A 3RD-ORDER ROTATIONAL ISOMERIC STATE MODEL FOR POLY(OXYETHYLENE) BASED UPON AB-INITIO ELECTRONIC-STRUCTURE ANALYSES OF MODEL MOLECULES

A new three-state rotational isomeric state (RIS) model has been derived for poly(oxyethylene) (POE), based upon ab initio electronic structure analyses of the model molecules 1,2-dimethoxyethane (DME) and diethyl ether (DEE). It is demonstrated that the low energy of the tg+/-g-/+ conformation of DME, resulting from strong O ... H attractions, as indicated by the ab initio studies, necessitates the inclusion of third-order interactions (depending on three consecutive torsional angles) in the RIS model. This is realized by adopting 9 x 9 statistical weight matrices. This third-order RIS model, with all the parameters derived from the conformational geometries and energies of DME and DEE, predicts the chain dimensions, the dipole moments, and their temperature coefficients for POE in good agreement with experiments. It was also found from analysis of the energies of the O-C-C-O, O-C-C-CH2-(O), and O-C-C-CH2-(CH2) gauche conformations (relative to trans) in DME, 1,3-dimethoxypropane, and methyl butyl ether, respectively, that the strength of the oxygen gauche effect for the C-C bond, i.e., the stabilization of the O-C-C-X gauche conformation relative to the trans conformation, correlates well with the degree of Coulombic repulsion or attraction between the oxygen and the ''X'' moiety. This explains the slightly higher energy (0.1 kcal/mol) found for the O-C-C-O gauche conformation (relative to trans) in DME from ab initio calculations while similar calculations for 1,3-dimethoxypropane and methyl butyl ether indicate that the O-C-C-CH2 gauche conformations are significantly lower in energy than their respective trans conformations.