For an isolated dissolved molecule or dispersed entity, an equation relating the rotational diffusion coefficient, \(\mathit{\Theta}\), to the equivalent hydrodynamic radius in rotational diffusive flow, \(r_{\mathit{\Theta}}\), and to the viscosity of the solvent continuum or the dispersion medium continuum, \(\eta_{0}\), with \[D = kT/(6\pi \eta_{\rm{0}} r_{\mathit{\Theta}}^3)\] where \(k\) is the Boltzmann constant and \(T\) the thermodynamic temperature.