Unraveling quantum mechanical effects in water using isotopic fractionation

TL;DR

This study demonstrates that quantum mechanical effects significantly influence isotopic fractionation in water, and that accurate modeling of anharmonicity is crucial for predicting these effects across temperatures.

Contribution

The paper shows that models incorporating anharmonicity in water's chemical bonds better predict isotope fractionation, revealing competing quantum effects.

Findings

Harmonic models over-predict isotope fractionation.

Anharmonic models provide more accurate predictions.

Quantum effects vary with temperature, affecting water's isotopic behavior.

Abstract

When two phases of water are at equilibrium, the ratio of hydrogen isotopes in each is slightly altered due to their different phase affinities. This isotopic fractionation process can be utilized to analyze water's movement in the world's climate. Here we show that equilibrium fractionation ratios, an entirely quantum mechanical property, also provide a sensitive probe to assess the magnitude of nuclear quantum fluctuations in water. By comparing the predictions of a series of water models, we show that those describing the OH chemical bond as rigid or harmonic greatly over-predict the magnitude of isotope fractionation. Models that account for anharmonicity in this coordinate are shown to provide much more accurate results due to their ability to give partial cancellation between inter and intra-molecular quantum effects. These results give evidence of the existence of competing quantum effects in water and allow us to identify how this cancellation varies across a wide range of temperatures. In addition, this work demonstrates that simulation can provide accurate predictions and insights into hydrogen fractionation.