AB-INITIO CALCULATIONS ON SMALL-MOLECULE ANALOGS OF POLYCARBONATES

In order to develop an ab initio force field for polycarbonates, exten sive quantum mechanical calculations were carried out on several model compounds: carbonic acid, methyl and dimethyl carbonates, phenyl carb onate, and 2,2-diphenylpropane. The calculations were performed with H artree-Fock (HF), Moller-Plesset second order perturbation (MP2), and density functional theory (DFT). Full geometry optimizations were perf ormed to characterize the global and local minimum energy structures a nd transition states of internal rotations. These geometry optimizatio ns reveal large differences in valence coordinates between different c onformational states, indicating the need to take molecular flexibilit y into account when calculating properties of these systems. The equil ibrium structures and energies are discussed in terms of substituent g roup effects and steric crowding. Finally, atomic partial charges were calculated via three methods: Mulliken population analysis, calculati on of atomic polar tensors, and by fitting the electrostatic potential surface. The latter calculations begin to uncover weaknesses in the i sotropic partial atomic charge model and quantitatively demonstrate th e possible importance of atomic dipoles and even quadrupoles in these systems.