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Remarkable stability is shown by the recently observed 3H isomer 1 of indole (2) (τ ≅ 100 s at pH ≅ 9 and 25°C). The equilibrium constant pKT of the tautomerization of 2 to 1 is 5.8 ± 0.2; in other words, the equilibrium concentration of 1 lies in the range of 1 to 2 ppm. The tautomerization is catalyzed by acid and—less efficiently—by base.