Raman and density functional theory studies of solvation structure and ion association of NaClO4 in 1,2-propanediamine

- Solvation structure of NaClO4 in a polyamine was studied by Raman and DFT studies.
- The fraction of contact ion pairs increases up to 0.5 with increasing NaClO4.
- The mean solvation number around ions of -NH2 is 3.1 for high NaClO4 concentration.
- The primary solvation shell around ions consists of around two solvent molecules.

The solvation structure and ion association of NaClO4 in 1,2-propanediamine were studied over a wide concentration range by Raman spectroscopy and density functional theory calculations (DFT-B3LYP). The local structure of the primary solvation shell around the contact ion pair is discussed on the basis of Raman and DFT-B3LYP results. The concentration of the contact ion pair increases with increasing molality of NaClO4, and its fraction is ca. 50% near the solubility limit. The mean solvation number of -NH2 groups around the ion pair or free ions is 3.1 for the high NaClO4 concentration region. DFT-B3LYP results for the corresponding ion pair and solvated complexes are consistent with the experimental data. The primary solvation shell around the ion pair consists of about two solvent molecules, and ion-solvent or ion-ion interactions are dominant over intrinsic hydrogen bonds between solvent molecules in highly concentrated solutions.