AB-INITIO CALCULATIONS ON SMALL-MOLECULE ANALOGS OF POLYCARBONATES

In order to develop an ab initio force field for polycarbonates, extensive quantum mechanical calculations were carried out on several model compounds: carbonic acid, methyl and dimethyl carbonates, phenyl carbonate, and 2,2-diphenylpropane. The calculations were performed with Hartree-Fock (HF), Moller-Plesset second order perturbation (MP2), and density functional theory (DFT). Full geometry optimizations were performed to characterize the global and local minimum energy structures and transition states of internal rotations. These geometry optimizations reveal large differences in valence coordinates between different conformational states, indicating the need to take molecular flexibility into account when calculating properties of these systems. The equilibrium structures and energies are discussed in terms of substituent group effects and steric crowding. Finally, atomic partial charges were calculated via three methods: Mulliken population analysis, calculation of atomic polar tensors, and by fitting the electrostatic potential surface. The latter calculations begin to uncover weaknesses in the isotropic partial atomic charge model and quantitatively demonstrate the possible importance of atomic dipoles and even quadrupoles in these systems.