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 1H
NMR results in D2O.
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 1H 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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