Abstract
Alkalides, the alkali metals in their −1 oxidation state, represent some of the largest and most polarizable atomic species in condensed phases. This study determines the solvation environment around the sodide anion, Na–, in a system of co-solvated Li+. We present isotopically varied total neutron scattering experiments alongside empirical potential structure refinement and ab initio molecular dynamics simulations for the alkali–alkalide system, LiNa·10MeNH2. Both local coordination modes and the intermediate range liquid structure are determined, which demonstrate that distinct structural correlations between cation and anion in the liquid phase extend beyond 8.6 Å. Indeed, the local solvation around Na– is surprisingly well defined with strong solvent orientational order, in contrast to the classical description of alkalide anions not interacting with their environment. The ion-paired Li(MeNH2)4+·Na– species appears to be the dominant alkali–alkalide environment in these liquids, whereby Li+ and Na– share a MeNH2 molecule through the amine group in their primary solvation spheres.
Highlights
The stability of group 1 metals in a formally negative oxidation state in polar solvents is rare, due to both the highly reducing nature of these alkalide anions or their being unstable with respect to dissociation to yield solvated cations and electrons such as in the case of metal−ammonia solutions.[1,2]
The sodide anion is the same approximate volume as that occupied by a solvated electron in dilute metal−amine solutions (∼5 Å diameter), yet comparing the neutron scattering spectra of Li+Na−· 10MeNH2 and Li+e−·10MeNH2, we see that the sodide templates the long-ranged liquid structure to a far greater extent than does the solvated electron
The lithium environment is consistent with that found for Li−MeNH2 liquids in both the electrolytic and metallic phases, and found in gas-phase investigations, being tetra-coordinate about the cation through the amine nitrogen
Summary
The stability of group 1 metals in a formally negative oxidation state in polar solvents is rare, due to both the highly reducing nature of these alkalide anions or their being unstable with respect to dissociation to yield solvated cations and electrons such as in the case of metal−ammonia solutions.[1,2] As such, the majority of alkalide systems are formed in nonpolar solvents via the dissolution of the alkali metal in the presence of a chelate, such as crown ether or cryptand.[3]. Structural models for the liquid system were refined to each unique neutron dataset simultaneously using the empirical potential structure refinement (EPSR) software.[20] The exact density of the alkalide solutions is not known but was estimated from the work of Pyper and Edwards to be 0.09275 atoms Å−3.8 This value was consistent with the total measured neutron scattering power of the samples. Trajectory data after a 0.5 ps equilibration period of the fully relaxed simulation was considered for analysis
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