Extended diffusion of rigid asymmetric molecules

Molecular reorientation in hydrogen-bonding liquids: Through algebraic ∼t−3∕2 relaxation toward exponential decay

We present a model for the description of orientational relaxation in hydrogen-bonding liquids. The model contains two relaxation parameters which regulate the intensity and efficiency of dissipation, as well as the memory function which is responsible for the short-time relaxation effects. It is shown that the librational portion of the orientational relaxation is described by an algebraic approximately t(-32) contribution, on top of which more rapid and nonmonotonous decays caused by the memory effects are superimposed. The long-time behavior of the orientational relaxation is exponential, although nondiffusional. It is governed by the rotational energy relaxation. We apply the model to interpret recent molecular dynamic simulations and polarization pump-probe experiments on HOD in liquid D(2)O [C. J. Fecko et al., J. Chem. Phys. 122, 054506 (2005)].