The spectral densities of p-methoxy-benzylidene-p-n-butylaniline (1O.4) were analysed using a new global target approach and a rotational diffusion model. The correlated internal motions are described by the decoupled model. We use the Nordio model and its extention to asymmetric molecules, the Tarroni and Zannoni (TZ) model, to analyse molecular reorientations. These diffusion models are compared in the nematic phase of 1O.4 and in the nematic and smectic A phases of 4O.8. Both models gave similar results for 1O.4, but for 4O.8 the TZ model appears to be more suitable.