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.