Investigation into the structures of some normal alkanes within the homologous series C13H28 to C60H122 using high-resolution synchrotron X-ray powder diffraction

Abstract

High-resolution X-ray powder diffraction using synchrotron radiation has been used to determine the unit-cell parameters of some of the n-alkanes within the range C₁₃H₂₈–C₆₀H₁₂₂. These have, in general, been found to conform to the structural patterns predicted by Nyburg and Potworowski (Acta Crystallogr., Sect. B, 1973, 29, 347). Polymorphic behaviour has been observed for the n= even alkanes associated with increasing symmetry from a triclinic structure (space group P1), to a monoclinic structure (space group P2₁/a) and to an orthorhombic structure (mostly space group Pca2₁) as the chain length increases. The n-alkanes C₃₈H₇₈,C₄₀H₈₂ and C₄₄H₉₀ were found to crystallize in the polytypic orthorhombic polymorph (space group Pbca) proposed by Boistelle et al.(Acta Crystallogr., Sect. B, 1976, 32, 1240). The n= odd alkanes all conform to an orthorhombic structure (space group Pbcm). A comparative examination of the data, as a function of chain length, suggests the existence of a degree of structural disorder along the crystallographic c-axis for the longer homologues, which implies that incipient chain folding takes place.