Beyond Relaxometry: ¹H TD-NMR Methods for Probing Molecular Structure and Dynamics in Complex Materials

Over the past years, Time-Domain Nuclear Magnetic Resonance (TD-NMR) at low magnetic fields has emerged as a versatile and widely used technique for a broad range of applications. Although it was primarily known for its use in relaxation studies, recent technological advances have significantly broadened the scope of TD-NMR, leading to widespread use in materials science, as well as in the food, pharmaceutical, and oil industries. In particular, the use of proton-based TD-NMR (¹H TD-NMR) in solid and soft materials has gained prominence. In these systems, through-space magnetic dipolar couplings between ¹H nuclear spins play a crucial role, making the NMR signals highly sensitive to molecular packing and mobility. This presentation focuses on a class of ¹H TD-NMR experiments that go beyond conventional relaxometry, including methods that explore local magnetic susceptibility variations as sources of internal field gradients, as well as techniques such as dipolar echoes, dipolar filters, and multiple quantum correlation experiments. Applications of these methods include the characterization of molecular motions, crystallinity, and crosslink density in polymers; the analysis of mesoporous structures via NMR cryoporometry; the monitoring of ion adsorption in biosorbents; and the study of water retention and fertilizer absorption in soils.