Dissolution Thermochemistry of Alkali Metal Dianion Salts (M2X1, M = Li+, Na+, and K+ with X = CO32-, SO42-, C8H82-, and B12H122-)

The dissolution Gibbs free energies (Delta G degrees(diss)) of salts (M2X1) 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 (Delta G(solv)(M+)) come from the literature, which coincide well with reduction potential versus SHE data. For solvation free energies of dianions (Delta G(solv)(X2-)), 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 (Delta G(latt)) of salts were determined by three different approaches: (1) volumetric, (2) a cohesive Gibbs free energy (Delta G(coh)) plus gaseous dissociation free energy (Delta G(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 CO32- and SO42-. Only the M05-2X/Pauling combination with the three different methods for estimating Delta G(latt) yields the expected negative dissolution free energies (Delta G degrees(diss)) of M2SO4. Salts with large dianions like M2C8H8 and M2B12H12 reveal the limitation of using static radii in the volumetric estimation of lattice energies. The value of Delta E-coh was very dependent on the DFT functional used.