HomeActivities > > Highlights > 2017 > Importance of a Fully Anharmonic Treatment of Equilibrium Isotope Fractionation Properties of Dissolved Ionic Species As Evidenced by Li+(aq)

- Romain Dupuis, Magali Benoît, Mark E. Tuckerman and Merlin Méheut.
*Accounts of Chemical Research* Equilibrium fractionation of stable isotopes is essential in fields ranging from chemistry, including medicinal chemistry, electrochemistry, geochemistry, and nuclear chemistry, to environmental science. The dearth of reliable estimates of equilibrium fractionation factors, from experiment or from natural observations, creates a need for accurate computational approaches. Because isotope fractionation is a purely quantum mechanical phenomenon, exact calculation of fractionation factors is nontrivial. Consequently, a severe approximation is often made, in which it is assumed that the system can be decomposed into a set of independent harmonic oscillators. Reliance on this often crude approximation is one of the primary reasons that theoretical prediction of isotope fractionation has lagged behind experiment.

A class of systems for which one might expect the harmonic approximation to perform most poorly is in the fractionation of ionic species between solid and liquid phases, particularly when the ionic species are relatively light. In order to illustrate this problem with the harmonic approximation, we have considered the fraction of Li+ between aqueous solution and phyllosilicate materials, where we find that the harmonic approximation overestimates isotope fractionation factors by as much as 30%. Lithium is a particularly interesting species to examine, as natural lithium signatures provide information about oceanic crust alteration and continental weathering, and separation of lithium isotopes is of growing interest in the nuclear industry due to a need for pure 6Li and 7Li isotopes.

Moving beyond the harmonic approximation requires tackling the problem of performing exact quantum calculations, which can be performed using the Feynman path integral formulation of quantum statistical mechanics. In the path integral approach, a system of quantum particles is represented as a set of classical-like ring polymer chains, whose interparticle interactions are determined by the rules of quantum mechanics. Because a classical isomorphism exists between the true quantum system and the system of ring polymers, classical-like methods can be applied.

In recent years, we have developed efficient path integral approaches for the exact calculation of isotope fractionation, which we apply to the aforementioned Li+ ion fractionation case. We find that the calculations yield results that are in good agreement with both experimental data and natural observations. Importantly, path integral methods, being fully atomistic, allow us to identify the origins of anharmonic effects and to make reliable predictions at temperatures that are experimentally inaccessible yet are, nevertheless, relevant for natural phenomena.

**Figure.**(top left panel) Snapshot of a Path Integral Molecular Dynamics of Li+(aq) surrounded by 4 water molecules. (bottom left panel) Li-O distance distribution versus Li-O-Li angular distribution at 365K. (right panel) Isotopic fractionation properties of Li, for a liquid/mineral equilibrium, computed with the harmonic approximation (red), the TI-PIMD method (green) and obtained by experiments (black). The figure shows the importance of a fully anharmonic treatment (TI-PIMD) for calculating the isotopic fractionation factor of dissolved ionic species.Publication reference:

**Importance of a Fully Anharmonic Treatment of Equilibrium Isotope Fractionation Properties of Dissolved Ionic Species As Evidenced by Li+(aq)**Romain Dupuis, Magali Benoît, Mark E. Tuckerman and Merlin Méheut.

Accounts of Chemical Research (2017)

DOI: 10.1021/acs.accounts.6b00607

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