Spectra of cellulose

The CP/MAS NMR spectra are an important source of information regarding the structure of cellulose and its polymorphos. A number of groups have investigated these spectra 11 15) and also reviews on the subject have been published 16 17>. For an orientation in the field Table 1 shows the most important features of the solid-state NMR spectra of cellulose I, II and IV and in Fig. 3 the numeration of the carbon atoms of the cellulose basic unit is given. It is evident that the polymorphs... 

The considerations and interpretation of the spectra have been discussed in detail by Earl and Van der Hart13). Here we shall follow Atalla s interpretation of spectra of celluloses from various origins algal cellulose, cotton linters, ramie, and the celluloses of pure polymorphic froms I and II 17,19). The experimental spectra are given in Fig. 4. 

Fig. 6. The CP/MAS spectra of cellulose acetate-butyrate (CAB) and of cellulose acetate (CA, degree of substitution = 1.97), 20. The observation frequency was 50.1 MHz and the irradiation frequency 199.5 MHz. The pulse repetition time was 5 s and the contact time 2 ms. For CAB 400 scans and for CA 60 scans were collected...

The affinity of cellulose and its derivatives for water can be explained by the existence of hydrogen bonds that bind the molecules of the cellulose chain either directly or with the help of water molecules, as outlined in Fig. 82. The existence of hydrogen bonds has been confirmed experimentally by studies of the infra-red absorption spectra of cellulose and its derivatives (the subject will be discussed later, on p. 287). 

Zhbankov, R. G. Infrared Spectra of Cellulose and Its Derivatives Consultants Bureau New York, 1966. 

Recent studies of the Raman spectra of Celluloses I, II, and IV have indicated that the different polymorphic forms involve two basically different molecular conformations in addition to the differences in crystalline packing (7,8,9). The conformation variations suggested by the Raman spectra are such that they could play an important role in determining the susceptibility of glycosidic linkages to attack by hydrolytic agents. The questions raised by this possibility will be addressed in this chapter. 

The author wishes to express appreciation to K. P. Carlson for acquisition of the Raman spectra of the disaccharides and for much valuable discussion. The spectra of celluloses were acquired by R. Whitmore. Support of this work from institutional research funds of The Institute of Paper Chemistry is gratefully acknowledged. 

Figure 1. ESR spectra of cellulose and cellulose derivatives irradiated with UV light of A > 280 nm for 60 min at 77 K. Spectra were recorded at 77 K. Key a, cellulose b, methylcellulose c, ethylcellulose d, acetylcellulose e, hydroxyethyl-cellulose f, carboxymethylcellulose.

Infrared Spectra (IR) - Infrared spectra of cellulose nitrate, poly (methyl methacrylate) and cellulose nitrate-g-poly(methyl methacrylate) films were recorded using a Perkin-Elmer 337 grating IR spectrophotometer. 

The nuclear magnetic resonance spectra of cellulose nitrate and of cellulose-g-poly(methyl methacrylate) after separation from cellulose nitrate and poly(methyl methacrylate), are shown in Figures 3 and 4, respectively. The graft copolymer shows the presence of C1CH3 proton at 1.06, indicating the presence of poly(methyl methacrylate) chains attached to cellulose nitrate. 

Figure 3. ESR spectra of cellulose which has reacted with Ceu+ ion and/or acrylonitrile recorded at —100°C.

Figure 9. ESR spectra of cellulose after copolymerization with acrylonitrile initiated by OH radicals formed in Fe2+-H202 system. Recorded at —40° C.

Michell AJ (1988) Second derivative FT-IR spectra of celluloses I and II and related mono and oligosaccharides. Carbohydr Res 173 185-195... 

Figure 2 C-NMR spectra of celluloses and Iq. (Reproduced from Ref. 4. Copyright 1984 American Chemical Society.)...

Figure 3 Rejection spectra of cellulosic (A) and polysulphone (B) membranes In saline (o) and plasma (e). See reference for methodology. Reproduced with permission from...

Figure 3 - Baman microprobe aectrum of a hazy film omtaminating a silicon wafer (upper trace). Reference spectra of Celluloses I and II shown below. (Reproduced with permission from Ref. 9b. Copyright 1976 John Wiley.)...

ESR spectra of cellulose irradiated with ultraviolet light have been reported by several authors (126-128). Phillips et al. (127) observed a singlet spectrum with a linewidth of 8-10 gauss after irradiation... 

In the analyses of the spectra of model compounds, changes of the magnitude indicated in Figure 8 were associated exclusively with the occurrence of differences in conformations. It seemed very probable therefore that the differences between the spectra of celluloses I, II, and III reflect changes in the skeletal molecular conformation accompanying the transition from one form to the other. Since the basic ring structure is not... 

One final consideration that was addressed is the possibility that rotations of the primary alcohol group at C6 could account for the spectral differences seen in the spectra of celluloses I, II, and III and in the spectra of the amyloses. The normal coordinate analyses of the hexoses showed that rotations about the C5-C6 bond can result in minor variations in the region below 600 cm but that the major impact of such rotations is expected in the spectral region above 700 cm . With all of the above considerations in mind, it became clear that the only plausible rationalization of the differences between the Raman spectra of celluloses I, II, and III had to be based on the possibility that differences between the skeletal conformations were the key. The key considerations have been presented elsewhere in greater detail and need not be repeated here. 

In view of the considerable variation observed in the Raman spectra of celluloses as a result of changes in molecular conformations, there can be little question that the spectra in Figure 9 represent evidence that the conformations of the cellulose molecules in Vahnia and Halocynthia are essentially identical. It is also important to note that the Raman spectra of the celluloses from V. macrophysa and V. ventricosa, both of which have been used in different studies as representatives of the Iq, form, are effectively indistinguishable in all regions of the spectra. This is also true of the Raman spectra of celluloses from the algae C. glomerata and Rhizoclonium heirglyphicum, which have also been used in many studies as representative of celluloses that are predominantly of the Iq, form. 

The studies undertaken on the basis of further examination of the solid-state NMR spectra of celluloses are in a number of categories. The first group is focused on further examination of the spectra of different native celluloses, in part aided by mathematical procedures for deconvolution of the spectra or for resolution enhancement. Another group relies on exploring the spectral manifestations of native celluloses that have been modified in different ways. Yet a third approach is based on investigation of celluloses subjected to different but well-known procedures for inducing structural transformations in the solid-aggregated state of cellulose. 

A different approach to mathematical analysis of the solid-state C NMR spectra of celluloses was introduced by the group at the Swedish Forest Products Laboratory (STFI). They took advantage of statistical multivariate data analysis techniques that had been adapted for use with spectroscopic methods. Principal component analyses (PCA) were used to derive a suitable set of subspectra from the CP/MAS spectra of a set of well-characterized cellulosic samples. The relative amounts of the I and I/3 forms and the crystallinity index for these well-characterized samples were defined in terms of the integrals of specific features in the spectra. These were then used to derive the subspectra of the principal components, which in turn were used as the basis for a partial least squares analysis of the experimental spectra. Once the subspectra of the principal components are validated by relating their features to the known measures of variability, they become the basis for analysis of the spectra of other cellulosic samples that were not included in the initial analysis. Here again the interested reader can refer to the original publications or the overview presented earlier. ... 

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