Computational Insights to Look for the Feasibility of Intramolecular Proton Transfer Reaction in some Imine-Functionalized Derivatives: A Comparative Ab-initio Analysis

Abstract

The present investigation reports the feasibility of occurrence of intramolecular proton transfer (IPT) reactions in four functional compounds, ortho vanillin and Schiff bases namely, salicylaldazine, 2-((E)-(ethylimino) methyl)-6 methoxy phenol (2EM6MP) and Bis 2-((E)-(ethylimino) methyl)-6 methoxy phenol (B2EM6MP) in their different electronic states. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations are performed on the ground (S₀) and two lowest excited states (S₁ and T₁) and S₂ state respectively to inquire the feasibility of the IPT reactions in these molecules. The calculations reveal that although none of the compounds are susceptible to the IPT process in the ground state; excited state intramolecular proton transfer (ESIPT) is feasible for all of them. The viability or non-viability of the proton transfer process has been judged from both the thermodynamic (enthalpy change, ΔH) and kinetic (activation energy, E_act) factors. Potential energy curves (PECs) and Frontier Molecular Orbital (FMO) diagrams have been generated to depict the propositions in the different electronic states. Feasibility of the double proton transfer (DPT) reaction in B2EM6MP and salicylaldazine is also inquired since both have the structural propensity to undergo DPT. Calculations project the viability of the ESDPT for B2EM6MP contrary to the non-occurrence of the same for the other molecular system, salicylaldazine. The differential proposition is rationalized since for ESDPT reaction the two proton transfer sites are in isolation through saturated carbon centres for B2EM6MP while they are directly connected in salicylaldazine.