A PM3 study of tautomerism in some 3,3′-methylenebis[4-hydroxycoumarin] systems

Abstract

This Parametric Method 3 (PM3) study represents one of the first attempts to investigate the relative stabilities of the tautomers of dicoumarol and its derivatives. The calculations have been carried out both in the gas-phase and in a continuum model of water, on isolated tautomers of 10 systems obtained by single proton transfer and their heats of formation (Δ H f), relative heats of formation ( δΔ H f) and dipole moments have been estimated. In most cases, the PM3 estimates of δΔ H f indicate the predominance of α,γ′-benzopyran tautomeric structure in gas-phase. Macroscopic solvent effects on the tautomers have been modeled within the framework of continuum solvation model using Conductor like Screening Model (COSMO) method. In aqueous solution, the calculations imply significant qualitative differences in the δΔ H f of the tautomers and predict the predominance of tetra-keto structure. However, in most of the cases regardless of the surrounding media, calculations indicate α,α′-benzopyran structure as the one having the largest dipole moment. The PM3 method has been demonstrated to be useful in studying such large systems which are otherwise difficult to be handled by ab initio methods for reasons of computational cost and time.