Study of the conformations and tautomerisation pathway in (Z)-4-(hydroxypropyl) isochroman-1, 3‑dione: Analysis through energy, vibrational signatures and hardness profiles

Abstract

• Tautomerisation pathways in (Z)−4-(hydroxypropyl) isochroman-1, 3‑dione are identified. • Several conformers are identified for both tautomers. • The role of substituents in the tautomerisation process in AICs is explored. • Maximum hardness principle and minimum electrophilicity principles are studied extensively. • Crystal structure data and FT-IR spectra corroborate our computational results. This article thoroughly probes the interconnectivity between all the conformations/configurations and the tautomerisation pathway in ( Z )-4-(hydroxypropyl) isochroman-1, 3‑dione (PIC) both in S 0 and S 1 states. The crystal structure data corroborates our computational results done with DFT and MP2 methods utilizing various basis sets. The tautomers are separated by 2.5 ± 0.3 kcal/mole in S 0 . The comparison of potential energy curves of close analogues shows that the inertia of the substituted functional groups (here C 2 H 5 ) plays a significant role in the proton transfer process. Intrinsic Reaction Coordinate (IRC) and frequency computations locate the transitions states (TS) in S 0 .The transition region in the tautomerisation pathway is clearly marked with the evaluation of reaction force and force constants. The excited state computations with CIS and TDDFT methods show some inconsistency regarding the relative energies of the tautomers. Maximum hardness principle (MHP) and minimum electrophilicity principle (MEP) are applied in this case. The frontier molecular orbitals in both the tautomers confirms the S 0 →S 1 transition as π-π* in nature. The plot of the difference in electron densities between S 0 and S 1 states in both the tautomers hint at the photophysical processes occurring on excitation. The vibrational signatures are detected and characteristic spectral lines and their relative intensities are discussed. The strong C = O stretching vibration appearing at 1740 cm −1 in the observed FT-IR spectra is computed to be at 1757 cm −1 confirming our assertion of PIC as a minimum energy tautomer.