Resolution of motions in the three solid phases of methylammonium chloride (CH3NH3Cl) by NMR

Abstract

The α, β, and γ phases of solid methylammonium chloride (CH₃NH₃Cl) and its partially deuterated analogs (CD₃NH₃Cl and CH₃ND₃Cl) were investigated using temperature dependent static and rotating frame proton spin‐lattice relaxation times, T₁ and ${\mathrm{T\hspace{0.17em}}}_{\text{1ρ}}$ , respectively. Broadline proton magnetic resonance studies were performed in the supercooled γ phases of two of the substances. Discontinuities in the spin‐lattice relaxation plots were observed at the reported $\mathrm{\beta \; \to \; \gamma }$ and $\mathrm{\gamma \; \to \; a}$ phase transitions for CH₃NH₃Cl. It was found in our experiments that partial deuteration of this salt causes the $\mathrm{\beta \; \to \; \gamma }$ transition temperature to shift to higher temperatures. The motions in the three modifications of these substances were analyzed using the spin‐lattice relaxation data and the second moments for the supercooled γ phase. A resolution of the CH₃ and ${\mathrm{NH}}_3^{+}$ rotations about the $\mathrm{C–N}$ axes was accomplished in the γ and β phases. The onset of CH₃ reorientations in the γ and β phases occurs at lower temperatures than those for ${\mathrm{NH}}_3^{+}$ , while the frequencies of these motions are quite close to each other in the β forms and further apart in the γ modifications. In the α phases CH₃ and ${\mathrm{NH}}_3^{+}$ seem to rotate relatively fast about the $\mathrm{C–N}$ axes and more or less as a unit. The various motional parameters were derived from calculated fits to our relaxation time data. The NMR results pertaining to the three phases of these compounds are discussed and related to other studies.