Fast intramolecular proton transfer in 2-(hydroxyaminomethylidene)-indan-1,3-dione

The structure of 2-carboxamide-indan-1,3-dione was investigated in gas phase, solution and solid states using a combination of quantum-chemical calculations, NMR spectroscopy and X-ray crystallography. Quantum-chemical calculations indicate that two tautomeric forms, 2-(hydroxyaminomethylidene)-indan-1,3-dione (A) and 2-carboamide-1-hydroxy-3-oxo-indan (B), are likely to coexist, with fast intramolecular proton transfer between them. The calculations were carried out at HF and MP2 level of theory using 6-31G** and 6-311G** basis sets. The energy barrier of the intramolecular proton transfer reaction was calculated as 2.30 kcal mol−1 at the MP4/6-311G**//MP2/6-311G** level. 1H and 13C NMR spectra are consistent with the coexistence of tautomers A and B in solution, while X-ray crystallography suggests the exclusive presence of tautomer A in the solid state.