Improved structural data of cellulose IIII prepared in supercritical ammonia

Highly crystalline cellulose IIII samples were prepared by subjecting oriented films consisting of an assembly of Cladophora cellulose microcrystals to supercritical ammonia. Diffraction data recorded on these specimens indicated that the crystals of cellulose IIII could be fully described with a one-chain unit cell and a P2(1) space group, with the cellulose chain axis on one of the 2(1) screw axes of the cells. The new cell had the following parameters: a = 0.448 nm, b = 0.785 nm, c (chain axis) = 1.031 nm, y = 105.1 degrees. In this cell, which is half of the one proposed so far for cellulose IIII, one glucosyl residue becomes the asymmetric unit. A good agreement between the diffraction analysis and spectroscopic data was observed. C-13 CP/MAS spectra of the samples presented only six sharp resonance peaks, attributed to the six carbons of the asymmetric glucosyl residue. In these spectra, the occurrence of the C6 signal at 62.3 ppm is a clear indication that the hydroxymethyl moiety is in the gt conformation. FT-IR spectra of cellulose IIII were recorded that showed that in the OH stretching region, there was only one very sharp absorption band that was polarized parallel to the fiber direction as opposed to two broad bands polarized perpendicular. A comparison of the spectroscopic data of cellulose IIII with those of the other cellulose allomorphs suggest that the single chain of cellulose IIII may have some conformational similarities with one of the two chains existing in the crystal of cellulose II.