Mapping structures and dynamics with frequency-correlated diffusion exchange

Understanding molecular motion in diffusion-driven complex environments is
critical for designing sustainable materials and improving chemical
processes. Here, we introduce a multidimensional nuclear magnetic
resonance (NMR) method that captures how molecular populations exchange
across different dynamic regimes. By extending the modulated gradient
spin-echo (MGSE) technique to include frequency–frequency correlations,
our approach reveals diffusion pathways that are otherwise obscured in
heterogeneous systems. Implemented on a unilateral NMR magnet, the method
eliminates gradient pulsing constraints and accesses dynamics in the
kilohertz regime. We apply this technique to swelling and acid-catalyzed
deconstruction of crosslinked and linear polymers to observe how
structural heterogeneity evolves over time. By linking molecular motion to
topology and chemical state, we extract physical metrics such as fractal
surface dimensionality and reaction wavefront velocity–properties
inaccessible with standard diffusion measurements. This work expands the
capabilities of NMR for probing soft matter, with implications for polymer
recycling and materials design.