We study the rotational dynamics of a supercooled molecular liquid by means of molecular dynamics simulations. The system under investigation is composed of rigid diatomic molecules with an associate dipole moment. At room temperature, orientational correlations decrease rapidly with increasing distances. Upon cooling, angles between dipole moments of molecules within the first coordination shell decrease. As for the dynamical properties, rotational diffusion coefficients decrease with temperature at a smaller rate than translational diffusion coefficients do, and the critical temperature associated with the former is lower than the one corresponding to their translational counterparts. Translation and rotation about an inertial axis are uncorrelated, whereas some coupling between translation and dipole reorientation is obtained.

Dielectric properties have been evaluated by means of Molecular Dynamics simulations on two model systems made up by dipolar molecules. One of them mimics methanol, whereas the other differs from the former only in the ability of forming hydrogen bonds. Static dielectric properties such as permittivity and the Kirkwood factor have been evaluated, and results have been analyzed by considering the distribution of relative orientations between molecular dipoles. Dipole moment time correlation functions have also been evaluated. The relevance of contributions associated to autocorrelations of molecular dipoles and to cross-correlations between dipoles belonging to different molecules has been investigated. For methanol, the Debye approximation for the overall dipole moment correlation function is not valid at room temperature. The model applies when hydrogen bonds are suppressed, but it fails upon cooling the non-associated liquid. Important differences between relaxation times associated with dipole auto and cross-correlations as well as their relative relevance are at the root of the Debye model breakdown. *

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