Dissolution thermochemistry of alkali metal dianion salts (M _2X_1, M = Li~+, Na~+, and K~+ with X = CO_3~(2-), SO_4~(2-), C _8H_8~(2-), and B_(12)H_(12)~(2-))

摘要

The dissolution Gibbs free energies (ΔG°_(diss)) of salts (M_2X_1) have been calculated by density functional theory (DFT) with Conductor-like Polarizable Continuum Model (CPCM) solvation modeling. The absolute solvation free energies of the alkali metal cations (ΔG _(solv)(M~+)) come from the literature, which coincide well with half reduction potential versus SHE data. For solvation free energies of dianions (ΔG_(solv)(X~(2-))), four different DFT functionals (B3LYP, PBE, BVP86, and M05-2X) were applied with three different sets of atomic radii (UFF, UAKS, and Pauling). Lattice free energies (ΔG_(latt)) of salts were determined by three different approaches: (1) volumetric, (2) a cohesive Gibbs free energy (ΔG _(coh)) plus gaseous dissociation free energy (ΔG_(gas)), and (3) the Born-Haber cycle. The G4 level of theory, electron propagator theory, and stabilization by dielectric medium were used to calculate the second electron affinity to form the dianions CO_3~(2-) and SO _4~(2-). Only the M05-2X/Pauling combination with the three different methods for estimating ΔG_(latt) yields the expected negative dissolution free energies (ΔG°_(diss)) of M _2SO_4. Salts with large dianions like M_2C _8H_8 and M_2B_(12)H_(12) reveal the limitation of using static radii in the volumetric estimation of lattice energies. The value of ΔE_(coh) was very dependent on the DFT functional used.