On Librational and Rotational Motions of Aromatic Rings in Layered Sn(IV) and Zr(IV) Phosphonate Materials: A Variable-Temperature ¹³C, ³¹P Solid-State NMR Study

According to the solid-state ¹³C, ³¹P NMR study and ¹³C chemical shift anisotropy (CSA) measurements, aromatic rings in the layered metal­(IV) phosphonate materials behave as low-energy rotors at rotation activation energy, E_act, of 1.4–3.0 kcal/mol. The rotational mechanism consists of 180° flips and librations around C(1)–C(4) axis. The amplitude of the librations, added to the flips, grows with temperature, shifting the reorientations toward rotational diffusion at high temperatures.