Structural information from progression bands in the FTIR spectra of long chain n-alkanes

The low temperature FTIR spectrum of long chain n-alkanes has been investigated in the region between the C-C stretching and CH₂ twisting fundamentals (1050-1133 cm⁻¹). With successive annealing and cooling stages, extended chain crystals of n-C₁₉₈H₃₉₈ show an improvement in the regularity of the progression bands observed. This is related to a ‘perfecting’ of the crystals. A once-folded sample of the same alkane shows additional features between 1050 and 1100 cm⁻¹, attributed to resonance modes from a tight (110) fold. These disappear on transformation to the extended form, to be replaced by progression bands. Assignment of the individual bands enables the length of the all-trans chain to be estimated and this method is used to show that centre-branched long chain n-alkanes have a folded conformation. It is also shown that the chain length derived from such FTIR data for a 1:1 molar mixture of n-C₁₆₂H₃₂₆ and n-C₂₄₆H₄₉₄ is consistent with a triple layer superlattice structure.