The proton free induction decay f(t) of ammonium selenate is measured from temperatures of no apparent effects of lattice motion (−185°C) upon f(t), to substantial motional effects (−145°C) upon f(t). Our measured results are consistent with a classical hindered rotational motion of the ammonium group. The deviation of f(t) from the rigid-lattice fid fo(t) can be expanded in a Taylor series as
From the Δ1f(t) data, a3 and a4 are derived, and they are well fitted by the simple relations
where ν is the jump frequency of the rotational motion, T1r is the rotating-frame spin-lattice relaxation time, and ω1 the strength of the rotating magnetic field in frequency units. α, C3 and C4 are calculable constants of order unity.