Studies of Ferroelectric Solids by Magnetic Resonance. XVIII. Proton and Deuteron Resonance of Thiourea

Abstract

The deuteron nuclear magnetic resonance of thiourea‐d₄ has been studied in the paraelectric Solid V at 300°K and above and in the ferroelectric Solid I at 120°K. The electric field gradient (EFG) tensors at 300°K demonstrate that the molecules are undergoing rapid 180° flip motion about the C–S bond as deduced earlier from proton second moments by Emsley and Smith. Complex deuteron spectra were observed at 120°K and fully analyzed. The quadrupolar coupling constant $e^2\mathrm{qQ\hspace{0.17em}/\hspace{0.17em}h}$ and asymmetry parameter $\eta$ for the inner and outer deuterons are $$\begin{array}{ccc}\mathrm{inner:}& e^2\text{qQ\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}212\hspace{0.17em}±\hspace{0.17em}3}\mathrm{kHz},& \text{η\hspace{0.17em}=\hspace{0.17em}0.154\hspace{0.17em}±\hspace{0.17em}0.007,}\\ \mathrm{outer}:& e^2\text{qQ\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}207\hspace{0.17em}±\hspace{0.17em}3}\mathrm{kHz},& \text{η\hspace{0.17em}=\hspace{0.17em}0.180\hspace{0.17em}±\hspace{0.17em}0.016.}\end{array}$$ The orientation of the principal coordinate systems of molecules 1 and 2 for the inner deuterons confirm the ferroelectric structure proposed by Goldsmith and White. The $x$ and $z$ principal axes of the outer deuterons deviate by about 10° from the plane defined by the heavy atoms. This appears to be due to an actual noncoplanarity of the outer N–D bonds with the plane of the molecule as indicated by earlier electron and neutron diffraction studies. Proton relaxation times $T_1$ and $T_{\text{1ρ}}$ have been measured from 240 to 420°K. $T_1$ reaches a minimum near 400°K and $T_{\text{1ρ}}$ a corresponding minimum near 270°K which are assigned to the 180° flip process. Near 330°K a secondary minimum in $T_{\text{1ρ}}$ is observed which is assigned to a ferroelectric mode process associated with Solid I. Below 290°K the deuteron resonance lines of the outer deuterons are observed to split into two components separated by 2.2 kHz. This splitting is ascribed to a slowing down of another motional process which involves a small change of the quadrupolar coupling constant due to a small change in the orientation of the outer N–D bonds.