Cellulose carbon-13 chemical shifts

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

Atalla RH, VanderHart DL. The role of solid-state carbon-13 NMR spectroscopy in studies of native celluloses. Solid State NMR 1999 15 1-19. Chen Y-Y, Luo S-Y, Hung S-C, Sunney I, Tzou D-LM. C solid-state NMR chemical shift anisotropy analysis of the anomeric carbon in carbohydrates. Carbohydr Res 2005 340 723-729. 

Fig. 4.12 Comparison of solution state and solid state NMR spectra of paracetamol (A) the C H solution state spectrum recorded on a ISmgmL solution of paracetamol in DMSO-d 125 MHz, (B) the solid state spectrum obtained on pure paracetamtd at 90.5 MHz, and (C) the solid state spectrum (90.5 MHz) of a crushed tablet of a paracetamol formulation. The solution state assignments are readily available from inverse corelation methods described in Section 4.2.4.1. The solid state isotropic chemical shifts are very similar to those obtained in the solution state, but note that the inequivalence of the chemically equivalent carbons e.g. 2,2 and 3, 3 in the solid state spectrum. The dmg resonances can easily be diffterentiated from those of the excipients in spectrum (C). The resonances between 60 and lOOppm in spectrum (Q originate from the cellulosic formulation ingredients. 

Our recent CP/MAS 3c NMR work(l, 2) on native and regenerated celluloses has shown that the resonance lines can be analyzed in terms of the contribution from the crystalline and noncrystalline components. The chemical shifts of Cl, C4, and C6 carbons thus ascribed to the respective components could be correlated to the torsion angles i(> and Tp, which define the conformation about the 3-1,4-glycosidic linkage, and x about the exo-cyclic C5-C6 bond,... 

The chemical shifts of Cl, C4, and C6 carbons of the crystalline and noncrystalline components of native and regenerated celluloses are tabulated in Tables I and II, respectively. Table II shows also the line width Av of each resonance line. Before we discuss these results, let us consider the meanings of the solid-state chemical shifts in the next section. 

Comparison of Chemical Shifts of Cl, C4 and C6 Carbons of Ramie In the Solid State with Those of Low Molecular Weight Cellulose In Solution... 

Figure 6 C chemical shifts of the Cl carbons vs. torsion angles . a g-D-celloblose, b 3-D-methyl cellobloslde CH OH, c a-D-lactose monohydrate, d 3-D-lactose, e cellulose I (29), f cellulose I ( ), g cellulose II (30), h cellulose II (32). (Data a-d, g-h Ref. 4.)...

Table II shows that the noncrystalline components of native cellulose have no significant difference in chemical shift and line width from each other. The chemical shifts of the regenerated cellulose fibers, except for the amorphous cellulose, are also almost the same as those of native cellulose. This may suggest that the most probable conformation of the regenerated fibers is very similar to that of native cellulose. However, there is a marked difference in line width Av of the Cl carbon between native and regenerated celluloses the line width of the Cl carbon of native cellulose is about half that of the regenerated cellulose. Although the cause of the line broadening in CP/MAS spectra is not clear as yet, it is most likely that the line width of the Cl carbon is primarily dependent on the distribution of the torsion angle <(). Thus, it is suggested that the distribution in the angle <() is relatively narrow for the non-...

In the amorphous cellulose sample the chemical shift of the C4 carbon is as low as 81.6 ppm and very close to the corresponding value of the low molecular weight cellulose in DMSO solutlon(see Table IV). This sample was prepared by dissolution of Watman cellulose powder CF-1 in DMSO-paraformaldehyde followed by precipitation in ethanol. Therefore, the molecular chains of this sample must be fully disordered in comparison with those of the regenerated cellulose fibers and native cellulose. The more detailed structure of the noncrystalline components of different cellulose samples will be discussed elsewhere. 

The contributions of the crystalline and noncrystalline components to the resonance lines of Cl, CA and C6 carbons of different native and regenerated cellulose samples can be analyzed by using the respective line shapes. The chemical shifts of both components thus obtained were precisely determined. 

The hypothesis that conformational differences occur is also supported by the differences between the CP-MAS NMR spectra of celluloses I and II. It is indeed not likely that the differences in the chemical shifts of the different carbons and the differences in the degrees of splittings of the Cl and C4 resonances can be accounted for in terms of structures adhering strictly to the assumption that the twofold screw axes coincide with the axes of the molecular chains. 

Figure 3 C chemical shifts of C6 carbons vs. torsion angles X a flt-D-glucose, b d-D-glucose H O, c 3 D-glucose, d B-D-cellobiose, e a-D-lactose H O, f 0-D-lactose, g cellulose I. o d-glucose and O-glucose residues,...

NMR spectral shifts for cellulose and some oligomers are sun-mar ized in Figure 2 ( 9). Horii et al. (9 ) discussed the chemical shifts of C-6 carbons, and proposed that the difference between Cellulose I and Cellulose II is caused by conformational differences at the 6-OH groups. The shifts in the crystalline parts of Cellulose I were assigned to t-g conformations (ca. 66 ppm for C-6), and those for Cellulose II and the amorphous parts of all celluloses to g-t conformations (ca. 63 ppm for C-6). 

Figure 2. -chemical shifts of carbon in various cellulose samples. 

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