Zero-point energy effects in anion solvation shells

Abstract

By comparing classical and quantum-mechanical (path-integral-based) molecular simulations of solvated halide anions X⁻ [X = F, Cl, Br and I], we identify an ion-specific quantum contribution to anion–water hydrogen-bond dynamics; this effect has not been identified in previous simulation studies. For anions such as fluoride, which strongly bind water molecules in the first solvation shell, quantum simulations exhibit hydrogen-bond dynamics nearly 40% faster than the corresponding classical results, whereas those anions which form a weakly bound solvation shell, such as iodide, exhibit a quantum effect of around 10%. This observation can be rationalized by considering the different zero-point energy (ZPE) of the water vibrational modes in the first solvation shell; for strongly binding anions, the ZPE of bound water molecules is larger, giving rise to faster dynamics in quantum simulations. These results are consistent with experimental investigations of anion–bound water vibrational and reorientational motion.