Molecular Vibrations and Structure of High Polymers. III. Polarized Infrared Spectra, Normal Vibrations, and Helical Conformation of Polyethylene Glycol

Abstract

The polarized infrared spectra of highly oriented crystalline films of polyethylene glycol were measured in the region 3500–400 cm—1. Five parallel bands and eleven perpendicular bands due to the fundamental vibrations were observed in the region 1500–600 cm—1. From the analysis of these bands, the polyethylene glycol chain was found to belong to the dihedral group and have twofold axes intersecting the helix axis at right angles. The structural models of polyethylene glycol were discussed by the use of the equations for the helical parameters (the second paper of this series). The most likely model (TGT) contains seven repeating units and two helical turns per fiber period of 19.25 Å. The internal rotation angles for this model are calculated to be 60° for the C–C bond and 191.5° for the C–O bond. The A1, A2, and E(θ) normal vibrations of polyethylene glycol were calculated by the general method for treating helical polymers (the first paper of this series). The normal vibrations of p-dioxane were also calculated. The modified Urey—Bradley force field was used for the calculations and the potential constants were transferred from polyethylene, dimethyl ether, and propyl alcohol. The frequencies calculated for the TGT model agreed well with the observed frequencies. The potential energy distributions were also calculated and the nature of the infrared bands was elucidated. The far-infrared spectra of crystalline films were also measured in the region 600–50 cm—1 and two parallel bands and two perpendicular bands were observed. The skeletal normal vibrations of the TGT model were also calculated, taking into account the torsional potential field. The torsional constants correspond to a potential barrier of 3 kcal/mole. The calculated frequencies agreed well with the observed, providing substantial support for the TGT model. The infrared bands in the region of 3000 cm—1 were assigned to the C–H stretching modes and the infrared active combination vibrations.