Figure 3.

¹H-NMR and ¹³C-NMR of compound 2 at pH values at approximately 0.8 and 0.9, respectively (left).Observed chemical shifts (δ_obs) of the signals arising from the tertiaryhydrogensand the carbon in position 1 associated with the oxo and hydrated forms of 2 plotted against pH (right).The four continuous lines with a sigmoidal shape are described by Eq. 14, fitting δ_obs values at different pH values.