Structural Dynamics by NMR in the Solid-State: The Unified MOMD Perspective Applied to Organic Frameworks with Interlocked Molecules.

The microscopic-order-macroscopic-disorder (MOMD) approach for NMR lineshape analysis has been applied to the University of Windsor Dynamic Materials (UWDM) of type 1, 2, α-3, β-3 and 5, which are metal-organic frameworks (MOFs) comprising mobile mechanically-interlocked molecules (MIMs). The mobile MIM components are selectively deuterated crown ethers - 24C6, 22C6 and B24C6. They serve as probe for internal mobility, described in MOMD by an effective dynamic mode characterized by a diffusion tensor, R, a restricting/ordering potential, u, expanded in the Wigner rotation matrix elements, DL0K, and features of local geometry. Experimental 2H lineshapes are available over 220 K (on average), and in some cases 320 K. They are reproduced with axial R, u given by the terms D200 and D20|2|, and established local geometry. For UWDM of type 1, β-3 and 5, where the MIM resides in a relatively spacious void, u is in the 1-3 kT, R∥ in the (1.0-2.5)×106 s-1, and R⊥ in the (0.4-2.5)×104 s-1 range; the deuterium is bonded to a carbon atom with tetrahedral coordination character. For UWDM of type 2 and α-3, where the MIM resides in a much tighter space, a substantial change in the symmetry of u, and the coordination character of the 2H-bonded carbon, are detected at higher temperatures. The activation energies for R∥ and R⊥ are characteristic of each system. The MOMD model is general; effective/collective dynamic modes are treated. The characteristics of motion, ordering and geometry are physically well-defined; they differ from case to case in extent and symmetry, but not in essence. Physical clarity and consistency provide new insights. A previous interpretation of the same experimental data used models consisting of collections of independent simple motions. These models are specific to each case and temperature. Within their scope generating consistent physical pictures and comparing cases is difficult; possible collective modes are neglected.