Structural analysis of cellulose triacetate polymorphs by two-dimensional solid-state 13C-13C and 1H-13C correlation NMR spectroscopies

For the elucidation of the crystal structures of the two crystalline allomorphs of cellulose triacetate (CTA), namely CTA I and CTA II, two-dimensional (2D) solid-state through-bond C-13-C-13 and H-1-C-13 correlations NMR techniques applied to the two crystalline allomorphs of CTA. As a result, the C-13 and H-1 chemical shifts of the glucopyranose ring of CTA I and CTA II were completely assigned by the 2D NMR spectra of these allomorphs. On the 2D C-13-C-13 correlation spectrum of CTA II, two sets of the C-13-C-13 correlations from C1 to C6 were observed. This indicated that the CP/MAS C-13 NMR spectrum of CTA II can be characterized by its overlapping of the C-13 subspectra of two kinds of 2,3,6-triacetyl-anhydroglucopyranose units and that there are two magnetically non-equivalent sites in the unit cell of CTA II. In the case of CTA I, the numbers of respective 13 C and 1H shifts of CTA I agreed with the those of the glucopyranose residue in the allomorph, strongly suggesting that the asymmetric unit of CTA I is only one glucose residue. In addition, conformational differences in the exocyclic C5-C6 bonds between CTA I and CTA II were strongly suggested by the notable differences in the H-1 and C-13 chemical shifts at the C6 sites of these allomorphs. (C) 2004 Elsevier Ltd. All rights reserved.