Fig. 2.

Linker dynamics upon melting. a The ¹³C MAS (black solid line) and ¹H-¹³C Lee–Goldburg cross-polarization MAS (red solid line) NMR spectra of crystalline ZIF-76-mbIm. Tentative assignment of individual signals is based on literature data for isotropic chemical shifts from the two molecular fragments. The assignment is further confirmed by comparing the resonances in the ¹³C MAS and ¹H-¹³C Lee–Goldburg cross-polarization MAS NMR spectra. In the latter, only resonances belonging to those carbons with hydrogens attached can be observed. b The ¹³C-detected 2D spin-diffusion NMR spectrum, recorded with a mixing time of 10 ms. Cross peaks due to polarization transfer between methyl protons of 5-mbIm on one hand and H1’ protons of Im and H1 protons of 5-mbIm on the other are denoted. c, d Spin-diffusion curves from the ¹³C-detected measurements on ZIF-76-mbIm and a_gZIF-76-mbIm (squares: inter-linker polarization transfer between methyl protons of 5-mbIm and H1’ protons of Im; circles: intra-linker polarization transfer between methyl and H1 protons of 5-mbIm). Solid lines in c and d indicate the initial slopes of the two spin-diffusion curves for crystalline ZIF-76-mbIm (i.e., lines in d are equal to lines in c)