Solutions of fatty acid potassium salts in formamide have been investigated using electron spectroscopy in the angular resolved mode. The variable surface sensitivity thus achieved allows details of the electric double layers formed at the solution surfaces to be investigated. The cation distribution is found to vary as a function of solution concentration. The simple diffuse layer theory based on the Poisson–Boltzmann equations is found inadequate in describing the observed features. The data suggest that structural changes occur at the higher concentrations forming closer bound cation states at the surface. These findings qualitatively confirm recent theoretical model predictions by other workers.
A sensitive bioanalytical method for the determination of melatonin in saliva by solid-phase extraction (SPE), high-performance liquid chromatography (HPLC) and fluorescence detection has been developed and validated. Saliva was collected with a Salivette sampling device (Sarstedt) and a mixed-mode SPE column was used for the extraction of melatonin and internal standard (N-acetyl-6-methoxytryptamine) from the saliva. Chromatographic separation was performed using a HyPurity C18 LC column (150 x 2.1 mm) with mobile phase acetonitrile-ammonium hydrogen carbonate buffer, 0.015 M, pH 6.8 (23:77, v/v). Excitation and emission wavelengths were set to 285 nm and 345 nm, respectively. The within-day precision for the method at 50 pmol/L was 7.9 % and the between-day precision was 10.5 %. The limit of quantification was 50 pmol/L.
The energy for deprotonation of a molecule in the gas phase may be divided into an initial-state
electrostatic part and a relaxation part. The electrostatic part is dominating in determining the relative acidities
of organic compounds, but the relaxation part is not negligible. The relaxation energy may be split up in two
parts, and accurate calculations (MP2/6-311++G**//RHF/6-311++G**) of these electronic and geometric
relaxation energies are presented for 13 small organic molecules (four alkanes, three alcohols, two enols, and
three carboxylic acids). It is shown that although the electronic relaxation energy is 2 orders of magnitude
larger than the geometric relaxation energy, the difference in electronic relaxation energy and the difference
in geometric relaxation energy between a pair of molecules may be of the same size. For example, while the
electronic relaxation energy of the acetate anion is 6.1 kcal/mol smaller than that of the 2-propanoxide anion,
the geometric relaxation energy is 4.6 kcal/mol larger. Hence, the two relaxation contributions partially cancel
each other, and the total difference in deprotonation energy is approximately equal to the shift in initial-state
electrostatic potential between the two compounds. The electronic relaxation energies are largest for the most
easily polarizable molecules, and the geometric relaxation energies are largest for molecules where classical
resonance arguments suggest strongly stabilized anions (carboxylic acids and enols). The MP2/6-311++G**//RHF/6-311++G** level of theory, including zero-point and thermal energy corrections, give computed absolute
acidities, ΔH(298), very close to experimental gas-phase acidities (root-mean square deviation 1.1 kcal/mol).
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