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.