The reason why HAlCl(4) acid does not exist.

The explanation of the hypothetical HAlCl(4) acid instability is provided on the basis of theoretical considerations supported by ab initio calculations. The equilibrium structures of LiAlCl(4), NaAlCl(4), and KAlCl(4) salts were examined and compared to that of their corresponding parent acid. The process of formation of the representative NaAlCl(4) salt was analyzed, and the interaction energy between NaCl and AlCl(3) was estimated to be ca. 55 kcal/mol while that between HCl and AlCl(3) (when the HAlCl(4) species is formed) was calculated to be smaller by an order of magnitude (ca. 8 kcal/mol). The hypothetical HAlCl(4) acid was identified as an HCl...AlCl(3) adduct (with the hydrogen chloride tethered weakly to the quasi-planar aluminum chloride molecule). The electron affinity of the neutral AlCl(4) superhalogen molecule was found to be the factor determining the ability to form a stable compound of MAlCl(4) type.