Abstract
Quinone–quinoneimine tautomerism of 7, 10–dihydroxy–2, 3, 4–trihydrobenzo f quinoxalin–6–one derivatives has been investigated by means of their n. m. r. and visible absorption spectra. The quinonoid ring protons gave signals in the range 5. 6–6. 1 p. p. m., while the benzenoid ring protons showed signals in the range 6. 9–7. 0 p. p. m. These differences in chemical shift were valuable in identifying the structure of the tautomer. Tautomerism was found to be largely influenced by the nature of the substituents and the polarity of the solvent, and the equilibrium shift to the quinoneimine form in polar solvents. The quinone form absorbed visible light at much longer wavelengths (by about 100 nm) than does the quinoneimine form. The isosbestic point of the tautomerism was observed at about 510 nm. The calculated absorption maximum of each tautomer obtained using the PPP MO method was correlated with the observed values to predict the principal tautomer in solution.
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