Rocking, methylene doublet

The band resulting from the methylene rocking vibration (p CH2), in which all of the methylene groups rock in phase, appears near 720 cm-1 for straight-chain alkanes of seven or more carbon atoms. This band may appear as a doublet in the spectra of solid samples. In the lower members of the w-alkane series, the band appears at somewhat higher frequencies. 

The 725 cm" rocking mode doublet is presented in Figure 3. The peaks at 731 and 720 cm for this sample are approximately 15 times the intensity of the amorphous wagging modes. The 720 cm limiting frequency is assigned to long trans sequences in the amorphous and crystalline phases. For these samples, most of the methylene structures are in the crystal, and the band is associated with this phase predomi-... 

Tuot and Lecomte [35] in 1943 gave data on a series of thirty alcohols in which they noted the appearance of a band near 720 cm when at least four adjacent straight-chain methylene groups were present, and Thompson and Torkington [17] assigned the doublet at 721 and 732 cm in polythene to this cause. The vibration has been shown to be due to a rocking mode of the CH2... 

---