Splitless injection of up to hundreds of microliters of liquid samples in capillary GC: Part II,experimental results

Sample evaporation in splitless injection of large volumes is rapid: depending on the experiment, results indicate that 200 μl of hexane, for instance, evaporates in 2–10 s, producing vapor at a rate of many hundreds of milliliters per minute. A 60 × 4 mm packed bed of 20–35 mesh Tenax TA enabled injection of 200 μl volumes of all solvents tested, and even 1 ml injections were possible provided they were performed over a period of 30 s. Retention of volatile sample components depends on the sample solvent, the injection volume, and the injection speed, but only little on the injector temperature. Losses of n-tridecane varied from hardly 15 % (when dissolved in pentane) to ca 60 % (ethyl acetate); losses of n-heptadecane were usually below 20 %. The column temperature during injection should be at least ca 20–30°C above the standard solvent boiling point.     

Correlation of the position of the bond functions in H-X with electronegativity

S type Gaussian bond functions are optimized for HF, H₂O, NH₃, and CH₄. The optimization is carried out with respect to the exponent and position in the H-X bond. The position is found to correlate well with the electronegativity of Pauling and Allred-Rochow.     

Substituent Effects on the C-C Bond Strength, 15[1] Geminal Substituent Effects, 7[2]. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)₃C⁻ anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol⁻¹, ΔS ( 10c ) = 58· 5 J mol⁻¹ K⁻¹]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol⁻¹), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol⁻¹), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol⁻¹), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol⁻¹), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol⁻¹), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol⁻¹) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol⁻¹ was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol⁻¹) and α,α-dicyanoalkyl (27 kJ mol⁻¹) radicals, which were recalculated from bond homolysis measurements^[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).     

Selective detection and identification of Se containing compounds--review and recent developments

The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine.     

Stereoselective formation of ansa-metallocenes in reactions of radical-anionic salts of acenaphthylene with lanthanide and calcium halides

ansa-Metallocenes (⁵:⁵-C₂₄H₁₆)M(THF)₂ (M = Sm (1), Yb (2), Ca (3)) and (⁵:⁵-C₂₄H₁₆)MI(THF) (M = Dy (8), Er (9), Tm (10), Lu (11)) were prepared in 50—90% yields by the in situ reactions of two equivalents of potassium acenaphthylenide K⁺C₁₂H₈ ^– with MI₂ or MI₃, respectively. Complexes 2 and 3 were also obtained by direct reduction of acenaphthylene with ytterbium and calcium naphthalenides, respectively. An ESR signal of the acenaphthylene radical anion, which was observed upon dissolution of compound 2 in THF, indicates that the [C₂₄H₁₆]^2– ansa-ligand dissociated into two [C₁₂H₈]^·– radical anions. Hydrolysis of complex 2 in benzene afforded 1,1",3,3"-tetrahydro-3,3"-biacenaphthylene (4) and 3,3",4,4"-tetrahydro-3,3"-biacenaphthylene (5). The reaction of complex 2 with ZrCl₄ and the reaction of compound 3 with Me₃SiCl proceeded with the cleavage of the C—C bond between two acenaphthylene fragments of the [C₂₄H₁₆]^2– ansa-ligand to produce (²-C₁₂H₈)ZrCl₂(THF)₃ (6) and bis(trimethylsilyl)acenaphthene (Me₃Si)₂C₁₂H₈ (7), respectively. Compounds 1—3, 6, 7, and 11 were characterized by ¹H and ¹³C NMR spectroscopy. The temperature dependence of the ¹H NMR spectrum of compound 11 in tetrahydrofuran is indicative of the dynamic exchange of the solvent molecules in the coordination sphere of the Lu atom. After cooling of the solution to 210 K, the dynamic process was terminated as evidenced by the nonequivalence of the ¹H signals of two acenaphthylene fragments. According to the X-ray diffraction data for complex 11, dimerization of two acenaphthylene radical anions at the Lu atom gave rise to the rac-ansa-metallocene structure. In compound 11, the Lu atom is ⁵-coordinated by two five-membered rings of the acenaphthylene ligands and also by the I atom and the THF molecule. The coordination environment about the Lu atom is a distorted tetrahedron. The average distance between the lutetium atom and the carbon atoms of the five-membered rings is 2.623 .     

Molekulares S=GeCl2 Matrix-IR-Untersuchungen und ab initio SCF-Rechnungen

The Molecule S?GeCl₂. Matrix IR Investigation and Ab initio SCF Calculation Molecular S?GeCl₂ is found in a matrix reaction between the high-temperature molecule Ge?S and Cl₂. A structure analog to that of phosgene can be derived from the isotopical shifts (⁷⁰Ge/⁷²Ge/⁷³Ge/⁷⁴Ge/⁷⁶Ge and ³⁵Cl/³⁷Cl) within the IR spectra. The normal coordinate analysis results for the Ge?S force constant a value of 4.21 mdyn/Å. The spectroscopic results are confirmed by ab initio SCF calculations.     

DFT study and Monte Carlo simulation on proton transfers of 2-amino-2-oxazoline, 2-amino-2-thiazoline, and 2-amino-2-imidazoline in the gas phase and in water

Density functional theory (DFT) and Monte Carlo (MC) simulation with free energy perturbation (FEP) techniques have been used to study the tautomeric proton transfer reaction of 2-amino-2-oxazoline, 2-amino-2-thiazoline, and 2-amino-2-imidazoline in the gas phase and in water. Two reaction pathways were considered: the direct and water-assisted transfers. The optimized structures and thermodynamic properties of stationary points for the title reaction system in the gas phase were calculated at the B3LYP/6-311+G(d, p) level of theory. The potential energy profiles along the minimum energy path in the gas phase and in water were obtained. The study of the solvent effect of water on the proton transfer of 2-amino-2-oxozoline, 2-amino-2-thiazoline, and 2-amino-2-imidazoline indicates that water as a solvent is favorable for the water-assisted process and slows down the rate of the direct transfer pathway.     

Hydration scheme of uracil and cytosine

The comparison of a pure electrostatic approximation and complete supermolecule SCF ab initio computations on the hydration scheme of uracil and cytosine shows that the electrostatic procedure is capable to reproduce the general aspects of the results of the supermolecule treatment provided that different distances of shortest approach be adopted for the distances between the oxygen of water and the nitrogen of NH₂ or NH groups or the oxygen of C-O groups on the one hand and the oxygen of water and pyridine-type nitrogens on the other hand.     

Thermodynamics of the conversion of aqueous glucose to fructose

The thermodynamics of the conversion of aqueous glucose to fructose has been investigated using both heat conduction microcalorimetry and high pressure liquid chromatography (HPLC). The reaction was carried out in both aqueous Tris/HCl buffer and in aqueous phosphate buffer in the pH range 7–8 using the enzyme glucose isomerase and the cofactors CoCl₂ and MgSO₄. The temperature range over which this reaction was investigated was 298.15–358.15 K. We have found that the enthalpy of reaction is independent of pH over the range investigated. A combined analysis of both the HPLC and microcalorimetric data leads to the following results at 298 15 K:ΔG° = 349 ± 53 J mol^-1, ΔH° = 2.78 ± 0.20 kJ mol^-1, and ΔC _p ^° = 76 ± 30 J mol^-1 K^-1. The stated uncertainties are based upon an analysis of both the random and systematic errors inherent in the measurements. Comparisons are made with literature data. The percent conversion of glucose to fructose has been calculated for the temperature range 300–373.15 K.