Framework mobility in the metal–organic framework crystal IRMOF-3: Evidence for aromatic ring and amine rotation

The framework motions in IRMOF-3 (Zn4O(BDC-NH2)3), where BDC-NH2 represents 2-amino-1,4-benzenedicarboxylate, have been investigated with 1H NMR relaxation measurements. Isotopic enrichment of the 2-amino group with 15N was critical in elucidating the lattice dynamics and enhancing spectral resolution. These results indicate a low energy process associated with rotation of the amino group, with an activation energy of 1.8 ± 0.6 kcal/mol, and full 180° rotation of the phenylene group in the BDC-NH2 moiety with an activation energy of 5.0 ± 0.2 kcal/mol. A relatively low pre-exponential factor for amine rotation (1.3 × 107 s−1) is tentatively associated with the need to break a hydrogen bond as the rate-limiting step. Both amine rotation and the aromatic ring flip occur at frequencies that provide an effective relaxation mechanism for the 99.6% natural abundance quadrupolar 14N in the amino group. Dipolar coupling of the 14N to adjacent spin-½ nuclei (both 1H and 13C) occurs not only in the static sample but also in the MAS experiments at the 7 T magnetic field used in this study. As a result, the spin dynamics and the cross-polarization dynamics are affected, resulting in spectral broadening. In the MAS experiments, isotopic replacement of the natural abundance 14N with 15N significantly improves resolution of the 15N spectra as well as in the 1H and 13C spectra.