AN EVALUATION OF THE OVERALL ROTATIONAL CORRELATION TIMES OF TETRAALKYLAMMONIUM IONS IN SOLUTION - C-13 NMR RELAXATION STUDY

The C-13 NMR spin-lattice and spin-spin relaxation times (T-1 and T-2) , and the nuclear Overhauser effect (NOE) were measured for tetraalkyl ammonium ions (R4N+, R= C2H5 (EI), n-C3H7 (n-Pr), n-C4H9 (n-Bu), and n -C5H11 (n-Pen)) in the concentrated aqueous solutions of their bromide s or chlorides. A minimum was observed in the temperature dependence o f T-1 for the alpha-carbon in each solution. The value was 1.2-1.4 tim es higher than that predicted assuming only the isotropic overall rota tion of the R4N+ ion. This increase in the T-1 minimum values was ascr ibed to the rotations around the N-alpha-C bonds with an restriction i n the azimuthal angle for each bond. The ranges where the N-alpha-C bo nds could rotate with the time scales much less than those for the ove rall rotations were determined by the values of the T-1 minima for the corresponding R4N+ ions as +/-20 degrees, +/-17 degrees, +/-20 degree s, and 1200 centering the gauche conformations, respectively, for Et4N +, n-Pr4N+, n-Bu4N+, and n-Pen(4)N(+). These values well reproduced th e T-2 and NOE values observed at various temperatures in the concentra ted aqueous solutions of the R4N+ halides. By using the thus obtained ranges of the azimuthal angles, the overall rotational correlation tim es in the dilute aqueous solutions (0.01 mol kg(-1)) were also determi ned. The obtained rotational correlation times were compared with thos e calculated using the Stokes-Einstein-Debye equation and the effect o f the hydrophobic hydration was discussed.