Unsubstituted benzotriazole, tautomerism

The tautomeric equilibrium of unsubstituted benzotriazole has been studied extensively as summarized in reviews (63AHC(2)27, 76AHCS295). In the crystalline state the sole existence of 1H-benzotriazole is demonstrated by X-ray (74AX(B)1490), by 13C NMR (83H(20)1713), and by microwave spectra (93JSP)161)136). The 1 //-form also predominates strongly under most other conditions. 

Annular tautomerism (e.g. 133 134) involves the movement of a proton between two annular nitrogen atoms. For unsubstituted imidazole (133 R = H) and pyrazole (135 R = H) the two tautomers are identical, but this does not apply to substituted derivatives. For triazoles and tetrazoles, even the unsubstituted parent compounds show two distinct tautomers. Flowever, interconversion occurs readily and such tautomers cannot be separated. Sometimes one tautomeric form predominates. Thus the mesomerism of the benzene ring is greater in (136) than in (137), and UV spectral comparisons show that benzotriazole exists predominantly as (136). 

---