A rare case of solid state tautomerism of 2-carbamido-1,3-indandione: Very fast intramolecular proton transfer

Solid-state tautomerism of 2-carbamido-1,3-indandione (CAID) was examined by periodic density functional calculations and infrared spectroscopy. Crystal structure models of A and B tautomers, consisting of four molecules in the unit cell, were used to simulate the intermolecular interactions in the solid state. The tautomer dynamic equilibrium in time (20 ps) at different temperatures was theoretically evaluated by first principle molecular dynamics (MD). The rare phenomenon in solid state was identified, which is expressed in the coexistence of two tautomers, the equilibrium of which varies with temperature due to simultaneous conversion via intramolecular proton transfer. MD calculations predicted a dominant presence of A , and an increase in the amount of B (3–6 %) at higher temperatures. It was shown that the coexistence of tautomers is evident in the experimental IR and UV–Vis spectra of CAID.