Studies on aliphatic polyesters.: Part II.: Ab initio, density functional and force field studies of model molecules with two carboxyl groups

This paper is a continuation of our study on the ester bond energetics. Here, model molecules for structural units of biodegradable polyglycolic and polylactic acids (HC(=O)-O-CH2-C(=O)-O-CH3 and HC(=O)-O-CH(CH3)-C(=O)-O-CH3), which contain two carboxyl groups in close vicinity to one another, have been considered. Rotations about the neighbouring C(sp(3))-O(sp(3)) and C(sp(3))-C(sp(2)) bonds adjacent to the CH2 or CH(CH3) group have been studied quantum chemically by ab initio and density functional methods (MP2, B3-LYP and B-LYP), using the standard Gaussian-type basis set 6-31G(d), as well as by the PCFF (Polymer CFF) force field. The quantum chemical results mostly are in good agreement with each other. However, for the C-O rotations, the Density Functional Theory (DFT) barriers are 0.5-3.1 kcal/mol lower than the corresponding MP2 ones. The conformational dependency of bond lengths, valence angles, and that of atomic charges also is similar in the MP2 and DFT methods. The changes in bond lengths with conformation are small, but the valence angles vary more and especially in high-energy states with low population they may open as much as 18 degrees. The conformational dependence of the most significant atomic CHELPG charges also was found to be small, and the largest relative changes occurred in small charges that do not have a major impact on the electrostatic potential. The PCFF force field produced torsional energetics that was in serious disagreement with the quantum chemical results, especially in the case of the C-C rotations. By reoptimizing the pertinent torsion parameters of the PCFF force field, these disagreements could be removed. (C) 2000 Elsevier Science B.V. All rights reserved.