| Abstract
| - This work reports studies of thermochemistry of pyran-2-thione (PT), a sulfur derivative of α-pyrone(AP). Moderate heating of PT results in scrambling of sulfur and oxygen atoms in the moleculeand formation of isomeric thiapyran-2-one (TP). The products of pyrolysis of PT were studiedexperimentally by a combined use low temperature matrix isolation and Fourier transform infraredspectroscopy. The infrared spectrum of the TP monomer isolated in solid argon at 10 K wascompletely assigned based on comparison with theoretical calculations undertaken at the DFT(B3LYP)/6-311++G(d,p) level. The upper limit of thermal stability of PT was investigated usingthe differential scanning calorimetry technique. It was found that pyrolysis of PT is already initiatedat temperatures below 130 °C. The mechanism of the observed pyrolytical conversion has beenstudied theoretically at the MP2/6-311++G(d,p) level, in the ground electronic state. The primarystep of the pyrolytical reaction in PT is the α-cleavage of the C−O single bond. It proceeds via anopen-ring thioketene−aldehyde structure, TK1. According to the calculations, the ring-openingreaction from PT to TK1 requires an activation energy less than 80 kJ mol-1, at 130 °C, being therate-determining step. Further steps of the pyrolytical reaction involve internal rotations aroundsingle bonds and [1,5] sigmatropic shift of the aldehydic hydrogen. Pyrolytical ring-opening reactionswere studied theoretically also for AP and TP and compared to the pyrolysis of PT. It is suggestedthat the relative ease of the pyrolytical transformation in PT can be explained in terms of existenceof the additional minimum TK1 in the reaction path. No counterparts for this structure could betheoretically located for AP and TP.
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