Protonation of ninhydrin and indan-1,2,3-trione revisited: A combined theoretical and experimental study

Abstract The literature data on protonation of indan-1,2,3-trione and ninhydrin (indantrione hydrate) in acidic media are inconsistent, formation of dications at pH 3 and very low extent of protonation in superacid CF 3 SO 3 H were claimed. Our combined theoretical (DFT, MP2, CCSD) and experimental (NMR) study has shown that indantrione undergoes single protonation in CF 3 SO 3 H resulting in monocation formation. In more strong superacid FSO 3 H-SbF 5 dication is formed, but, contrary to the literary data, trication is not formed even in this superacid. Significant dependence of 13 C NMR chemical shifts of indantrione on media acidity allows using this compound as a convenient indicator of acidity in a broad range, from H 0  −3 to −25.