Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures

Abstract

The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S 0 ) and excited (S 1 ) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S 0 and enol form with a monocarbonyl structure in S 1 indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S 1 . Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S 0 and S 1 . The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800 cm −1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S 0 . Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π * in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

Harnessing the duality of bases toward controlled color and fluorescence

“…As shown in Fig. 3C, before adding the base, two peaks were observed at 1803 and 1754 cm −1 , which correspond to the two carbonyls of the keto form of HA, and the shoulder at 1728 cm −1 is consistent with the carboxylic C═O of homophthalic acid ( 24 ), indicating a small degree of conversion of HA to the diacid. Upon addition of DBU, the two anhydride C═O peaks rapidly diminished then disappeared, and a peak at 1693 cm −1 appeared, which is assigned to C═O vibration from the enolate form of HA.…”