Probing the Dielectric Transition and Molecular Dynamics in the Metal–Organic Framework [(CH 3 ) 2 NH 2 ]Mg(HCOO) 3 Using High Resolution NMR

We present a detailed multinuclear ( 13 C, 15 N, 25 Mg) NMR study on a metal–organic framework material with a dimethylammonium (DMA + ) cation, [(CH 3 ) 2 NH 2 ]Mg(HCOO) 3, that shows a dielectric phase transition at 270 K. The mechanism underlying this phase transition is not fully understood as there are contrasting reports attributing the phase transition to the order–disorder dynamics of the dimethylammonium cation in the cavity or to the contraction of the metal formate framework. In this work, we use high-resolution solid-state NMR to understand the mechanism of this phase transition by analyzing the motional dynamics of nitrogen and carbon. Spin–lattice relaxation time ( T 1 ) and BPP theory yield the correlation time for nitrogen hopping of 10 –9 s and activation energy of 28.62 kJ/mol. The T 1 behavior and the chemical shifts of 13 C and 15 N show that the phase transition has both order–disorder and displacive characteristics. 25 Mg NMR spectra show a change in line width with the change in DMA + cation dynamics, but there is no change in its chemical shift implying that its local environment remains the same.