Conformational Properties Of Poly(vinyl Fluoride) Based Upon Ab Initio Electronic Structure Calculations

Recently, with the development of electronic structure theory, the studies on the rotational isomerc state (RIS) model attract the extensive attention again. In past, because of the restriction of the computer and electronic structure theory, it is popular to study conformational energies and geometry properties of model molecules by molecular mechanics (MM) calculations. But the MM calculations do not explicitly treat the electrons in a molecular system. Therefore, the MM method can not accurately describe the molecule systems where the electronic effects are predominant. In order to better quantify the conformational energies and geometry properties, many researchers try to employ ab initio electronic structure (quantum chemistry) calculations on the model segment of polymer where electronic effects are important. The geometry properties and conformational energies are used to determine the statistical weight parameters in a rotational isomeric state model. In this paper, the geometry properties and conformational energies of model molecular of poly (vinyl fluoride) (PVF) were obtained by quantum chemistry calculations and compared with the result from MM calculations. Then we calculate characteristic ratio ( C_∞). According to geometries properties and conformational energies of model molecules, C_∞can be determined. Furthermore, we calculate the value of ?C ∞?lnw. The calculated characteristic ratio is in agreement with available experimental value. The following is the main results: 1. The geometries properties and conformational energies of model molecule of PVF are determined at MP2 level with a 6-311++g** basis set. Vibrational frequencies are calculated to verify the stationary points. All quantum chemistry calculations are performed with the quantum chemistry package Gaussian 03. By fully optimizing the original molecule geometries, we obtained 11 stationary conformers for 2, 4-difluoropentane (DFP) and 11 stationary conformers for 2, 4, 6-trifluoroheptane (TFH). Other conformers having higher energies are not stable. All the conformational energies are respect to the lowest energy conformation of each model molecule. Based on the high level quantum chemistry calculations, we can get the average of C-C skeleton bond length and C-C-C skeleton valence angle for model molecules which will be used in RIS model. We also find that the C-C-C-C skeleton torsion angles deviate generally from the standard values in order to alleviate steric and electrostatic repulsions, some torsion angles deviate from the standard values by 5-15o and some more than 30o. The deviations can be contributed to first-order and second-order interactions respectively. This is important in determining the interaction energies and statistical weight parameters. 2. By means of the geometry properties and conformational energies of...