Exploring the first‐shell and second‐shell structures arising in the microsolvation of Li⁺ by rare gases

Abstract

An evolutionary algorithm was used to search for the low-energy structures of Li + Ar n and Li + Kr n (n = 1 − 14). Two functions were used to describe the interaction potential at the CCSD(T)/augcc-pVQZ level of theory: one is based on a sum of all pair potentials, whereas the other includes three-body interactions. In general, the global minimum structures are similar for both Li + Ar n and Li + Kr n . Modifications in the octahedral structure of the first solvation shell lead to a high-energy penalty. Conversely, the second solvation shell shows a panoply of minima with similar energies that are likely to be interconverted. Post-optimization at the MP2 level confirmed that, for n = 2 and 3, one has to include three-body terms in the potential to reproduce the low-energy structures. Additionally, MP2 calculations indicate that energy reorder of the global minimum structure observed for Li + Kr 8 is related to the Kr 3 Axilrod-Teller-Muto term included in the potential.ab initio calculations, clusters, evolutionary algorithm, geometry optimization, microsolvation | INTRODUCTIONSolvation is a complex phenomenon with great relevance for the broad area of chemical physics. It is intimately related with the interactions that can be established among solute and solvent molecules, although the contribution from entropic effects can only be correctly described when incorporating all degrees of freedom of the system. Two main theoretical approaches are followed in the study of solvation: (1) continuum solvation models that do not treat solvent molecules as individual entities; and (2) explicit microsolvation methodology, where the solvent molecules are successively added to the system. Although the continuum models are able to incorporate the polarization of the solvent, they cannot explicitly describe interactions with a strong directional character and, in this way, environments comprising several solvents, ionic solutes, and biological systems are difficult to be accounted for. To somehow reduce these drawbacks, continuum-solvent methods [1] have been improved to approximately incorporate the structure of the solvent, [2] as well as the formation of the cavity for solute and the dispersion interaction by Abbreviations: ATM, Axilrod-Teller-Muto; aug-cc-pVQZ, augmented correlation consistent with polarized-valence quadruple-zeta basis set.; BSSE, basis set superposition error; cc-pVQZ, correlation consistent with polarized-valence quadruple-zeta basis set; CCSD(T), couple cluster with singles and doubles and perturbated triples excitations; EA, evolutionary algorithm; MP2, second-order Moller-Plesset.

A thermodynamic view on the microsolvation of ions by rare gas: application to Li+ with argon

“…Accordingly, this results clearly show that the structure formed by Li + and its six nearest-neighbor Ar-atoms can be viewed as an independent sub-aggregate of the growing microsolvation cluster. This picture is compatible with a "rigid"-like ("fluid"-like) first (second) solvation shell, which was suggested in a previous global optimization study 39 .…”

“…Indeed, the strong increase of the Lindemann index (i.e., reaching values greater than 0.3) can be attributed to the large-amplitude motion of the argon atoms that are farther apart from Li + . This corroborates the idea of a "fluid"-like second solvation-shell, which has been suggested in our previous global optimization study 39 .…”

“…In the case of Li + Ar 12 , the PDF has two distinct peaks at T = 39 K, which correspond to the six argon atoms in the first solvation shell (the one at smaller distances) and the other six in the second solvation shell; the corresponding integrated-PDF curve shows that the two sets of six argon atoms are essentially localized at the corresponding equilibrium distances, i.e., the position of the two PDF-peaks. We should note that the peak associated to the second solvation shell is broader than the one for the first shell, which is consistent with a more fluid-like behavior, as we had anticipated in a previous work 39 . At T = 70 K, the smallest peak becomes broader due to a larger mobility of the secondshell atoms, while the straight peak associated to the first shell keeps essentially unchanged.…”

“…It is important to underline, at this stage, the common feature associated to the "structural transition" occurring in all of the three clusters, which we have just discussed: Li + Ar 8 , Li + Ar 9 and Li + Ar 10 show low-energy minima (other than the global one) that were already reported in our previous global optimization study 39 . Nonetheless, the existence of low-energy minima cannot be seen as a sine qua non condition to observe a c V -peak at low temperatures.…”

“…Indeed, the relevant interactions depend essentially on the polarizability of the rare-gas, as well as on the charge and radius of the ion. In this context, we have performed global optimization studies for describing the microsolvation of Li + by argon [36][37][38] and krypton 38 , or a mixture of both 39 .…”