Intranodal Proton Hopping in Defect-Free UiO-66: Evidence from Operando NMR and ML-Accelerated Enhanced Sampling

Proton conduction in metal–organic frameworks (MOFs) is frequently attributed to defects or guest molecules, leaving the intrinsic behavior of pristine frameworks unresolved. Here, we combine operando 1H MAS NMR, DFT-NMR calculations, and machine learning–accelerated molecular dynamics simulations to characterize water–framework interactions and proton dynamics in defect-free UiO-66. Operando NMR shows that μ3–OH groups remain intact up to 250 °C but rearrange to yield a distinct ~6.4 ppm resonance and EXSY cross-peaks, providing direct evidence of intranodal proton hopping. DFT-NMR confirms the assignments of μ3–OH and μ3–O–H species, while simulations not only reproduce water adsorption isotherms, but also resolve a stepwise, linker-mediated proton transport mechanism. These findings demonstrate that UiO-66 exhibits intrinsic proton mobility, and they establish a framework-mediated mechanism with broad implications for proton transport in porous solids, membranes, and biological systems.