Raman study of tetraglyme–LiClO4 solvate structures

Abstract

Lithium perchlorate has been dissolved in tetra(ethylene glycol) dimethyl ether (tetraglyme or G4) to obtain (G4)_n–LiClO₄ mixtures with 1/2 ≤ n ≤ 80. The conformational changes and ionic interactions occurring in the crystalline and liquid phases as a function of temperature and salt concentration have been investigated by Raman spectroscopy. This spectroscopic characterization is based on previous vibrational assignments of “free” and coordinated perchlorate anions and on structural results obtained for the (G4)₁:LiAsF₆ and (G4)_1/2:LiBF₄ crystalline complexes. In crystalline (G4)₁:LiClO₄, the ligands adopt, as in (G4)₁:LiAsF₆, a double-helix dimer structure in which two six-coordinated Li⁺ cations are coordinated by two G4 molecules forming solvent separated ion pairs (SSIP) with the two anions. The two corresponding anions, however, are spectroscopically distinct as shown by their ν₁ vibrations at ∼931 and 928 cm⁻¹. In the (G4)_n–LiClO₄ liquid phase for n ≥ 1, an equilibrium occurs between SSIP and monodentate contact ion pairs (CIP-I) characterized by ν₁ components at 932 and 939 cm⁻¹, respectively. For n < 1, a new ν₁ component appears at 945 cm⁻¹ in both the crystalline and liquid phases. It suggests the formation of aggregate interactions consisting of linear [Li⁺⋯(ClO₄⁻)₂]_n ionic chains.