The microsolvation of Li+ by both argon and krypton atoms has been studied based on a new potential energy surface that includes two- and three-body interactions; the potential terms involving the lithium ion were calibrated with CCSD(T)/aug-cc-pVQZ energies after being corrected for the basis-set superposition error. The structures of the Li+ArnKrm (n + ≤ m 20) clusters arising from global optimization show a first solvation shell preferentially occupied by krypton atoms. These binary-solvent microsolvation clusters are most stable when the total number of krypton (argon) atoms occupy the first (second) solvation shell.
Solvation is a ubiquitous phenomenon in chemistry, which has been treated under several perspectives from both experimental and theoretical sides. Perhaps the most detailed way to look at such a physical process is by using the microsolvation approach, where the solvent entities (atoms or molecules) are added stepwise to the solute. This leads to the formation of clusters that grow and whose structural and energetic properties explain about the way the solvent surrounds the solute. From the theoretical side, the major challenges to set up the study of microsolvation are 2-fold: (i) accurately describe the interaction potential, and (ii) efficiently optimize the clusters to discover the lowest-energy structure (and, eventually, other low-energy minima). In general, the former requires the use of state-of-the-art methods to perform electronic-structure calculations that are, then, employed in a least-squares fit 2 to an adequate analytical potential function. In turn, the latter is a very active area of research whose endeavor has given fruitful results, with the development of state-of-the-art global-optimization algorithms that have been already applied in the study of microsolvation clusters. The most used approach comprises using state-of-the-art techniques to carry out global geometry optimization of the microsolvation clusters modeled with an adequate analytical interaction potential that has been previously fitted to ab initio data the obtained minima structures are, then, reoptimized at a higher level of theory to get more accurate energy values. Although alternative approaches have been proposed they are computationally more expensive. Among the great number of publications related to the study of solvation, those involving the formation and characterization of small ionic clusters have attracted much interest. Indeed, a great variety of clusters with ionic species and atomic and molecular systems, respectively, as solutes and solvents have been the subject of recent studies. These charged clusters can be used to gain detailed insight into solvation phenomena at the molecular level by establishing the bridge connecting the isolated ion in the gas-phase and the solvated ion in solution. In previous studies, we have assessed the importance of including up to three-body interactions for describing the microsolvation of Li+ with either argon or krypton atoms. Here, we explore the ability of a similar potential model to reproduce the main structural features of microsolvation clusters that contain Li+ and atoms of both argon and krypton. To our knowledge, this is one of the first works on the solvation of an alkaline ion by mixtures of rare gases. In contrast, heterogeneous rare-gas clusters have received the attention of theoreticians and experimentalists. One of the central issues discussed in such studies is the preferential site occupancy of the different types of atoms in the rare-gas aggregates. In the case of large heterogeneous Ar−Kr clusters, though it can occur that some krypton atoms are present on the surface and some argon atoms in the bulk, the most favorable structure shows preferentially krypton atoms in the bulk and argon atoms on the surface.36 In this paper, we investigate solvent-selectivity effects in heterogeneous clusters resulting from the microsolvation of Li+ with both argon and krypton. To achieve that purpose, we have employed an evolutionary algorithm to perform a global optimization study on the Li+ Ar Kr clusters. More specifically, we have investigated the preferential location of argon and krypton atoms around the lithium ion and whether the two rare gases are mixed together or, conversely, tend to be separated when forming the microsolvation cluster.
