Infrared spectra of normal paraffins in the hexagonal crystal form

Abstract

At a certain temperature, at most a few degrees below the melting point, long-chain normal paraffins suffer a first-order transition to a hexagonal crystal form. The infrared spectra of hexagonal crystals of n-C 24H 50, n-C 29H 60, n-C 28H 58 and n-C 36H 74 have been investigated with linearly polarized radiation. In addition, the spectra of a polycrystalline aggregate of hexagonal n-C 29H 60 and of hexagonal n-C 36H 74 in potassium bromide pellet have been observed. The spectrum of the hexagonal crystal form is compared with those of the orthorhombic or monoclinic forms and with the spectrum of the liquid phase. The results support the assumption that the molecules execute hindered rotation about their axes in the hexagonal crystal form.