  In contrast to simple monatomic alkali and halide ions, complex polyatomic
ions like nitrate, acetate, nitrite, chlorate etc. have not been studied in any
great detail. Experiments have shown that diffusion of polyatomic ions exhibits
many remarkable anomalies, notable among them is the fact that polyatomic ions
with similar size show large difference in their diffusivity values. This fact
has drawn relatively little interest in scientific discussions. We show here
that a mode-coupling theory (MCT) can provide a physically meaningful
interpretation of the anomalous diffusivity of polyatomic ions in water, by
including the contribution of rotational jumps on translational friction. The
two systems discussed here, namely aqueous nitrate ion and aqueous acetate ion,
although have similar ionic radii exhibit largely different diffusivity values
due to the differences in the rate of their rotational jump motions. We have
further verified the mode-coupling theory formalism by comparing it with
experimental and simulation results that agrees well with the theoretical
prediction.
