FTIR studies of conformational disorder: crystal perfecting in long chain n-alkanes

The low temperature FTIR spectra of long chain n-alkanes C198H398 and C246H494, crystallised from solution in both extended and once-folded forms, have been investigated in the CH2 wagging region. Successive annealing and cooling stages result in a significant reduction in the number of non-planar C–C bond conformers in all cases. This is evidence of a ‘perfecting’ process within the crystals. The time dependence of this behaviour shows that, on raising the annealing temperature, the level of disorder reaches a maximum before declining. In addition, in a continuous heating run on once-folded C198H398, all types of conformational disorder except end-gauche bonds are shown to pass through a maximum near 118 °C, in the region of the transformation to extended chain crystals.