Crystal Structure Refinements of Cellulose Polymorphs using Solid State 13C Chemical Shifts
Force field methods in combination with chemical shift target functions are used to investigate the structures of cellulose I and II. Since diffraction investigations of biopolymers like cellulose II are only of poor resolution, different models for the structure are discussed in the literature. These models were used as the starting point for force field optimizations with 13C chemical shift target functions. In these optimizations additionally to the total energy a pseudo-energy is minimized that depends harmonically on the difference between calculated and observed chemical shifts. In the case of cellulose II all four criteria: (i) total energy, (ii) pseudo-energy, (iii) chemical shift rms (root mean square) difference, and (iv) deviation from the diffraction data favour the structure model of Kolpak and Blackwell with two antiparallel carbohydrate chains. The CH2–OH group of one chain is in tg and that of the other chain in gt conformation. The chemical shift optimized fractional coordinates for cellulose II, Iα and Iβ are presented together with the calculated and experimental 13C chemical shifts.
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