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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Physical Chemistry

Background:

• Metal-organic frameworks (MOFs) with mobile mechanically interlocked molecules (MIMs) exhibit complex dynamics.
• Previous analyses using independent simple motion models lacked consistency and generality.

Purpose of the Study:

• To apply the microscopic-order-macroscopic-disorder (MOMD) approach to analyze the 2H NMR lineshapes of various UWDM MOF types.
• To provide a unified and physically consistent model for the motion, ordering, and geometry of mobile MIMs in MOFs.

Main Methods:

• Analysis of experimental 2H NMR lineshapes for five UWDM MOF types (1, 2, α-3, β-3, 5) over a temperature range (220-320 K).
• Modeling molecular motion using an effective/collective dynamic mode within the MOMD framework, incorporating a diffusion tensor and ordering potential expanded in Wigner rotation matrix elements.
• Characterization of the diffusion tensor (R∥, R⊥) and ordering potential (u) based on experimental data and local geometry.

Main Results:

• The MOMD model accurately reproduces experimental 2H NMR lineshapes for all studied UWDM types.
• Distinct differences in the ordering potential symmetry and carbon coordination were observed for MOFs with tighter spaces (types 2 and α-3) at higher temperatures.
• Activation energies for molecular motion were found to be characteristic for each specific MOF system.

Conclusions:

• The MOMD approach offers a general and physically insightful method for analyzing complex molecular dynamics in MOFs.
• This unified model overcomes limitations of previous case-specific and temperature-dependent models.
• The MOMD framework reveals consistent underlying principles of motion, ordering, and geometry across different MOF systems.