Determination of reorientational correlation functions in ordered fluids: IR absorption spectroscopy

Fourier band analysis is applied to obtain the reorientational correlation functions from IR spectra of ordered nematic liquid crystals. Applying the equations derived in this work on vibrations not symmetric about the long molecular axis the rotational diffusion coefficient Drot of 4, n-octyl 4′-ethoxytolane molecules in nematic phase was determined and its temperature dependence was investigated. It is shown that Drot can also be obtained using the differences in the half-widths of a band recorded with E∥n and E⊥n. This was done for both 4, n-pentyl 4′-methoxytolane and 4, n-octyl 4′-ethoxytolane. The molecular dynamics in the investigated aligned nematic phase is also discussed.