The tautomerism of benzotriazole has been studied extensively both experimentally and theoretically. From quantum chemical calculations it is predicted that 1H-benzotriazole is more stable than 2H-benzotriazole. Only a few gas phase experiments have been reported. The microwave absorption spectrum of benzotriazole was recorded in a heated cell by Velino et al., and was attributed to 1H-benzotriazole. The same group recorded the band contour of the S1<--S0 transition. This band contour was analyzed under the assumption that this spectrum has to be from 1H-benzotriazole (that is, the microwave data of 1H benzotriazole was used in the analysis). We have recorded the same transition in a jet expansion (rotational and vibrational cooling!) and showed that this transition should be assigned to 2H-benzotriazole. Note that the microwave spectrum of 2H-benzotriazole is not identified as results of a small permanent dipole moment.
At the moment the status is as follows: 1) the microwave spectrum of 1H benzotriazole has been recorded in a heated cell and a cold jet expansion; thus showing the existence of ground state 1H-benzotriazole in the gas phase. 2) the S1<--S0 spectrum of 2H-benzotriazole has been recorded; thus showing the existence of ground and excited state 2H-benzotrizole (our work) 3) the S1 state of 1H-benzotriazole is not jet identified/detected.
G. Berden, E. Jalviste, and W.L. Meerts.
Rotationally resolved UV spectroscopy on
the 2H-tautomer of benzotriazole in a molecular beam.
Chem. Phys. Lett. 226 (1994) 305-309. G. Berden, W. Leo Meerts and E. Jalviste.
Rotationally resolved UV spectroscopy of indole, indazole
and benzimidazole, inertial axis reorientation
in the S_1(1^L_b) <-- S_0 transitions.
J. Chem. Phys. 103 (1995) 9596-9606
K. Remmers, E. Jalviste, I. Mistrik, G. Berden and
W.L. Meerts.
Internal rotation effects in the rotationally resolved
S1(1Lb) <- S0 origin bands of 3-methylindole and
5-methylindole.
J. Chem. Phys. 108 (1998) 8436-8445