Proton Magnetic Resonance of Ammonia at Low Temperatures

Abstract

Proton relaxation times (T1, T2, T1ρ) were measured for liquid and solid ammonia from 50 to 280°K. Second moments (M2) were determined from the melting point (195°K) to 10°K. In the liquid T1 ≠ T2 due to spin–spin coupling of the proton and nitrogen-14. The spin–spin interaction is modulated by chemical exchange above 220°K which has an activation energy of 1.9 kcal mole−1. T1 decreases discontinuously by an order of magnitude at the melting point with decrease in temperature and reaches a minimum at 119°K. The activation energy for the threefold axis reorientation of the NH3 molecule is 2.41 kcal mole−1 and the intramolecular H–H distance is 1.63 Å. T1ρ decreases with increase in temperature from about 160°K to the melting point. This corresponds to self-diffusion in the solid with an activation energy of 4.94 kcal mole−1. M2 increases from 14 G2 above 90°K to 20 G2 below 70°K but does not attain the calculated rigid lattice value of 48 G2. This effect appears to be due to quantum-mechanical tunneling at low temperatures with a frequency of about 60 kHz. The self-diffusion observed in the solid is interpreted with reaction rate theory models. It is necessary to introduce a orientational transmission factor that is equal to 5 × 10−3 in order to account for the magnitude of the observed self-diffusion coefficients and jump times.