Water Modulated Framework Flexibility in NH2-MIL-125: Highlights from 13C Nuclear Magnetic Resonance

The influence of adsorbed water on the dynamics of the organic linker 1,4-benzenedicarboxylate (BDC) in the metal organic framework NH2-MIL-125 was examined by applying 13C Nuclear Magnetic Resonance (NMR) spectroscopy on samples loaded with different amounts of water. In particular, the analysis of (i) cross-polarization (CP) 13C Magic Angle Spinning (MAS) NMR spectra in terms of chemical shift and line width of the carbon signals, (ii) variable contact time 13C CP-MAS experiments, and (iii) longitudinal 13C relaxation times indicated that, upon hydration, a dynamic process occurring on the microseconds timescale accelerates. This process could be identified with the rotation of the BDC benzene ring about its C2 axis, with water competing with the carboxylic oxygen for hydrogen bonding with the aminic group. Other motions occurring at frequencies on the order of the 13C Larmor frequency, i.e. 75 MHz, which contribute to the flexibility of the three-dimensional network, were detected, and identified with the twisting, libration and translation of the BDC linker.