Quantum mechanical study of energetics and mechanisms of cis–trans isomerization of some four-membered heterocycles

Abstract

Ab initio quantum mechanical calculations using density functional (B3LYP) method and 6-311G** basis set have been performed on two cis and trans conformers of 2,4-diphenyl thietane dioxide (DPTD), 2,4-diphenyl thietane (DPT), 2,4-diphenyl azetidine (DPA) and 2,4-diphenyl oxetane (DPO). The calculated stability energy for cis–trans isomerization in gas phase and in different solvents such as benzene, DMSO, water and methanol indicated that the cis conformer is more stable than trans in all above-mentioned compounds about 11–2 kcal mol−1. In the next step, a transition states for cis–trans inter-conversion for all four-membered heterocycles (DPTD, DPT, DPA and DPO) were proposed in methanol as solvent. Thermodynamic functions such as standard enthalpies of isomerization (∆Hoiso), standard entropy of isomerization (∆Soiso) and standard Gibbs free energy of isomerization (∆Goiso) for all studied compounds were also evaluated. The calculation showed that the conversion of trans to cis isomer is exothermic and spontaneous. In all calculations, solvent effects were considered using a polarized continuum model.