Abstract
The molecular and crystal structure of cellulose propanoate diacetate (CPDA, 2,3-di-O-acetyl-6-O-propanoyl cellulose) has been determined through combined X-ray fibre diagrams and electron diffraction patterns of single crystal analysis, aided by stereochemical restraints by using a constrained linked-atom least-squares refinement. The unit cell of CPDA is orthorhombic with space group P2 12121 and parameters:a =1.239 nm,b =2.498 nm and c (fibre axis)=1.044 nm. Based on these data, coupled with the observed density of the crystals, there are four chains per unit cell, distributed in two antiparallel pairs of parallel chains, and the independent repeat is the disaccharide unit in each chain. A preliminary CPDA disaccharide unit was derived based on the centre residue of cellotriose undeca-acetate, and this model was refined through a conformational analysis. The best model obtained by combining the stereochemical refinement with the diffraction intensities gave R=0.272 (R″=0.259) for the three-dimensional information from the X-ray fibre diagram and R=0.248 (R″=0.246) for the base plane data resulting from electron diffraction analysis.
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