proton spin couplings. These results are interpreted in terms of a monomer-dimer equilibrium. In the dimers of the type-I and -II chelates, the rings lie directly one above another, whereas with the type-III and -IV complexes, steric repulsions of the propionate side-chains cause lateral displacement of one molecule in the dimer relative to the other, resulting in the observed fine structure in the spectra. The inter-porphyrin distances and lateral displacements are calculated on this basis and are compared with the corresponding dimers of the parent coproporphyrins, with which there are considerable similarities.     

Overlap population and the homopolar dipole

The expression {[^S_AB/(1 + S_AB)]R/2} 2e is shown to be an upper limit approximation to the homopolar dipole. It results from adding the overlap population 4C₁C₂S_AB to the atom with the smaller orbital radius. Dipole moments calculated from this partitioning are closer to the experimental values than those computed with equally sophisticated techniques from the standard gross atomic populations. Atomic charges are identical to Löwdin's in the limiting case when S_ABR/2 = $\text{x}$_AB, but the calculations are over non-orthogonalized AO's.     

The development of an automated HPLC system for the determination of trace-metals in concentrated brines and its application to process monitoring

Methodology based on high performance liquid chromatography (HPLC) has been developed for the determination of trace-metals in concentrated brines. Metal ions, (Al, Ba, Ca, Mg, Sr and Zn) were concentrated quantitatively and isolated from the sodium chloride matrix using a chelating ion-exchange column dynamically coated with the chelating dye xylenol orange. The pH dependence of the uptake of the metals was studied. Using the chelating ion-exchange preconcentration column followed by ion chromatography all the metals studied were determined at the low ng ml^–1 level in concentrated brines and linear/workable calibrations were obtained in the concentration range of interest (<100 ng ml^–1). The methodology developed was successfully transferred to an automated on-line monitoring system for the determination of Ba, Ca, Mg and Sr in concentrated feed brines used in the chlor-alkali industry.