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

Abstract

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 Ea = 23 kJ/mol, and a pre-exponential factor of A = 3.6 × 1012. As expected, there was no evidence of quantum tunneling.