Conformational Analysis of Poly(propylene oxide) and Its Model Compound 1,2-Dimethoxypropane

Abstract

Conformational characteristics of poly(propylene oxide) (PPO) and its model compound, 1,2-dimethoxypropane (1,2-DMP), have been investigated. For 1,2-DMP, ab initio molecular orbital (MO) calculations at the MP2/6-31+G*, MP2/6-311+G*, and MP4/6-31+G* levels were carried out, and the free energies and dipole moments of all the possible conformers were evaluated. The bond conformations and the average dipole moments were calculated from the MO data and compared with the corresponding experimental values. The MO calculations, in general, reproduced the observations quite well. The unknown parameters in the Karplus equation expressing the dihedral-angle dependence of the vicinal coupling constant 3 J COCH were determined by comparison of the 3 J COCH values observed for 1,2-DMP with those calculated from the MO results. The free energies were broken down into conformational energy contributions corresponding to first-, second-, and third-order intramolecular interactions. All the first-order-interaction energies were found to be positive. On the other hand, the second-(E ω1 and E ω2 ) and third-(E χ ) order-interaction energies, which reflect nonbonded C-HO...D contacts, were found to be negative ; for example, the E ω1 , E ω2 , and E χ values determined for gaseous 1,2-DMP at the MP2/6-31+G* level are -1.236, -1.884, and -1.266 kcal mol -1 , respectively. As pointed out for 1,2-dimethoxyethane, the gauche stability of the C-C bond in the O-C-C-O sequence appears to arise from the nonbonded C-H...0 interactions. Using the rotational isomeric state (RIS) scheme with up to third-order interactions, the characteristic ratio and dipole moment ratio of isotactic PPO were calculated and found to be in good agreement with the observations.