The NMR. spectra of the five monomethylazulenes and of some polysubstituted azulenes, as well as those of their conjugate acids (azulenium cations), are recorded. It is shown that protonation of azulenes lacking C2v symmetry leads to a tautomeric equilibrium of two conjugate acids. The position of this tautomeric equilibrium is in essential agreement with the one previously predicted from the UV. spectra on the basis of a MO treatment.
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Meuche et al. (1963) studied this question.
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