The Role of Solid State13C NMR Spectroscopy in Studies of the Nature of Native Celluloses
R H. Atalla, David L. VanderHart
Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current level of understanding about crystalline polymorphy in these materials. Emphasis is placed on observations from solid state 13C NMR which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled Iα and Iβ. Historical background is presented, highlighting the structural controversies which mainly arose because different native celluloses were used, each one representing a different mixture of allomorphs. Input from Raman, infrared and electron diffraction data is included in the discussion of our current understanding of polymorphy in native celluloses. Also noted is the input from more recently studied celluloses (e.g., Halocynthia) as well as from newer processes that convert the Iα to the Iβ form. On the basis of Raman and IR observations, it is argued that the Iα and Iβ allomorphs differ in hydrogen bonding patterns only and that backbone conformations are nearly identical. Also, the point is made that the absence of correlation field splittings in the Raman spectra calls into question (although it does not disprove) whether the normal 2-chain-per-unit-cell, monoclinic Ibeta allomorph really possesses 2 equivalent chains. 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 self consistency of inferences that are based on lineshape analyses for different carbons (particularly C1 and C4). It is concluded that higher plant celluloses most likely possess a minor amount of the Iα allomorph where the Iα/Iβ ratio is probably less than 0.25.
allomorphs, carbon-13, cellulose, electron diffraction, infrared, native, NMR, Raman, solid state
and VanderHart, D.
The Role of Solid State<sup>13</sup>C NMR Spectroscopy in Studies of the Nature of Native Celluloses, Solid State Nuclear Magnetic Resonance, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=851629
(Accessed June 9, 2023)