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The Debye-Stokes-Einstein (DSE) model of rotational diffusion predicts thatthe rotational correlation times $\tau_{l}$ vary as $[l(l+1)]^{-1}$, where $l$is the rank of the orientational correlation function (given in terms of theLegendre polynomial of rank $l$). One often finds significant deviation fromthis prediction, in either direction. In supercooled molecular liquids wherethe ratio $\tau_{1}/\tau_{2}$ falls considerably below three (the Debye limit),one usually invokes a jump diffusion model to explain the approach of the ratio$\tau_{1}/\tau_{2}$ to unity. Here we show in a computer simulation study of astandard model system for thermotropic liquid crystals that this ratio becomesmuch less than unity as the isotropic-nematic phase boundary is approached fromthe isotropic side. Simultaneously, the ratio $\tau_2/\eta$ (where $\eta$ isthe shear viscosity of the liquid) becomes {\it much larger} than hydrodynamicvalue near the I-N transition. We have also analyzed the break down of theDebye model of rotational diffusion in ratios of higher order rotationalcorrelation times. We show that the break down of the DSE model is due to thegrowth of orientational pair correlation and provide a mode coupling theoryanalysis to explain the results.Comment: Submitted to Physical Review

physical review. e, statistical, nonlinear and soft matter physics

Complete breakdown of the Debye model of rotational relaxation near the isotropic-nematic phase boundary: Effects of intermolecular correlations in orientational dynamics