A multi-field variable contact time study of CPMAS ¹³C NMR of cellulose nanocrystals

The characterization of hydrated polysaccharide assemblies by solid-state NMR (ssNMR) remains challenging because hydration modifies proton networks, molecular mobility, and polarization transfer pathways. In this study, we investigate how key experimental parameters-static magnetic field (B₀), radiofrequency field strength (B₁), magic-angle spinning (MAS) frequency, and cross-polarization (CP) ramp conditions-affect the polarization transfer and proton-mediated spin diffusion in cellulose samples with different hydration states. Using comparative measurements on dry and hydrated cellulose, we analyze how these parameters influence CP efficiency and kinetics, signal intensity, and the spin-diffusion values. The results show that experimental conditions strongly affect the values associated with the proton-driven polarization transfer pathways in hydrated systems. In particular, variations in MAS frequency and RF matching conditions modify the relative contribution of different spin-dynamic processes (T_HHa, T_HHb and T_HHc). The reduced heterogeneity gained at higher magnetic fields results in changes in CP conditions that makes challenging the determination of NMR observables (spin-diffusion and relaxation times). These findings, that cannot be directly assigned to a modification of the underlying molecular dynamics, highlight the importance of carefully optimizing ssNMR experimental conditions when studying hydrated polysaccharide materials. The present work demonstrates how hydration-dependent proton environments influence polarization transfer behavior and the resulting spectroscopic observables. This work provides practical guidance for the design of ssNMR experiments aimed at investigating hydrated cellulose and related biopolymer systems.