Gas chromatography on a self-associating component of a binary phase. Retention model by formal analogy with conductance of electrolytes in dilute solution

Summary The dependence of specific retention by a binary stationary phase in GC can be expressed as the sum of the products of the specific retention of the pure components times their respective volume fractions. In this study, however, one component has a site, which is not only mainly responsible for the selectivity, but also participates in strong self-association. This requires introduction of a concentration-dependent factor (μ^x) in the corresponding term of the equation correlating Vg _mix ^x with x. In the GC resolution of N-trifluoroacetyl-amino acid isopropyl esters on a binary phase, N-lauroyl-L-valine t-butyl amide-squalane, data for the values of μ^x were obtained. Adapting a previously developed LC retention model to the above GC data, an equation was derived for the dependence of μ^x on the weight fraction (x) of the selector, namely μ^x=√1/x. This relationship permits calculation of retention volumes on the binary phase for a given x, as well as corresponding resolution coefficients of enantiomeric amino acid derivatives with generally excellent agreement with experiment. The chirality of the system is not relevant to application of the equation.     

2,6-Dialkoxy-7-methylpurines

The preparation of unsymmetrical 2,6-dialkoxy-7-methylpurines (2), and 2-alkoxy-1,7-dialkyl-6-oxo-1,6-dihydropurines (5) is described. In contrast to1 and2, a facile thermal lactim-lactam rearrangement from hypoxanthines5 and7 into xanthines6 was observed.

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2,6-Dialkoxy-7-methylpurine

Zusammenfassung Die Darstellung von unsymmetrischen 2,6-Dialkoxy-7-methylpurinen (2) und 2-Alkoxy-1,7-dialkyl-6-oxo-1,6-dihydropurinen (5) wird beschrieben. Im Gegensatz zu den Verbindungen1 und2 erfolgt die thermische Lactim-Lactam-Umlagerung der Hypoxanthine5 und7 zu den Xanthinen6 glatt.

     

Thermal properties of Fe-fullerene systems is reaction C60+Fe→Fe@C60 in solid fullerite possible?

Iron in the fullerite lattice binds fullerences in a sandwich type C₆₀FeC₆₀ complexes. At the concentration C₆₀Fe₂ it crystallizes in the monoclinic lattice. The structure is thermally unstable, with the energy release of 606 kJ·mol^?1 it returns to fee lattice. Two possible sites in the reconstructed fee lattice are discussed, Fe bond to C₆₀ and Fe inside the C₆₀ cage.     

The influence of the mobile phase pH and the stationary phase type on the selectivity tuning in high performance liquid chromatography nucleosides separation

Analysis of the modified nucleosides is particularly important in the medical area because of a possibility of cancerogenic processes studies. The aim of this work was to study the selectivity tuning of modified nucleosides through the investigations of interactions analyte (modified nucleoside) <==> stationary phase <==> mobile phase. A series of homemade stationary phases with different surface properties has been utilized. All of them contain various interaction sites such as: cholesterol (SG-CHOL); n-acylamide (SG-CHOL, SG-AP); aminopropyl (SG-CHOL, SG-AP, SG-NH2, SG-MIX); cyanopropyl, phenyl, octyl (SG-MIX), octadecyl (SG-MIX, SG-C18) and silanols localized on the silica gel surface of all packings. The attempt to predict the main interactions responsible for the retention between nucleosides and stationary phase ligands was done on the basis of the elemental analysis, and proportional part of an individual ligand bonded to silica surface results. In order to study the influence of different packing types on the analyzed nucleosides retention, the relationship between pH of the mobile phase buffer and the selectivity of a stationary phase was investigated.     

Selective interaction of lower rim calix[4]arene derivatives and bivalent cations in solution. Crystallographic evidence of the versatile behavior of acetonitrile in lead(II) and cadmium(II) complexes

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.     

Estimation of total heterogeneity properties of carbon nanotubes

Physico-chemical properties (adsorption capacity, desorption energy distribution and pore-size distribution functions) of nanomaterial surfaces from selected materials, based on sorptometric and liquid thermodesorption measurements under quasi-equilibrium conditions, are presented. The fractal dimensions of nanotubes using sorptometric and AFM data have been evaluated. Comparison of thermogravimetric and other data provide new information about the adsorption and pore structure of the studied materials. The fractal dimensions of nanomaterial surfaces using sorptometry are in good agreement with those from AFM.     

Mathematical models of solute retention in gas chromatography as sources of thermodynamic data. Part 1. Methyl n-alkyl ketones as the test analytes

Although gas chromatography (GC) is mainly used for analytical and preparative separation of complicated mixtures and then identification of the separated components, it can also be used as a relatively simple tool for the determination of physicochemical (including thermodynamic) properties. In this study, we discuss three different retention models devised specially for GC. They are tested for a homologous series of methyl n-alkyl ketones chromatographed on stationary phases of low and medium polarity at several different isothermal temperatures. The statistical performance of each model is excellent, which makes them a convenient tool for the evaluation of analyte retention under the given working conditions. They can also be used for the determination of the numerical values of thermodynamic properties (e.g., the enthalpy of vaporization of the analytes and the standard molar chemical potential of the partitioning of one methylene group (-CH2-) between the stationary and mobile phases).     

Cellulose sorbents in the investigation of self-association of higher fatty acids

The possibility was established of using cellulose sorbents for qualitative and semiquantitative evaluation of intermolecular interactions with higher fatty acids.     

Determination of Calcium and Iron and Measurements of Ash Content in the Brown Coal

Applying the XRF technique and the low-energy X-ray backscattering method an attempt of calcium and iron determination, as well as measurement of the ash content in the brown coal was made. A ²³⁸Pu source, an argon filled propotional counter and a three channel pulse height analyser were used. A simple theoretical model is proposed and obtained results are discussed.