Temperature and phase behavior of infrared intensities: The poly (methylene) chain

The infrared band intensities of crystalline n-alkanes and polyethylene decrease nonlinearly with increasing temperature. For all modes except the C-H stretches, the decrease is large and far exceeds that expected from density and refractive index effects. Similar anomalous decreases occur for the odd n-alkanes at their principal solid-solid (orthorhombic-to-hexagonal) phase transition. Further decreases occur in going to the liquid and gas phase. The intensities of the methylene bending and rocking fundamentals for the gas at 300 K are about 1/3 those of the crystalline solid at 77 K. A good correlation between the temperature coefficients of the intensity and of the lateral expansion is found for the crystal. This relation suggests that low-frequency modes play an important role in determining the temperature behavior of intensities. A mechanism involving low-frequency modes is proposed that appears to qualitatively explain our experimental results. The sensitivity of intensities to temperature and phase must be taken into account in infrared studies of poly(methylene) chain systems and in the transfer of observed and calculated gas-phase intensities to the condensed state. Similar temperature behavior is expected for Raman intensities and for other flexible chain molecules.