Rotations around the neighboring C–C(O) and C(O)–O bonds in two esters, H3C–CH2–C(O)–O–CH3 (I) and H3C–CH(CH3)–C(O)–O–CH3 (II) that represent model molecules for aliphatic main chain and side group polyesters were studied by ab initio and density functional methods (MP2, B3-LYP and B-LYP) using the standard Gaussian basis set 6-31G(d). The performance of the PCFF force field, developed for polymers, to reproduce the conformational behaviors of the C–C and C–O rotations in question was evaluated. Disagreements between the quantum chemical and force field results were removed by reoptimizing the PCFF force field torsion parameters of current interest. The conformational dependence of geometrical parameters and electrostatic potential derived (CHELPG) atomic charges was also studied.
Blomqvist, J., Ahjopalo, L., Mannfors, B., & Pietilä, L-O. (1999). Studies on aliphatic polyesters I: Ab inito, density functional and force field studies of esters with on carboxyl group. Journal of Molecular Structure, 488(1-3), 247-262. https://doi.org/10.1016/S0166-1280(99)00038-X