The nature of low-frequency (0 to 200 cm−1) dynamics of liquid crystal molecules is explored using a combination of high-resolution Raman spectroscopy and first principles computer simulations implemented on a parallel computer. The frequencies and displacement patterns associated with these vibrations have been determined by diagonalisation of a dynamical matrix. Results indicate that in the prototypical nematogen 5CB, complex internal modes have frequencies comparable to those of lattice modes in the low-temperature solid polymorphs. We expect this result is not specific to 5CB and that such behaviour is common to many liquid-crystal systems. Such low-frequency modes may be related to the conformational changes which have been observed to occur near liquid crystalline phase transitions. In smectic 8CB, very low-frequency features (near 10 cm−1) in the Raman spectrum are observed and are attributed to interlayer interactions in the smectic phase.
