Theoretical models are used to study pH-dependent equilibria of 2,4-diamino-5-phenylthiazole tautomer molecules in water. A complete screening of semiempirical SCF multiple minimums of hypersurfaces, corresponding to several solute-water supermolecules, has been made. Multiple minimum hypersurface searching confirms experimental NMR results indicating that the native diamine tautomer predominates in aqueous neutral and basic media. This tautomeric structure, protonated either in N3 and N4, also predominates in aqueous acid media with a minor presence of a protonated monoimine tautomer, in agreement with 1 H NMR results in D 2 O. High-level ab initio SCF MO of the main structures, where solvent reaction field effects are taken into account with a dielectric constant equivalent to that of water, predict a nonconjugated protonated monoimine tautomer in nonprotic solvents, according to 1 H NMR data in polar aprotic solvents. Calculated electron excitation patterns of hydrated species in water agreed with the experimental UV spectra at different pH values. The quantum chemical procedures for calculating total energies and frontier orbital eigenvalues in local minimum geometries of the relevant supermolecules provide an appropriate model for comparisons of theoretical results with experimental facts in the case of analytical voltametry. The frontier orbital eigenvalues of the most populated minimums discard the appearance of electroanalytic signals in the case of acidic samples because of the similarity of the predicted values for all protonated isomers and water. Experimental measurements confirm the oxidative character of electroanalytic signals.

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A rapid multiresidue method has been developed for the analysis of eight pesticides (triazines, triazinones and ureas) in environmental waters. A simple end-column electrochemical detector was used in combination with an available commercial capillary electrophoresis instrument with UV detection. The determination of these pesticides using micellar electrokinetic capillary chromatography (MEKC) with dual electrochemical and UV detection is the first time reported. In both detection systems, a linear range was obtained for the eight pesticides, concentrations lower than 5.0 Â 10 À5 mol L À1, in 0.020 mol L À1 boric acid at pH 8.3 and containing 0.025 mol L À1 of sodium dodecylsulfate, to obtain selectivity in the additional separation by micellar distribution process. Under these conditions a lower detection limit than 2.0 Â 10 À6 mol L À1 (0.15 pmol of pesticide) was achieved for the most of them. The eight pesticides are resolved in less than 14 min.

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