Raman spectra of Celluloses

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. 

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. 

In the first detailed comparison of the Raman spectra of celluloses I and II, it was concluded that the differences between the spectra, particularly in the low frequency region, could not be accounted for in terms of chains possessing the same conformation but packed differently in the different lattices (33). As noted above, that had been the general interpretation of diffrac-tometric studies of Che two most common allomorphs. The studies of the Raman spectra led Co the proposal Chat two different stable conformations of the cellulose chains occur in the different allomorphs. 

Raman Spectra of Cellulose. In laser excited Raman spectroscopy, a sample is exposed to monochromatic light, and the scattered light is analyzed. The frequency of a small fraction of the scattered light is shifted relative to the exciting light. The magnitude of the frequency shift corresponds to the vibrational frequencies of the molecules in the sample. Therefore, Raman spectroscopy provides information similar to that provided by infrared spectroscopy. 

According to X-ray measurements, the cellulose chains are arranged parallel in cellulose I and antiparallel in cellulose II. In contrast, celluloses III and IV possess mixed structures their Raman spectra represent superpositions of the Raman spectra of celluloses I and II. 

The IR and Raman spectra of cellulose acetate are presented in Reference Spectrum 60. A decrease (or even disappearance) in band intensity of the... 

The remaining questions in relation to the comparison made above is concerned with the appearance of two sharp bands in the IR and Raman spectra of mercerized cellulose, while only one such band is seen in the... 

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 n.4 Molecular structures and Raman spectra of (a) acetaminophen, an active ingredient (b) Cellulose, an organic inactive ingredient (c) Calcium carbonate, an inorganic inactive ingredient. 

It is important at this point to address the need for a new paradigm that was not recognized in the early work of Atalla and VanderHart. The title of the early articles was still defined in terms of the classical approach to cellulose structure in that the two forms of cellulose, and 1,3, were referred to as two distinct crystalline forms. Note was not taken at that point of the rapidly developing evidence that the lateral dimensions of most native cellulose fibrils were very limited and that cellulose nanofibrils have an inherent tendency to develop a right-handed twist when cellulose chain molecules aggregate. While this important development had shed some light on the controversies associated with many of the prior interpretations of diffractometric characterizations of native celluloses, it had not yet provided conclusive evidence that the interpretations based on the symmetry of the P2i space group for crystalline cellulose cannot be valid for native celluloses. It was the acquisition of the Raman spectra of Tunicate and Valonia celluloses that provided the conclusive evidence. 

Figure 8 Raman spectra of high-crystallinity samples of celluloses I, II, and I...

Figure 9 Raman spectra of tunicate (Halocynthia roretzi) and Valonia macrophysa celluloses in the Raman active regions.

The samples of cellulose IV obtained through regeneration from solution were shown to have Raman spectra that could be represented as linear combinations of the spectra of celluloses I and II, at first suggesting that it may be a mixed lattice in which molecules with two different secondary structures coexist. However, a mixed crystal would be expected to have Raman spectra that are quite different from those of celluloses I and II. It appears more likely that cellulose IV consists of nanonuclei of celluloses I and II. Such a mixture would explain the observation that the spectra of cellulose IV are linear combinations of the spectra of celluloses I and II. 

In later studies, the Raman spectra and corresponding infrared spectra indicated that the primary differences between the I and I/j forms of native cellulose were in the pattern of hydrogen bonding. Furthermore, the Raman spectra of the two forms raise questions as to whether the structures can possess more than one molecule per unit cell since there is no evidence of any correlation field splittings of any of the bands in the spectra of the two forms. 

In studies of the Raman spectra of different native celluloses, Atalla (32) concluded that the two forms 1 and Ig consist of molecular chains which have the same molecular conformation. In the chapter by Wiley and Atalla in the present volume, evidence is presented to suggest that though the molecular conformations are the same, the hydrogen-bonding patterns differ in the two forms. 

The structural differences between Ig and Ig are not understood yet. Atalla (18) compared the Raman spectra of various native celluloses with different Ig to Ig ratios. He also compared the native cellulose spectra with the spectrum of cellulose II. The spectra of... 

Two classes of experiments were conducted. In both sets of experiments, fibers in which the cellulose chains are oriented parallel to the fiber axis were used. In the first class of experiments, the plane of polarization of the incident light was changed relative to the axis of the fibers by rotating the fibers around the optical axis of the microscope (see Figure 2a). The dependence of the band intensities on the polarization of the incident light was studied to determine the directional character of the vibrational motions. This information was used to advance the assignment of the Raman spectrum of cellulose. Spectra from Valonia, ramie, and mercerized ramie fibers, which have different allomorphic compositions, were compared to study the structural differences between the allo-morphs. 

Polymorphy. Cellulose polymorphy within the cellulose I family was studied by comparing the Raman spectra of VaIonia and ramie cellulose. Solid state NMR spectra indicate that the I, form predominates in Valonia while the Ig form predominates in ramie (17-18). 

The cellulose I spectra were also compared with spectra of ceTTulose II recorded from a mercerized ramie fiber. Figures 7 and 8 show the Raman spectra of these three celluloses. Spectra were recorded with the electric vector of the incident light parallel and perpendicular to the chain axis. These spectra can be divided into two regions. The region below 1600 cm (Figure 7) is most sensitive to the conformation of the cellulose backbone (especially below 700 cm" ). 

Based on the number and location of the maxima and minima in the relationship between the band intensities and the polarization of the incident light relative to the chain axis, the bands in the Raman spectrum of cellulose could be divided into four groups. The about the direction of the vibrational motions in cellulose. The directions of the vibrations are such that the major change in polarizability associated with the motions is either parallel or perpendicular to the chain axis. Raman spectra recorded from deu-terated celluloses allowed the vibrational modes involving C-H and 0-H motions to be identified. These spectra demonstrated that most of the modes are complex coupled vibrations. Normal coordinate analyses of cellulose model compounds were done to determine the types of motion most likely to occur in each region of the spectrum. The calculations also suggested that the vibrational motions are very complex. The information from the normal coordinate calculations. 

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