4- and 4,5-substituted N-methoxythiazole-2(3H)-thiones -: Preparation, UV/Vis spectra, and assignment of electronic transitions in comparison to N-Methoxypyridine-2(1H)-thione using time-dependent density functional theory calculations

Experimentally observed absorptions in UV/Vis spectra of N-methoxy-4-methylthiazole-2(3H)-thione, N-methoxy-5-(p-methoxyphenyl)-4-methylthiazole-2(3H)-thione, N-methoxypyridine-2(1H)-thione, and selected N-hydroxy derivatives thereof have been assigned to pi-->pi*-type transitions as dominating character, using the results from ab initio calculations [time-dependent density functional theory (TD)RI-BLYP/TZVPP]. Theory further predicts that electronic excitations in N-methoxythiazole-2(3H)-thiones on one side and N-methoxypyridine-2(1H)-thione on the other side differ significantly with respect to character and statistical weight of contributing transitions. These effects originate predominantly from contributions of the endocyclic sulfur atom onto orbital energies and shapes in thiazole-2(3H)-thiones, and may be intensified by substituents such as a p-methoxyphenyl group located in position 5. Since the majority of the calculated spectral differences between thiazole- and pyridinethiones refers to excitations of low intensity, the findings from the present study correlate with two important experimental facts: (i) Apart from minor shifts in the exact spectral location of UV/Vis absorptions, electronic spectra of N-hydroxy- or N-methoxy-substituted pyridine-2(1H)-thiones and thiazole-2(3H)-thiones are surprisingly similar in shape. (ii) N-alkoxypyridine-2(1H)-thiones and N-alkoxythiazole-2(3H)-thiones liberate upon UV/Vis excitation oxygen-centered radicals with a comparable efficiency. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).