Transannular S...N interactions in 10-ethynyl-10 H -phenothiazine 5-oxide and 5,5-dioxide

The title compounds, C 14 H 9 NOS, (1), and C 14 H 9 NO 2 S, (2), are oxidation products of the parent compound 10-ethynyl-10 H -phenothiazine. They differ with respect to transannular interactions, the intramolecular S...N contact being shorter in (2). Intermolecular C sp —H...O hydrogen bonds were detected in both crystals, and (1) was found to form stronger hydrogen bonds. These results are in agreement with the lower acidity of C sp —H in (2), caused by an increase in π-electron density due to the transannular S...N interaction.