Abstract: The associated species and process in the NaClO₄ solution were investigated at the HF/6-31+G* level. The ClO₄^- ions binding with hydrated Na⁺ ions can form solvent-shared ion pairing, contact ion pairing and multiple(including triple) ion clusters. When there are abundant water molecules, the regular octahedral hexacoordinated structure of Na⁺ ion can be kept while the irregular penta- and tetra-coordinated structures will form with lacking water. The ν₁-ClO₄^- frequencies in the species above are 4.7, 8.5 and 14—17 cm^-1 larger than the reference value in hydrated ClO₄^-, which is corresponding with the results in experiment after multiplied by a scale factor. The associated process in the NaClO₄ solution can be expressed as “free hydrated ion→solvent-shared ion pairing→mono-dentate contact ion pairing→cation-anion-cation triple ion cluster→catenulate multiple ion cluster→reticular multiple ion cluster→amorphous crystal”. The process is not analogous with those in the Mg(NO₃)₂, Mg(ClO₄)₂ and NaNO₃ systems but the MgSO₄ system.

Key words: Ab-initio, Sodium perchlorate, Associated species, Associated process