Frequency Shifts and Band Contours of Vibrational Transitions of Vapors Trapped in High-Density Polyethylene. The ν3, ν5, ν6, ν5−ν3, and ν2−ν6 Bands of CHCl3 and the ν2, ν3 Modes of CS2

Abstract

Frequency shifts and band contour changes of far-infrared vibrational transitions of chloroform and carbon disulfide vapor upon entrapment in high-density polyethylene above the glass transition temperature have been studied between 240 and 420 cm−1. The data were applied to elucidate the interactions between the trapped molecules and the polymer. The ν6, ν5, ν3 fundamentals and the ν2−ν6, ν5−ν3 difference bands of chloroform and the ν2, ν3 fundamentals of carbon disulfide were measured with the molecules in the free vapor state and each included in polyethylene. The two difference bands and the ν5 fundamental of chloroform undergo large red shifts upon entrapment of the molecules in polyethylene. A qualitative evaluation of the red shift of the ν5 band based on the Kirkwood—Bauer—Magat theory shows that the interaction between the trapped chloroform and the polyethylene can be described in terms of a polarization of the polymer by the dipoles of chloroform. There is no appreciable amount of localized bonding between chloroform molecules or chloroform and polyethylene. The carbon—chlorine deformation modes ν6 and ν3 of chloroform undergo slight blue shifts upon entrapment in polyethylene. Based on Buckingham's quantum-mechanical model of intermolecular interactions, the smallness of these shifts may be explained by the relatively minor degree of anharmonicity of the ν6 and ν3 modes compared to that of the other modes of chloroform. Two hot bands, which are hidden under the R and P branches of the ν2 mode of free carbon disulfide molecules, appear in the spectrum of the ν2 mode of the trapped molecules. Intensity measurements indicate that the hot bands arise from the levels ν2 and 2ν2, respectively. It is shown that the solvent shifts of the ν2 and ν3 modes of carbon disulfide in polyethylene, which amount to 2.8 and 8.0 cm−1, are insufficient data to afford a quantitative evaluation of the intermolecular potential in Buckingham's model.