Rotation and solvation of ammonium ion.

From nitrogen-15 spin-lattice relaxation times and nuclear Overhauser enhancements, the rotational correlation time (c) for (15)NH(4)(+) was determined in a series of solvents. Values of (c) range from 0.46 to 20 picoseconds. The solvent dependence of (c) cannot be explained in terms of solvent polarity, molecular dipole moment, solvent basicity, solvent dielectric relaxation, or solvent viscosity. The rapid rotation and the variation with solvent can be accounted for by a model that involves hydrogen bonding of an NH proton to more than one solvent molecule in a disordered solvation environment.