Reorientation of NH₄⁺in ammonium salts studied with NMR relaxation and inelastic neutron scattering

The following activation energies for NH 4 + reorientation have been derived from proton spin-lattice relaxation T 1 and T 1 rho : E a =9.4 kcal mol -1 in NH 4 F, E a =8.0 kcal mol -1 in NH 4 CH 3 COO, E a =6.8 kcal mol -1 in (NH 4 ) 2 (COO) 2 .H 2 O, E a =6.1 kcal mol -1 in (NH 4 ) 2 CO 3 , E a =3.7 kcal mol -1 in (NH 4 ) 2 CrO 4 , and E a =2.6 kcal mol -1 in (NH 4 ) 2 S 2 O 3 . The following frequencies for NH 4 + torsional oscillations have been found from inelastic neutron scattering: nu v =450 cm -1 for (NH 4 ) 2 (COO) 2 .H 2 O, nu v =330 cm -1 for (NH 4 ) 2 S 2 O 3 and nu v =250 cm -1 for (NH 4 ) 2 Ce(NO 3 ) 6 . The relation between E a and nu v has been calculated for a cubic potential with an improved zero-point energy correction. Comparisons between E a and nu v for these and other ammonium salts show that the reorientation is by 90 degrees around twofold axes in most high-barrier salts.