Solvent Effect on Rotational Correlation Times of Symmetric Tetraalkylammonium Ions

The overall rotational correlation times of symmetric tetraalkylammonium ions, R ₄N⁺ (R = ethyl, n-propyl, n-butyl, and n-pentyl), in various solvents were determined by the measurements of the ¹³C NMR spin-lattice relaxation times and the nuclear Overhauser enhancement factors of each α-carbon, considering the contribution of the internal rotation around the N—C bond. Except in water, the observed solvent dependencies of the rotational correlation times, τ_r, showed good correlations with those predicted from an electrohydrodynamic (Hubbard–Onsager–Felderhof) model. The correlation times of R ₄N⁺ increased as the size of the alkyl groups became larger. In the case of the n-Bu₄N⁺ and the n-Pen₄N⁺ ion, the τ _r values were similar to or even higher than those predicted by the HOF model under the stick hydrodynamic boundary condition, in spite of the fact that the ions were too small to allow the solvent to be regarded as a hydrodynamic or a dielectric continuum. A comparison of the results with the rotations of other pseudotetrahedral ions, e.g., tetraphenylborate and tetraphenylarsenium ions and with the translation of the R ₄N⁺ ions suggests that a considerable part of the rotational friction for R ₄N⁺ is brought about by pushing aside the solvent in the spaces between the alkyl groups of R ₄N⁺. A significant slowing in the rotation in water was observed for the n-Pr₄N⁺, n-Bu₄N⁺, and n-Pen₄N⁺ions; the extent of this effect increased with increasing size of the alkyl group. The increase in friction was related to the hydrophobic hydration of the R ₄N⁺ ions.