Supersalts Na2AlX5 (X = F, Cl): Quantum Chemical Study of the Structure, Vibration Spectra and Thermodynamic Properties

Abstract

Theoretical investigation, prediction and design of novel supersalts Na2AlX5 (X = F, Cl) have been performed. The formation of the supersalts was considered through association reactions between different building blocks: Superalkalies and superhalogens, ionic (Na2X+ and AlX4−), as well as dimers Na2X2 and traditional salts AlX3. The optimization of geometrical structures, and determination of vibrational spectra of supersalts and their respective building blocks was carried out by the DFT/B3P86 and MP2 methods; The McLean-Chandler basis set and the extended basis set (cc-pVTZ for Na, Al; aug-cc-pVTZ for F, Cl) were used. Different possible geometrical configurations for the supersalts Na2AlX5 were considered. Consequently, two structures were proved to be isomers: A structure with two cycles in perpendicular planes with a tail (C2v symmetry) and a compact structure with a tail (Cs symmetry). Evaluation of their relative concentrations in equilibrium vapour was done, and the energies and enthalpies of the association reactions were determined. The enthalpies of formation ∆fH°(0) of gas-phase supersalts are: -2250±45 kJ×mol-1 (Na2AlF5) and -1380 ±20 kJ×mol-1 (Na2AlCl5).