Conformational analysis of symmetric carbonic acid esters by quantum chemical calculations and dielectric measurements

Abstract

Geometries, energies and dipole moments of the conformers of carbonic acid and carbonic acid esters (−CH 3,−CH 2CH 3) are calculated by semi-empirical (MINDO/3, MNDO, AM1 and PM3) and ab initio methods (STO-3G, 3–21G, 4–31G, 6–31G* and MP2 MP4). Only two stable conformers (gauche—trans and gauche—gauche) are found. The gauche—gauche conformer with the lower dipole moment is energetically favoured. The best agreement with experimental results for the geometry, the energy difference between the conformers and the dielectric permittivity is obtained with the 6–31G* basis set. The results obtained with the semi-empirical method PM3 and with the minimal basis set STO-3G are remarkably good, while the results using other basis sets or semi-empirical methods are not satisfactory. Applying the Møller-Plesset perturbation treatment the STO-3G and the 6–31G* calculations give almost the same energies. Assuming an equilibrium between the conformers, good agreement between theoretical and experimental results is found for the temperature dependence of the average dipole moment in the gas phase and also in the liquid phase, where the reaction field stabilizes the conformer with the higher dipole moment.