Water dynamics in deuterated gypsum, CaSO₄⋅2D₂O, investigated by solid state deuterium NMR

NMR relaxation theory and NMR lineshape calculations were used to characterize the rates of C₂ symmetry jumps of deuterium nuclei in partly deuterated gypsum powder. The experimental data consisted of variable temperature deuterium NMR powder line shapes and deuterium T₁ relaxation times. All of the Mathematica© notebooks used to simulate the spectra and match the experimental T₁ values are included as supplementary material, and are suitable templates for similar calculations on other systems. Our simulations show that the deuterium nuclei of D₂O in Gypsum undergo a two-site C₂ 180° jump about the D-O-D bisector angle of 54.8°. The jump rate stays in the fast motion regime down to about 218 K. Below 193 K the powder lineshapes change, the spectral intensities drop significantly, and the motion slows into the intermediate motion regime. The best fit quadrupole coupling constants (QCC's) vary between 216 kHz at the highest temperatures to 235 kHz at the lowest temperatures. The asymmetry parameters (ɳ) vary between 0.11 at the highest temperatures to 0.15 at the lowest temperatures. Knowledge of the C₂ jump rates allowed us to calculate activation parameters for the jumps, namely ΔH^‡ = 22 kJ/mol, and ΔS^‡ = -10 J/mol·K which indicate a non-spontaneous activation process, an activation energy of E_a = 23 kJ/mol, and a pre-exponential factor of A = 3.6 × 10¹². As expected, there was no evidence of quantum tunneling.