In situ C-13 NMR measurements are reported on C-13-enriched powdered poplar wood that is subjected to pretreatment with 0.5 M sulfuric acid as a function of time and at two temperatures. C-13 MAS (magic-angle spinning) spectra were obtained in both the DP (direct polarization) and CP (cross-polarization) modes, the contrasts in this combination yielding valuable qualitative information on the effect of pretreatment on local molecular mobilities. T-1 values for C-13 and for H-1, as well as T-CH and T-1 rho H, were measured at various stages of treatment with 0.5 M H2SO4 for lignin peaks and for cellulose peaks in the C-13 NMR spectra, as quantitative indicators of the degree of molecular motion for those two structural entities. The results show that a substantial fraction of the solid/semisolid biomass is converted at elevated temperatures to (a) chemically different and more mobile structures and (b) locally similar structures with enhanced atomic-level mobilities and that some fraction of this "mobilized" biomass does not return to the original level of immobility upon cooling the biomass back to room temperature. Analysis of the T-1 results by a rather simple model indicates that, for poplar wood in 0.5% H2SO4, the estimated ("global") motional correlation time (at the multiatom level), tau(c), is in the range of about 0.7-1.5 ns at various stages and temperatures of the treatment.
