Tautomeric Equilibria of [1]Benzopyrano[3,4-d]imidazol-4(3H)-ones, a Theoretical and NMR Study

Benzopyranoimidazolones could virtually exist in four tautomeric forms, namely N3−H, N1−H, coumarin O−H, and C2−H. Experimental evidence reported thus far has been unable to lead to a unique statement about the preferred tautomeric forms in solution. In this work, tautomeric equilibria for a series of 2-substituted [1]benzopyrano[3,4-d]imidazol-4(3H)-ones were investigated by DFT calculations, in both gas phase and solution. The influence of the solvent was included in the calculations by the CPCM solvent model. ¹³C chemical shifts of all tautomers were computed at different levels of theory and then compared with experiments to assign the preferred tautomers. Theoretical findings were then compared to dynamic ¹H NMR experiments results.