An Evaluation of the Overall Rotational Correlation Times of Tetraalkylammonium Ions in Solution. 13C NMR Relaxation Study

Abstract

Abstract The 13C NMR spin-lattice and spin–spin relaxation times (T1 and T2), and the nuclear Overhauser effect (NOE) were measured for tetraalkylammonium ions (R4N+, R = C2H5 (Et), n-C3H7 (n-Pr), n-C4H9 (n-Bu), and n-C5H11 (n-Pen)) in the concentrated aqueous solutions of their bromides or chlorides. A minimum was observed in the temperature dependence of T1 for the α-carbon in each solution. The value was 1.2—1.4 times higher than that predicted assuming only the isotropic overall rotation of the R4N+ ion. This increase in the T1 minimum values was ascribed to the rotations around the N–α-C bonds with an restriction in the azimuthal angle for each bond. The ranges where the N–α-C bonds could rotate with the time scales much less than those for the overall rotations were determined by the values of the T1 minima for the corresponding R4N+ ions as ±20°, ±17°, ±20°, and ±20° centering the gauche conformations, respectively, for Et4N+, n-Pr4N+, n-Bu4N+, and n-Pen4N+. These values well reproduced the T2 and NOE values observed at various temperatures in the concentrated aqueous solutions of the R4N+ halides. By using the thus obtained ranges of the azimuthal angles, the overall rotational correlation times in the dilute aqueous solutions (0.01 mol kg−1) were also determined. The obtained rotational correlation times were compared with those calculated using the Stokes–Einstein–Debye equation and the effect of the hydrophobic hydration was discussed.