The possible thione–thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV–VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione–disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione–thiol tautomerism, absorption spectra, thiol–disulfide oxidation, mercaptopyridines and pyrimidines.

No abstract

No abstract

The electronic absorption and emission spectra of some symmetrical heterocyclic disulfides are investigated. The reversible disulfide — thione transformation in water is discussed in view of the complex equilibrium processes present. UV irradiation and pH influence on the above transformation is also studied. The emission properties at room and low temperature are related to the computed molecular geometries of the ground and low excited states of the compounds.

A series of substituted pyridinium betaines of squaric acid have been prepared by quaternization reaction of 3‐ and 4‐substituted pyridines with squaric acid. The products were characterized by means of elemental analysis, melting points, and their UV‐vis and IR spectra. The UV‐vis spectra of all investigated compounds in five different solvents were studied in detail. The UV‐vis spectral elucidation has given evidence of the clearly distinct negative solvatochromism of the pyridinium betaines of squaric acid, characteristic for compounds with a dipolar electronic ground state structure. The two absorption bands observed in the visible region show a charge‐transfer character, which determines their strong dependence on the polarity of the medium and the nature of the substituent. The investigation of the spectral behavior of pyridinium betaines of squaric acid has revealed their potential nonlinear optical and electro‐optical properties. Furthermore, these compounds exhibit remarkable thermal stability, required for such applications. The eventual technical application of these colored single crystals is discussed.