CONFORMATIONAL CHARACTERISTICS OF POLY(OXYMETHYLENE) BASED UPON AB-INITIO ELECTRONIC-STRUCTURE CALCULATIONS ON MODEL MOLECULES

A revised rotational isomeric state (RIS) model for poly(oxymethylene) (POM) chains has been developed on the basis of the conformational energetics of the model molecules, dimethoxymethane (DMM) and 1,3-dimethoxydimethyl ether (DEE), as determined from ab initio electronic structure calculations. Due to the strong preference for gauche (g) conformations in POM chains, the energy minima for the g states were found to be deep and narrow, necessitating the inclusion of preexponential factors for the statistical weights in the RIS model that deviate from unity. Therefore, both the energy minima and preexponential factors for all of the important conformations of the model molecules were estimated by performing ab initio electronic structure calculations using a D95+(2df,p) basis set. Electron correlation effects were included at the MP2 level. The revised RIS model, with all parameters estimated from ab initio calculations, predicts the unperturbed chain dimensions and the dipole moment of POM chains as a function of temperature in good agreement with experiments. Therefore, it is demonstrated that ab initio calculations at the level of theory employed (D95+(2df,p) basis set at the MP2 level) are sufficient to provide accurate estimates of the conformational energetics of POM chains. Moreover, it is also shown that proper description of the conformation-dependent properties of POM chains requires consideration of not only the energy minima but also the energy well profile for each of the important conformations.