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

摘要

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 pi -> pi*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.