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.

In our previous paper,' helical parameters of infinite polymer chains were expressed as functions of bond lengths, bond angles, and internal rotation angles.1 In making use of these equations, likely structure models were deduced for polyethylene glycol. The most probable model belongs to the dihedral group, and the internal rotation configurations are trans and gauche for the CH2-0 bond and for the CH2-CHz bond respectively (TGT model).2 This model contains seven chemical units and two turns of the helix per fiber period of 19.25 A. In the present study the infrared active vibrations belonging to the Az and E (e) species have been treated. The AZ vibrations give rise to parallel bands and the E(s) vibrations give rise to perpendicular ones. The normal frequencies and normal modes were calculated by the general method for treating the molecular vibrations of helical polymers belonging to the dihedral group.3 The C-H stretching modes were separated from the low-frequency modes. The modified UreyBradley force field4 was used as the potential function and the potential constants were transferred from p~lyethylene,~ dimethyl ether,5 and propyl alcohol.6 The calculated frequencies and the vibrational assignments are shown in Table I along with the observed infrared freq~encies.~,7 The average deviation between the calculated and observed frequencies is 3% although no adjustments were made for a better fit.For the Az species, the hybridization of the CH2 wagging and twisting modes may be remarked. This hybridization arises from the gauche configuration about the CHZ-CH2 bond and is considered to be responsible for the two strong parallel bands observed at 1344 and 1244 em.-' (On the other hand, for the trans configuration, the hybridization of the CH2 twisting and wagging modes will not occur and the infrared bands due to the pure twisting mode should be very weak.)The parallel band a t 960 em.-' and the perpendicular bands a t 948 and 844 ern.-' were assigned to the CHI rocking modes and used in deducing the configuration about the CHz-CH2 b~n d .~, T ? Davison assigned the bands a t 960 and 948 em.-' to the symmetric CHz rocking mode of the gauche form, and the band at 844 em.-' to the antisymmetric rocking mode of the gauche form.7 On the other hand, Miyake assigned the bands at 060 and 948 em.-' to the symmetric and antisymmetric rocking modes, respectively, of the gauche form, and the band a t 844 cm.-l to the CH2 rocking mode of the trans form, and concluded that the polirethylene glycol chain in the crystalline state contains CH2-CHz groups in the trans form as well as in the gauche form The present normal coordinate treatment has clearly indicated that, for both the A2 and E(s) species, the hybridization of the CH2 rocking mode and the C-0 stretching mode is appreciable, and that not only the bands a t 960,948, and 844 em.-' but also the parallel band at I I02 and the perpendicular bands a t 1150 and 1061 em.-' are due to these hybridized modes. The difficulties encountered in the empirical vibrational assignm...

No abstract

No abstract