Spin-Lattice Relaxation of Protons in Ammonium Sulfate: Influence of the Natural Abundance of ¹⁷O Nucleus on T_1ρ

Abstract

Proton spin-lattice relaxation time T_1ρ in the rotating frame was measured in ammonium sulfate between 370 K and 118 K. The minimum value of T_1ρ (2∼4 sec) in the paraelectric phase of ammonium sulfate is attributed to the reorientation of SO₄⁻² which modulates the dipolar interaction between ¹H and the natural abundance of ¹⁷O spins (0.037%) and not the effect of the ferroelectric mode proposed by Knispel et al. and Nordland et al. The activation energy E_a∼4 kcal/mol for the reorientation of SO₄⁻² was determined.