Infrared active methyl group vibrations in tetratetracontane: a probe for chain end organization and crystal structure

By crystallizing tetratetracontane (n-C44H90) from solution in toluene under a variety of conditions, the orthorhombic I and (011) tilted monoclinic forms were identified by X-ray diffraction (XRD) and the orthorhombic II form was inferred from infrared spectroscopy. Application of pressure to the crystal mat resulted in a proportion of triclinic structure. At 75 °C, the (011) monoclinic form transforms to a (101) tilted monoclinic phase. Differing extents of chain end and translational disorder are believed to be responsible for these different crystal forms, which were studied using the conformational sensitivity of several infraredactive methyl and methylene group vibrations. Depending on the crystal phase, the symmetric methyl deformation around 1378 cm-1 shows differing characteristics, and both the rocking and the asymmetric C-H stretching vibrations show different components. An increase in the end-gauche methylene wagging absorbance in the (101) monoclinic structure with increasing temperature together with the presence of even order bands from the CH2 methylene rocking-twisting progression are also attributed to chain end disorder. The implications of the results on the methyl symmetric deformation for the use of the methylene wagging region of the spectrum to characterize disorder in alkyl chains are discussed.