The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses
The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses
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DOI：
10.1016/s0926-2040(99)00042-9
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发表时间：
1999-10-01
影响因子：
3.2
通讯作者：
VanderHart, DL
中科院分区：
文献类型：
--
作者：
Atalla, RH;VanderHart, DL
已发表的关于天然纤维素晶体结构的光谱观察结果进行了回顾，目的是确定我们目前对这些材料中晶体多态性的理解水平。重点放在固态C-13核磁共振（NMR）的观察上，这首先导致了大多数天然半结晶纤维素是两种晶体异形体的复合材料的假设，标记为i - α和i - β。介绍了历史背景，突出了结构上的争议，主要是因为使用了不同的天然纤维素，每一种纤维素都代表了不同的异形体混合物。从拉曼，红外（IR）和电子衍射数据的输入被包括在我们目前对天然纤维素多态性的理解的讨论中。还注意到来自最近研究的纤维素（例如，盐藻）以及将i - α转化为i - β形式的新工艺的输入。基于拉曼光谱和红外光谱的观察，认为i - α和i - β异构体仅在氢键模式上不同，而主链构象几乎相同。此外，作者还指出，拉曼光谱中没有相关场分裂，这让人质疑（尽管没有反驳）正常的单斜i - β异形体是否真的拥有两条等效链。对高等植物中纤维素微晶的异胚组成进行了大量的讨论。从线形理论的角度和基于不同碳（特别是C1和C4）线形分析的推论的自洽性的角度，对已发表的用于高等植物纤维素的核磁共振线形分析方法进行了回顾和批评。由此得出结论，高等植物纤维素极有可能具有少量的i - α异形体，其中i - α / i - β的比值可能小于0.25。(C) 1999 Elsevier Science B.V.版权所有
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 polymorphism in these materials. Emphasis is placed on observations from solid state C-13 nuclear magnetic resonance (NMR), which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled I-alpha and I-beta. 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 (IR) and electron diffraction data is included in the discussion of our current understanding of polymorphism 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 I-alpha to the I-beta 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 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 two-chain-per-unit-cell, monoclinic I-beta allomorph really possesses two 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 I-alpha allomorph where the I-alpha/I-beta ratio is probably less than 0.25. (C) 1999 Elsevier Science B.V. All rights reserved.