Chromatographic and related studies of alkylamide phases

Summary A series of chemically bonded phases has been prepared with amide groups localized by means of hydrophobic ligands. The physicochemical and chromatographic properties of such phases with chain lengths ranging from C₅ to C₈ have been examined by porosimetry, elemental analysis, solid-state NMR, and liquid chromatography. Subsequently, the conformational dynamics of these phases have been investigated by analyzing the dependence of the capacity factor (k) on the reciprocal of temperature for different organic compounds. Special emphasis has been given to the reproducibility of retention data obtained before and after temperature-dependent measurements.This work was presented at the 9th International Symposium: Advances and Applications of Chromatography in Industry, Bratislava, Slovakia, August 29–September 3, 1993.     

Applications of fluorinated compounds as phases and additives in chromatography and their uses in pharmaceutical analysis

A review is presented of the use of fluorinated phases and fluorine-containing surfactants in chromatography. Additionally, new information is provided on the application of two nonionic perfluorosurfactants as reversed-phase mobile phase additives to enhance chromatographic performance of aromatic amines and some amines of pharmaceutical significance.     

quantitation of p- and m-methylstyrenes by liquid chromatography and 13C nuclear magnetic resonance spectrometry

The molar ratio of p- and m-methylstyrene in mixtures is determined by liquid chromatography and ¹³C NMR spectrometry. In the former instance, separation and quantitations are achieved using an adamantyl-modified column with a mobile phase of ethanol/water (30 + 70, v/v) containing 0.01 M β-cyclodextrin. In the latter instance, a ¹H-decoupled ¹³C NMR technique is described for the determination of the positional isomers. The results obtained by both methods are self-consistent, with a standard deviation for the molar ratio of the para and meta isomers of less than ±0.01.     

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate

he hypophosphito derivative of Co^III, (NH₃)₅CoO₂PH₂ ²⁺, decomposes in basic media, yielding Co(II) quantitatively, along with a 1:1 mixture of hypophosphite and phosphite. Earlier studies point to a cobalt (I) intermediate, which rapidly reduces a second molecule of Co(III) reactant to Co(II). In the presence of an external cobalt (III) oxidant (Co³X), the latter competes with the hypophosphito complex for Co(I), lowering the yield of free hypophosphite. From the ratio of phosphorus products (P³/P¹), evaluated by proton decoupled ³¹P NMR, the relative reactivities (k_t/k_c) of the external Co(III) “traps” and the hypophosphito complex may be determined. A log-log plot comparing values of k_t/k_c with rates for reductions of the same series of Co(III) oxidants with Ru(NH₃)₆ ²⁺ (k_Ru values) is badly scattered with a slope of only 0.23, well below the value of unity stipulated by the Marcus model, indicating that an outer-sphere mechanism cannot operate for all of the Co(III)-Co(I) reactions and may not operate for any, except for that with Co(NH₃)₅(py)₃₊. Among the carboxylato-substituted oxidants, there are no rate enhancements by neighbouring pyridine, -SR,-OH, -CHO, or -SO₃H functions, in contrast to the accelerations previously observed for reductions by Cr(II), Eu(II) and Ti(III), which are attributed to intermediacy of chelate-stabilized precursor complexes. It is suggested that the Co(III)-Co(I) reactions, which determine the ratio of phosphorus products, are much more rapid than the ligand substitution at the Co(I) center which must precede chelate formation, and that the latter therefore does not intervene significantly in the redox processes.     

Nuclear magnetic resonance spectrometry of chemically and physically altered porous silica surfaces under gas chromatographic conditions

Conventional nuclear magnetic resonance spectrometry in solution, in combination with (13)C labelling, has been utilized to investigate differences in freedom of motion of similar chemically altered and physically modified porous silica sorbents under dry conditions, and the results have been correlated with previously reported results for chemically altered solvated surfaces. Spin-lattice relaxation time of the labelled terminal methyl groups decreases in the order chemically modified solvated ligands, chemically bonded unsolvated ligands, and physically sorbed unsolvated molecules.     

An unusual phase of some thermotropic liquid crystals. A carbon-13 NMR study

Upon slow cooling, a small fraction of the thermotropic liquid crystal S1496 exhibits a new phase which shows a high-resolution carbon-13 NMR spectrum with isotropic chemical shifts. This new phase is probably either cubic phase or a plastic phase. Its formation is facilitated by the addition of small amounts of solutes. Some other liquid crystals containing alkylcyclohexanes behave similarly.