Asymmetry in three-site relaxation exchange NMR

Area of Science:

• Physical Chemistry
• Chemical Engineering
• Computational Chemistry

Background:

• Asymmetric peak integrals in 2D relaxation maps suggest non-reciprocal exchange between sites.
• This observation contradicts detailed balance, a fundamental principle of thermodynamic equilibrium.
• Understanding particle exchange dynamics is crucial for various chemical processes.

Purpose of the Study:

• To investigate the impact of topological constraints on particle exchange between three relaxation sites.
• To model and understand the mechanisms behind observed circular molecular flow.
• To explore potential applications in enhancing heterogeneous catalysis.

Main Methods:

• Monte Carlo simulations were employed to model confined diffusion on a 2D checkerboard grid.
• Confined gas diffusion was also simulated to explore different topological constraints.
• Analysis focused on particle exchange pathways and density variations within the simulated pore.

Main Results:

• Simulations revealed density variations across the pore, indicating non-uniform particle distribution.
• Up to 1% of molecules were observed to move in circular paths between relaxation pools.
• This circular motion was found to be thermodynamically silent, meaning it does not affect overall equilibrium.

Conclusions:

• The study confirms that multi-site exchange maps can be symmetric even with circular flux, challenging traditional interpretations.
• Coherent flux is attributed to stochastic pore resonance linked to diffusion eigenmodes.
• Experimental control of this phenomenon using external fields could offer a novel route for enhancing heterogeneous catalysis.