Isotopic Preferential Solvation of I- in Low-Temperature Water Nanoclusters

We present results from ring polymer molecular dynamics experiments that provide microscopic insights into the characteristics of the isotopic stabilizations of H and D aqueous species in the first solvation shell of a halide I- ion in water nano clusters at low temperatures. The analysis of the simplest I- (HOD) dimer shows a clear propensity for the light isotope to lie at the non-hydrogen-bonded dangling position. Our results predict that, at T similar to 50 K, I-center dot(DOH) isomers are three times more abundant than I-center dot(HOD) ones. The reasons for such stabilization can be traced back to differences in the nuclear kinetic energy projected along directions perpendicular to the plane of the water molecule. Dynamical implications of these imbalances are shown to be reflected in the characteristics of the corresponding bands of the infrared spectroscopic signals. A similar analysis performed in larger aggregates containing similar to 20 water molecules reveals, in contrast, a stabilization of the light isotope along I-center dot center dot center dot HO hydrogen bonds. Effects derived from the consideration of smaller halide anions with larger electric fields at the surface are also examined.