SOLVENT EFFECT ON ROTATIONAL MOTION OF PERCHLORATE ION

The rotational relaxation times of perchlorate ion, tau(2r), in 15 sol vents and at various temperatures were determined from the measurement s of the O-17 NMR spin-lattice relaxation times. The obtained tau(2r) values were much smaller than those predicted fi om the hydrodynamic m odel (Stokes-Einstein-Debye, SED, equation). Comparison between the ob served solvent dependence of the tau(2r) value and those predicted by the continuum models, including the SED hydrodynamic model, the Hubbar d-Onsager-Felderhof (HOF) electrohydrodynamic model, and the Alavi-Wal tieck (AW) dielectric friction model for multipole rotation, demonstra ted that solvent viscosity is an expedient indicator for representing the overall trend of the solvent dependence of the rotational relaxati on time; the observed tau(2r) values showed a fractional power depende nce on the viscosity (i.e., tau(2r) proportional to eta(alpha), where alpha similar to 0.25 and eta is the solvent viscosity). Site-site int eractions between the perchlorate ion and solvent molecules, however, provided a significant effect on the perchlorate rotation in some solv ents with a large imbalance of the electronic donor and acceptor prope rties (e.g., hexamethylphosphoric triamide). The values for tau(2r) ca lculated 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 de pendence of the perchlorate rotation by the electrohydrodynamic model was also compared with that for the perchlorate translation, and the d ifference in the validity of the continuum models for the ionic rotati on and the translation was discussed.