The tautomeric equilibrium of benzotriazole: new evidence from the jet-cooled rotational spectrum and first principles calculations

The microwave spectrum of jet-cooled benzotriazole is presented and, similarly to the room temperature spectrum, it is shown to be due to the 1H-tautomer. With the help of ab initio quantum chemical calculations of the electronic excitation energies and relative stability of the two tautomers, we show that the microwave results do not disagree with the conclusions of the recent rotational analysis of the 0-0 fluorescence band in jet-cooled molecular beams, which was attributed to the 2H-species.