The tautomerism of benzoxazoline-2-thione (BOT; thio-keto) and 2-mercaptobenzoxazole (MBO; thio-enol) has been thoroughly investigated by means of Raman (3600-100 cm-1), infrared (4000-200 cm-1), 1H and 13C NMR spectra and X-ray powder diffraction (XRD). In addition, temperature-dependent 1H NMR spectra from -90 to +90 °C were acquired. To complement experimental results with theoretical predictions, we performed Density Functional Theory (DFT) calculations utilizing B3LYP, B3PW91 and SVWN methods at 6-31G(d) basis set. Both computational and spectral results were in favor of thio-keto BOT structure with no evidence for the existence of thio-enol (MBO) tautomer which firmly eliminates the possibility of an existing equilibrium between keto and enol forms. Moreover the dimerization percentage of thio-keto benzoxazoline-2-thione (BOT) and benzothiazoline-2-thione (BTT) were found to be 11.9% and 2.5%, respectively which favors strong hydrogen bonding interactions in BOT. Aided by normal coordinate analysis, force constants in internal coordinates and potential energy distributions (PEDs), a complete vibrational assignment for all fundamentals was obtained. The results are compared with the sulfur analogue, benzothiazoline-2-thione (BTT) whenever appropriate.
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