Solvent Effect on Rotational Motion of Perchlorate Ion

Abstract

The rotational relaxation times of perchlorate ion, τ2r, in 15 solvents and at various temperatures were determined from the measurements of the 17O NMR spin−lattice relaxation times. The obtained τ2r values were much smaller than those predicted from the hydrodynamic model (Stokes−Einstein−Debye, SED, equation). Comparison between the observed solvent dependence of the τ2r value and those predicted by the continuum models, including the SED hydrodynamic model, the Hubbard−Onsager−Felderhof (HOF) electrohydrodynamic model, and the Alavi−Waldeck (AW) dielectric friction model for multipole rotation, demonstrated that solvent viscosity is an expedient indicator for representing the overall trend of the solvent dependence of the rotational relaxation time; the observed τ2r values showed a fractional power dependence on the viscosity (i.e., τ2r ∝ ηα, where α ∼ 0.25 and η is the solvent viscosity). Site−site interactions between the perchlorate ion and solvent molecules, however, provided a significant effect on the perchlorate rotation in some solvents with a large imbalance of the electronic donor and acceptor properties (e.g., hexamethylphosphoric triamide). The values for τ2r calculated for alcohols (methanol, ethanol, and n-propanol) according to the HOF and the AW models were appreciably overestimated to a greater degree than those in the other solvents, and this result was ascribed to a predominant contribution from the interactions with the hydroxyl groups of the alcohols. The result of the analysis for the solvent dependence of the perchlorate rotation by the electrohydrodynamic model was also compared with that for the perchlorate translation, and the difference in the validity of the continuum models for the ionic rotation and the translation was discussed.