Computational Investigation on the Tautomerisation between (Z)-4-[Hydroxy(phenyl)methylene]isochroman-1,3-dione and 4-Benzoyl-3-hydroxy-1H-isochromen-1-one in S0 and S1 States

Abstract

The tautomerization pathway between (Z)-4-[hydroxy(phenyl)methylene]isochroman-1, 3-dione (PHIC) and 4-benzoyl-3-hydroxy-1H-isochromen-1-one (PHOC) was explored both in the S₀ and S₁ states through DFT and ab initio methods involving 6-31 G(d) basis. The former was found to be the global minimum in both the states, but the orientation of the phenyl ring changes substantially on excitation. Potential energy curves were explored with the variation of proton transfer coordinate and the phenyl rotation. IRC computations identify the transition states and the reaction force and force constants specify the transition region. PHOC is about 3.5±0.5 kcal/mol higher in S₀. This difference increases in S₁. Maximum hardness principle is obeyed only by the TS. The frontier molecular orbitals confirm the S₀→S₁ transition as π–π* in nature. All the vibrational signatures in the tautomers were identified. A number of low-frequency modes below 100 cm^–1 arises due to the phenyl rotations.