Chemical peculiarities and structure of 2-(indane-1?, 3?-dionid-2?-yl)pyridinium (?-pyrophthalone)

Solution Structures of Pyrophthalones, I Structure and Conformation of 1,3-Indandionato-2(2-pyridinium)-betaïn — a 1H/13C NMR Approach

“…This may have its reason in molecular deformations already found by X-ray analysis [12], which all tend to separate H3 from the neighboured oxygen atom.…”

“…The structure of a-pyrophthalone (a-PPH; l,3-indandionato-2(2-pyridinium)-betain) is a matter of controversy since the first synthesis of the compound [1][2][3][4], The tautomeric and mesomeric structures shown in Scheme 1 have been used to explain the numerous analytical data from UV [5][6][7][8][9], IR [7,9,10], EPR [9,11], X-ray [12] and NMR spectroscopy [10,12,13]. The same problem exists for the dye-stuff quinophthalone (QPH) [14,15], which is included for comparison.…”

“…The same problem exists for the dye-stuff quinophthalone (QPH) [14,15], which is included for comparison. Although charge separation and protonation of the nitrogen has been mentioned sometimes [3,5,6,11,12,14,15], many papers stated a planar structure and inferred its stabilization by hydrogen bonding between O and N.…”