Conformational Landscape and Tautomerisation in (Z)-4-(hydroxymethylene) isochroman-1,3-dione: Analysis through Energy and Hardness profiles

Abstract

• Tautomerisation pathways in ( Z )-4-(hydroxymethylene) isochroman -1,3-dione are identified in S 0 and S 1 . • Several conformers are distinguished for both tautomers. • The variation of reaction force and force constants are studied and transition region is characterized. • Maximum Hardness Principle and Minimum Electrophilicity Principles are studied both in S 0 and S 1 . ( Z )-4-(hydroxymethylene) isochroman-1,3-dione (HIC) and 3-hydroxy-1-oxoisochroman-4-carbaldehyde (HOC) exhibits a number of conformers. Out of these, we are primarily interested on the one showing tautomerisation between them through the migration of H atom. The tautomerisation reactions are theoretically characterized by DFT and ab-initio calculations in S 0 and by CIS and TDDFT calculations in S 1 . All the computational techniques confirm the HIC to be the most stable conformer as well as tautomer in S 0 . In S 1 state, TDDFT computations hint at HOC and CIS indicates HIC as the minimum with an energy difference of 1-2 kcal/mole between the tautomers. All the conformers/configurations and the transition state structures are identified and respective potential energy curves connecting these structures are drawn in S 0 . IRC calculations in S 0 and S 1 show the evolution from HIC to HOC form and vice versa through the transition state. The energy difference between the tautomers is less in S 1 indicating larger probability of intramolecular proton transfer in S 1 than in S 0 . The vibrational signatures are identified and all the modes are assigned in S 0 . Electron distribution in HOMO and LUMO are calculated and interpreted for both the forms. The energy and hardness profiles are opposite to each other, verifying the principle of maximum hardness for all the structures in S 0 . The minimum electrophilicity principle is followed by the transition state structure only in S 0 , while both the principles are obeyed in S 1 . The electron density difference plots indicate the transfer of charges on excitation.