The role of solid state C-13 NMR spectroscopy in studies of the nature of native celluloses

Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current lev el of understanding about crystalline polymorphism in these materials. Emph asis is placed on observations from solid state C-13 nuclear magnetic reson ance (NMR), which first led to the postulate that most native, semicrystall ine celluloses are composites of two crystalline allomorphs, labeled I-alph a and I-beta. Historical background is presented, highlighting the structur al controversies which mainly arose because different native celluloses wer e used, each one representing a different mixture of allomorphs. Input from Raman, infrared (IR) and electron diffraction data is included in the disc ussion of our current understanding of polymorphism in native celluloses. A lso noted is the input from more recently studied celluloses (e.g., Halocyn thia) as well as from newer processes that convert the I-alpha to the I-bet a form. On the basis of Raman and IR observations, it is argued that the I- alpha and I-beta allomorphs differ in hydrogen bonding patterns only and th at backbone conformations are nearly identical. Also, the point is made tha t the absence of correlation field splittings in the Raman spectra calls in to question (although it does not disprove) whether the normal two-chain-pe r-unit-cell, monoclinic I-beta allomorph really possesses two equivalent ch ains. Considerable discussion is devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses are reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of se lf-consistency of inferences that are based on lineshape analyses for diffe rent carbons (particularly C1 and C4). It is concluded that higher plant ce lluloses most likely possess a minor amount of the I-alpha allomorph where the I-alpha/I-beta ratio is probably less than 0.25. (C) 1999 Elsevier Scie nce B.V. All rights reserved.