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Structural physico-chemistry of cellulose macromolecules. Vibrational spectra and structure of cellulose
Journal of Molecular Structure 614 (2002) 117–125 www.elsevier.com/locate/molstruc
Structural physico-chemistry of cellulose macromolecules. Vibrational spectra and structure of cellulose R.G. Zhbankova, S.P. Firsova, D.K. Buslova, N.A. Nikonenkoa, M.K. Marchewkab, H. Ratajczakb,c,* b
a B.I. Stepanov Institute of Physics, Academy of Sciences of Belarus, Skaryna Pr. 70, Minsk 220072, Belarus Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland c Faculty of Chemistry, University of Wrocław, 50-383 Wroclaw, Poland
Received 10 November 2001; revised 12 March 2002; accepted 12 March 2002
Abstract By the methods of Fourier transform infrared and Raman spectroscopy and theoretical conformational analysis, the conformational peculiarities of macromolecules of cellulose of various structural modifications have been studied. It has been shown that celluloses of structural modifications I, II and III differ in rotary isomerism of elementary links (form of macromolecules), rotamers of C6H2OH groups as a consequence of rotations around C5C6 and C6O6 bonds and system of intra- and intermolecular hydrogen bonds. The results obtained are of great importance for the development of a new scientific direction—structural physico-chemistry of cellulose macromolecules which consists in the establishment of interrelationship between the physical structure and conformational properties of macromolecules determined by their chemical structure and intra- and intermolecular interactions. q 2002 Published by Elsevier Science B.V. Keywords: Fourier transform IR and Raman spectroscopy; Cellulose; Conformations; Rotary isomers; Structural physico-chemistry
1. Introduction Cellulose is the most important natural polymer. The specifically valuable properties of cellulose, the unlimited raw material source which is replenished by nature, and the low cost determine the wide practical use of this polymer in house, in technology and medicine. Great interest in research on the structure and properties of * Corresponding author. Address: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland. Tel.: þ 48-71-343-5020; fax: þ 4871-441029. E-mail address: [email protected] (H. Ratajczak).
cellulose and its derivatives is apparent. The cellulose structure and properties are discussed in several international journals and original monographs [1–16] at specialized conferences and symposia. At a fixed chemical structure materials on the basis of cellulose and its derivatives can differ widely in physico-mechanical, technological and operational characteristics. This is fully determined by the peculiarities of their physical structure. A change in the physical structure—the spatial arrangement of macromolecules and their components, intra- and intermolecular interactions—is the main source of upgrading quality of polymer products with a specified chemical structure.
0022-2860/02/$ - see front matter q 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 5 2 - 1
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Fig. 1. Scheme of the dimer fragment of the cellulose macromolecule showing the types of intramolecular hydrogen bonds and angles of internal rotation.
By structural physico-chemistry of cellulose macromolecules, we mean a system of knowledge around the relationship between its physical structure and spatial-conformational properties of macromolecules which are determined by peculiarities of the chemical constitution of cellulose, its intra- and intermolecular interactions. In our monographs [9 – 11,13,15], we summarized and systematized in reviews the main concepts on the physical structure of cellulose: conformations of pyranose cycles and lateral groups; rotary isomerism of elementary links; the system of intra- and intermolecular hydrogen bonds; the mechanism of structural transitions; orientation and crystallization, etc. It is these structural characteristics which underlie the structural physico-chemistry of cellulose macromolecules that determine many specific properties of cellulose, the diversity of forms of physical structure and structural transitions. Research at macromolecular level is one of the most difficult tasks and therefore the problems of the physics and structural physico-chemistry of cellulose have not been tackled well. This is due to the methodological difficulties of direct studies of conformations of natural celluloses in their natural fibrous state, the complicated combination in cellulose of
Fig. 2. Conformation of the pyranose cycle of 2,3-anhydro-methylD -mannopyranoside.
regions with a different degree of structural order, the difficulty of obtaining monocrystals of cellulose and other polysaccharides. Obviously, under these conditions the conformational properties of macromolecular components of cellulose and hydrogen bonds of hydroxyl groups can only be investigated on the basis of comparative analysis of data obtained by a complex of nondestructive physical (including spectroscopic) methods, a combination of theoretical and experimental studies, wide use of model systems. Particular consideration should be given to those physical methods which are most sensitive to geometry of molecules and parameters of hydrogen bonds, have a developed theoretical and experimental basis, and permit investigating substances in their natural state. Exactly such features characterize the method of vibrational spectroscopy which has now radically new potentialities due to the application of optical quantum generators, computer methods of obtaining and processing spectral information.
2. Results and discussion Let us dwell briefly on the principal results obtained. On the basis of numerous experimental and theoretical studies, it has been firmly established that in cellulose and other polyglucanes only one most stable ‘chair’ conformation of glucopyranose cycles C1 (4C1) is realized (Fig. 1). No substantial distortions of this conformation as a result of substitution of hydroxyl groups have been discovered. Considerable distortions of conformation C1 take place only on the introduction into the glucopyranose cycle of additional closed bonds (for example, in compounds with a-oxide cycles, 2,3- and 3,6-anhydride derivatives of cellulose). Our theoretical studies of model systems point to a strong simplification of the chair conformation of the pyranose cycle (Fig. 2). In our works [17 – 24], we discovered and systematized a strong dependence of vibrational energy distribution over C –C and C – O bonds of the skeletal base of carbohydrate molecules on the steric factors which leads to specific changes in band intensities in IR and Raman spectra. This explained the so-called ‘phenomenon’ of vibrational spectra of carbohydrates—their strong dependence on the change in
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Fig. 3. Conformational chart of the dimer fragment of a cellulose chain. The position of the global minimum is asterisked; (- - -) curves of equal distances O4· · ·O10 . The numbers indicate energies in kcal/mol of glucopyranose residues.
the spatial arrangement of C – O and C – H bonds in individual carbon atoms. A characteristic feature of spectra of carbohydrates containing C – O bonds only in the equatorial position relative to the plane of pyranose cycles (accordingly, of C –H bonds in the axial position) is the presence in the structuresensitive range 750– 950 cm21 of only one band at 900 ^ 20 cm21. Equatorial disposition of C –O bonds in all carbon atoms of molecules of b-D -glucopyranose and their derivatives is only possible in the conformation C1 (4C1) of pyranose cycles. At fixed conformations of pyranose cycles a change in the form of macromolecules of cellulose and other polysaccharides can only take place due to
Fig. 4. IR spectra of celluloses I (1) and II (2).
Fig. 5. Raman spectra of celluloses I (1), II (2), and III (3) and cellulose I, reconstructed from cellulose III (4).
rotary isomerism of the spatial arrangement as a result of rotations around the glycoside bonds C1 – O – C0 4 (C1, C2; Fig. 1). The results of the investigations point to a limited freedom of rotation of glucopyranose residues. The most probable conformation features a relatively strong intramolecular hydrogen bond O0 3 –H· · ·O5 (Fig. 1). Hydrogen bonds and positions of lateral groups have a considerable influence on stability of spatial forms of cellulose macromolecules. However, the question of the degree of this influence had to be elucidated. In our calculations for the first time all kinds of rotamers of OH and CH2OH groups and intramolecular hydrogen bonds were taken into account. The calculated values of potential energy depending on the values of angles of rotations around the glycoside bond C1 –O – C0 4 C1 and C2 are given in Fig. 3 [13]. The rotary isomer of pyranoside cycles with C1 ¼ 22108; C2 ¼ 21508 is characterized by the most stable state. At excess energy DU # 2 kcal=mol variations of angles C1 and C2 by 208
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Fig. 6. Intramolecular and intermolecular hydrogen bonds in ordered regions of natural cellulose.
are possible. Intramolecular hydrogen bonds considerably limit the mobility of elementary links. In this connection, of great interest was experimental detection of rotary isomerism of elementary links of cellulose by an example of celluloses of various structural modifications. As found earlier, celluloses I and II differ considerably in IR spectra in the region of stretching vibrations of OH groups included in hydrogen bonds, especially in the frequency range of intramolecular bonds [19] (Fig. 4). Fig. 5 shows for the first time the Fourier transform Raman spectra of microcrystalline celluloses of various structural modifications in the 200 – 700 cm21 range. The spectrum of natural cellulose I is distinguished from the spectra of celluloses II and III by the opposite intensity ratio of bonds at 380 and 350 cm21. When cellulose I is obtained back from cellulose III, the initial intensity ratio of these bands is also reconstructed. Such a kind of interrelated change in the intensity ratio of closely spaced bands is a typical manifestation of the phenomenon of rotary isomerism. According to the results of theoretical calculations, the 330 – 350 cm21 spectral range contains vibrational frequencies with prevailing contribution to the potential energy distribution (PED) of C1C2, C3C4 bonds which are directly connected to the glycoside bridges between the elementary links [20]. The earlier-mentioned spectral changes should be definitively attributed to the rotary isomerism of
cellulose pyranose links—the change in their relative position caused by rotations around the glycoside bond C1 –O –C0 4 (angles C1 and C2). Fourier Raman spectroscopy opens up unique possibilities of analyzing rotary isomers of elementary links of cellulose and other polysaccharides. As mentioned earlier, in cellulose and its derivatives only one most stable chair conformation C1 (4C1) of pyranose cycles is realized and their mutual mobility is strongly limited. In specific structural modifications specific types of rotary isomers of elementary links are realized, which determine the form of macromolecules. From this a conclusion of fundamental importance follows: the formation in cellulose of regions with a different degree of ordering, most ordered (the so called ‘crystalline’) and least ordered (the so-called ‘amorphous’) ones, the processes of orientation and crystallization in products on the basis of cellulose and its derivatives are largely determined by the rotamers of hydroxyl groups (their substitutes) and by the system of intra- and intermolecular interactions formed by them. The study of the specificity of these rotamers in various cellulose materials is of great scientific and practical importance [25]. Of particular interest is analysis of most voluminous CH2OH groups whose hydroxyl groups are capable of entering simultaneously into intra- and intermolecular hydrogen bonds (Fig. 6). It was
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Fig. 7. Raman spectra of celluloses I (1), III (2) and II (3).
mentioned earlier that celluloses I, II and III differ in the form of macromolecular chains, the system of intra- and intermolecular hydrogen bonds. Naturally, they should also differ in the specificity of rotamers of oxymethyl groups which can be of two types: rotations around C5 –C6 and C6 – O6 bonds. From Figs. 7 and 8 it is readily seen that Raman and IR spectra of celluloses I, II and III differ considerably in the range of vibration frequencies with prevailing contribution to PED of atoms and bonds of oxymethyl CH2OH groups. According to the data of theoretical calculations, the greatest contribution to the band intensity of vibrational spectra of carbohydrates in the 1400 – 1500 cm21 range is made by deformation vibrations of HC6H and COH groups. The bands caused by vibrations of methylene groups are highly characteristic (up to 80% of the contribution to PED) and lie in a sufficiently narrow spectral range 1460– 1490 cm21 [23,24]. The band of internal deformation vibrations of methylene groups in the Raman spectrum of cellulose
Fig. 8. IR spectra of celluloses I (1), III (2) and II (3).
I with a maximum at 1479 cm21 has a complicated origin—an asymmetric contour with a projection near 1465 cm21. In the spectra of celluloses III and II, instead of this projection sharply defined bands appear at 1465 and 1460 cm21, respectively, while the band at 1479 cm21 disappears. As mentioned earlier, such a kind of interrelated change in intensities of closely spaced bands in vibrational spectra of compounds identical in chemical structure should be attributed to the phenomenon of rotary isomerism, in our case, the change in the spatial arrangement of methylene groups caused by rotations around the C5 – C6 bond. Another highly characteristic vibration band of groups of atoms in C5 and C6 elementary links of cellulose is the band at 900 cm21. The contribution of C5C6H and O6C6H groups to the PED of vibrations with this frequency is 45 and 35%, respectively. The increase in this band intensity on passing from cellulose I to celluloses II and III points to a change in the surrounding of the above groups of atoms due to the
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Fig. 9. IR spectra of cellulose I (1), cellulose I after treatment with 13% NaOH (2), cellulose II (3) (A) and the results of their deconvolution (B).
rotary isomerism of oxymethyl groups. This also explains the gradual disappearance of the sharp band at 1120 cm21 in the region of stretching vibrations of C5C6 and C6O6 bonds, the disappearance of the band at 1290 cm21 and the appearance of a new band at 1266 cm21 in the region of frequencies of non-plane deformation vibrations of methylene groups. In the IR spectrum of cellulose I, as opposed to the Raman spectrum, the main maximum of the band in the 1400 –1500 cm21 range lies at 1433 cm21 (Fig. 8). According to the results of calculations the lowfrequency range in this region has bands caused by vibrations with prevailing contribution to the PED of deformations of angles of C6O6H and C3O3H. The shift of this band maximum from 1433 cm21 (cellulose I) to 1425 cm 21 (cellulose III) and 1420 cm21 (cellulose II) should be attributed to the change in the form of rotamers of hydroxyls in positions ‘3’ and ‘6’ (rotations round bonds C3 –O3
and C6 – O6). As mentioned earlier, exactly these hydroxyls form intramolecular hydrogen bonds between the elementary links of cellulose I stabilizing the form of its macromolecular (O0 3 – H· · ·O5 and O2 –H· · ·O0 6). Since modifications II and III differ from modification I in the form of macromolecules (rotary isomers of elementary links), such a transition presupposes breakage of the former system of intramolecular H-bonds and the formation of a new one with a different spatial arrangement of the corresponding hydroxyl groups. As in the case of Raman spectra, the transition of cellulose I to celluloses II and III leads, in their spectra, to a decrease in the 1115 cm21 band intensity and an increase in the intensity of the 900 cm21 band. In this case, however, the main contribution to the intensities of these bands is made by the C5 –C6 bonds and C5C6H groups. As a result of mathematical treatment (deconvolution) of IR spectra of celluloses I and II, components of the complex band in the 1400 –1500 cm21 range have been revealed. As seen from Fig. 9, by analogy with Raman spectra, at transition cellulose I ! cellulose II the disappearance of the band at 1480 cm21 and the appearance of a new band at 1468 cm21 is observed. The shift of the main maximum of the band at 1430 cm21 is due to the decrease in intensity of the bands at 1430 and 1425 cm21 and the appearance of a new band at 1415 cm21. The components of the complex band at 900 cm21 (900, 895 and 885 cm21) retain their position, but there occurs a considerable increase in intensity of the band at 895 cm21. As seen from the experimental data obtained, cellulose III is intermediate between celluloses I and II as to spectral changes caused by rotary isomerism of oxymethyl groups. The spectral changes caused by rotary isomerism of elementary links in the case of celluloses II and III are identical. It should be concluded that cellulose III is a transitional structural modification in passing from cellulose I to II. To additionally substantiate this basic conclusion, we investigated the kinetics of cellulose II transition from cellulose I by an example of celluloses regenerated after treatment with water solutions of NaOH with a gradual increase in the alkali concentration. From Figs. 9 and 10 it is seen that within the range of
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Fig. 10. Raman spectra of cellulose I (1), cellulose, regenerated after treatment with water solution of NaOH of 13 (2), 16 (3) and 17.5% concentration (4).
Fig. 11. IR spectra of celluloses I (1), and III (2) and cellulose I, reconstructed from cellulose III (3).
concentrations from 13 to 17.5% there occurs a gradual low-frequency shift of 1432 and 1480 cm21 bands in IR and Raman spectra and other spectral changes observed in the cellulose III spectrum, which are typical of the structural transition cellulose I ! cellulose II take place. Convincing evidence that the above differences between the spectra of celluloses I, II and III are due to conformational changes in the structural fragments of cellulose macromolecules is the clearly defined tendency of cellulose I spectrum reconstruction on obtaining cellulose I back from cellulose III (Fig. 11). It should be noted, however, that the 900 cm21 band intensity decreases only partially and the band of internal deformation vibrations of methylene groups (HC6H) in the Raman spectrum retains its position (Fig. 12). It should be concluded that when cellulose I is reconstructed from cellulose III the former system of rotamers of CH2OH groups caused by rotations around C5 – C6 bonds is not reconstructed. Further development and improvement of our
knowledge of rotary isomers of cellulose macromolecules which determine its structural modifications, intra- and intermolecular interactions, the system of molecular and supermolecular ordering is of great importance for the development of structural physicochemistry of cellulose macromolecules and, consequently, for improving products on its basis and their quality. As mentioned earlier, an important role in this process is played by non-destructive spectroscopic methods whose theoretical and experimental base has accumulated an enormous analytical potential and is improving steadily. The combination of Raman and IR spectroscopy and theoretical studies of vibrational spectra open up unique possibilities of studying the specificity of rotary isomers of macromolecules of cellulose and its derivatives. As shown earlier, this phenomenon can manifest itself as a shift of individual bands, a decrease or increase in their intensities. Bands, which are due to vibrations of rotating groups of atoms, change their position at rotation isomerism,
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Fig. 12. Raman spectra of celluloses I (1), and III (2) and cellulose I, reconstructed from cellulose III (3).
and the change in intensities of bands caused by stretching and deformation vibrations has an opposite direction. As we see it, this dependence can be of general importance for spectroscopy of rotary isomers.
3. Conclusion The results obtained enable us to draw the following important conclusions: 1. Celluloses of various structural modifications I, II and III differ in rotary isomerism of elementary links (form of macromolecules), rotamers of C6H2OH groups as a result of rotations around C5C6 and C6O6 bonds. 2. As to the specificity of these differences, the structural modification cellulose III is intermediate between the main modifications, celluloses I and II. 3. The most probable sequence of conformational
transformations of cellulose I to other modifications is as follows: (a) break of intramolecular hydrogen bonds O0 3 – H 0 · · ·O5 and O2 – H· · ·O 0 6 which stabilize the form of cellulose macromolecules and, therefore, of interrelated intermolecular hydrogen bonds O0 6 – H· · ·O00 3 (Fig. 6); (b) change in the form of macromolecules caused by the formation of a new system of rotary isomers of elementary links; (c) the formation of a new system of hydroxyl rotamers; (d) a change in the spatial arrangement of oxymethyl groups (as a result of rotations around C5C6) caused by the change in the form of macromolecules and rotamers of hydroxyl groups; (e) the formation of a new system of intra- and intermolecular hydrogen bonds which stabilize the new structural modification of cellulose. 4. At structural transitions of cellulose I (natural modification) complete reduction of its original structure is impossible. The natural structural modification of cellulose cannot be recognized to be the most stable one. The results obtained are of great importance for the development of a new scientific direction—structural physico-chemistry of cellulose macromolecules which consists in the establishment of interrelationship between the physical structure and conformational properties of macromolecules determined by their chemical structure and intra- and intermolecular interactions.
References [1] P.P. Shorigin, Cellulose Chemistry, Moscow, 1939. [2] P.H. Hermans, Physics and Chemistry of Cellulose Fibres, Elsevier, New York, 1949. [3] Z.A. Rogovin, N.N. Shorigina, Chemistry of Cellulose and its Satellites, Moscow, Leningrad, 1953. [4] N.I. Nikitin, Chemistry of Wood and Cellulose, Moscow, Leningrad, 1961. [5] R.G. Zhbankov, Infrared Spectra of Cellulose and its Derivatives, Minsk, 1964.
R.G. Zhbankov et al. / Journal of Molecular Structure 614 (2002) 117–125 [6] R.G. Zhbankov, Infrared Spectra of Cellulose and its Derivatives, New York, 1966. [7] N.M. Bikales, L. Segal (Eds.), Cellulose and Cellulose Derivatives, New York, 1971. [8] Z.A. Rogovin, Cellulose Chemistry, Moscow, 1972. [9] R.G. Zhbankov, Infrared Spectra and Structure of Carbohydrates, Minsk, 1972. [10] V.P. Panov, R.G. Zhbankov, Conformations of Sugars, Minsk, 1975. [11] R.G. Zhbankov, R.M. Marupov, N.V. Ivanova, et al., Spectroscopy of Cotton, Minsk, 1976. [12] N.I. Klenkova, Structure and Reactivity of Cellulose, Leningrad, 1976. [13] R.G. Zhbankov, P.V. Kozlov, Physics of Cellulose and its Derivatives, Minsk, 1983. [14] I.A. Tarchevskij, G.N. Marchenko, Biosynthesis and Structure of Cellulose, Moscow, 1985. [15] V.P. Panov, R.G. Zhbankov, Intra- and Intermolecular Interactions in Carbohydrates, Minsk, 1988. [16] G.A. Petropavlovskij, Hydrophilic Partially Substituted Cel-
[17] [18] [19] [20] [21]
[22]
[23]
[24] [25]
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lulose Ethers and its Modification by Chemical Cross-linking, Leningrad, 1988. R.G. Zhbankov, J. Mol. Struct. 270 (1992) 523. R.G. Zhbankov, J. Mol. Struct. 272 (1992) 347. R.G. Zhbankov, J. Mol. Struct. 275 (1992) 65. R.G. Zhbankov, V.M. Andrianov, M.K. Marchewka, J. Mol. Struct. 436 –437 (1997) 637. R.G. Zhbankov, L.K. Prihodchenko, T.E. Kolosova, V.M. Andrianov, M.V. Korolevich, H. Ratajczak, M.K. Marchewka, J. Mol. Struct. 450 (1998) 29. R.G. Zhbankov, S.P. Firsov, E.V. Korolik, P.T. Petrov, M.P. Lapkovski, V.M. Tsarenkov, M.K. Marchewka, H. Ratajczak, J. Mol. Struct. 555 (2000) 85. R.G. Zhbankov, V.M. Andrianov, H. Ratajczak, M.K. Marchewka, Zh. Struct. Khim. 36 (1995) 322 see also pages 430, 443, 456. R.G. Zhbankov, V.M. Andrianov, H. Ratajczak, M.K. Marchewka, Zh. Phys. Khim. 69 (1995) 553. Y. Marechal, H. Chanzy, J. Mol. Struct. 523 (2000) 183.
Structural physico-chemistry of cellulose macromolecules. Vibrational spectra and structure of cellulose R.G. Zhbankova, S.P. Firsova, D.K. Buslova, N.A. Nikonenkoa, M.K. Marchewkab, H. Ratajczakb,c,* b
a B.I. Stepanov Institute of Physics, Academy of Sciences of Belarus, Skaryna Pr. 70, Minsk 220072, Belarus Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland c Faculty of Chemistry, University of Wrocław, 50-383 Wroclaw, Poland
Received 10 November 2001; revised 12 March 2002; accepted 12 March 2002
Abstract By the methods of Fourier transform infrared and Raman spectroscopy and theoretical conformational analysis, the conformational peculiarities of macromolecules of cellulose of various structural modifications have been studied. It has been shown that celluloses of structural modifications I, II and III differ in rotary isomerism of elementary links (form of macromolecules), rotamers of C6H2OH groups as a consequence of rotations around C5C6 and C6O6 bonds and system of intra- and intermolecular hydrogen bonds. The results obtained are of great importance for the development of a new scientific direction—structural physico-chemistry of cellulose macromolecules which consists in the establishment of interrelationship between the physical structure and conformational properties of macromolecules determined by their chemical structure and intra- and intermolecular interactions. q 2002 Published by Elsevier Science B.V. Keywords: Fourier transform IR and Raman spectroscopy; Cellulose; Conformations; Rotary isomers; Structural physico-chemistry
1. Introduction Cellulose is the most important natural polymer. The specifically valuable properties of cellulose, the unlimited raw material source which is replenished by nature, and the low cost determine the wide practical use of this polymer in house, in technology and medicine. Great interest in research on the structure and properties of * Corresponding author. Address: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw, Poland. Tel.: þ 48-71-343-5020; fax: þ 4871-441029. E-mail address: [email protected] (H. Ratajczak).
cellulose and its derivatives is apparent. The cellulose structure and properties are discussed in several international journals and original monographs [1–16] at specialized conferences and symposia. At a fixed chemical structure materials on the basis of cellulose and its derivatives can differ widely in physico-mechanical, technological and operational characteristics. This is fully determined by the peculiarities of their physical structure. A change in the physical structure—the spatial arrangement of macromolecules and their components, intra- and intermolecular interactions—is the main source of upgrading quality of polymer products with a specified chemical structure.
0022-2860/02/$ - see front matter q 2002 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 5 2 - 1
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Fig. 1. Scheme of the dimer fragment of the cellulose macromolecule showing the types of intramolecular hydrogen bonds and angles of internal rotation.
By structural physico-chemistry of cellulose macromolecules, we mean a system of knowledge around the relationship between its physical structure and spatial-conformational properties of macromolecules which are determined by peculiarities of the chemical constitution of cellulose, its intra- and intermolecular interactions. In our monographs [9 – 11,13,15], we summarized and systematized in reviews the main concepts on the physical structure of cellulose: conformations of pyranose cycles and lateral groups; rotary isomerism of elementary links; the system of intra- and intermolecular hydrogen bonds; the mechanism of structural transitions; orientation and crystallization, etc. It is these structural characteristics which underlie the structural physico-chemistry of cellulose macromolecules that determine many specific properties of cellulose, the diversity of forms of physical structure and structural transitions. Research at macromolecular level is one of the most difficult tasks and therefore the problems of the physics and structural physico-chemistry of cellulose have not been tackled well. This is due to the methodological difficulties of direct studies of conformations of natural celluloses in their natural fibrous state, the complicated combination in cellulose of
Fig. 2. Conformation of the pyranose cycle of 2,3-anhydro-methylD -mannopyranoside.
regions with a different degree of structural order, the difficulty of obtaining monocrystals of cellulose and other polysaccharides. Obviously, under these conditions the conformational properties of macromolecular components of cellulose and hydrogen bonds of hydroxyl groups can only be investigated on the basis of comparative analysis of data obtained by a complex of nondestructive physical (including spectroscopic) methods, a combination of theoretical and experimental studies, wide use of model systems. Particular consideration should be given to those physical methods which are most sensitive to geometry of molecules and parameters of hydrogen bonds, have a developed theoretical and experimental basis, and permit investigating substances in their natural state. Exactly such features characterize the method of vibrational spectroscopy which has now radically new potentialities due to the application of optical quantum generators, computer methods of obtaining and processing spectral information.
2. Results and discussion Let us dwell briefly on the principal results obtained. On the basis of numerous experimental and theoretical studies, it has been firmly established that in cellulose and other polyglucanes only one most stable ‘chair’ conformation of glucopyranose cycles C1 (4C1) is realized (Fig. 1). No substantial distortions of this conformation as a result of substitution of hydroxyl groups have been discovered. Considerable distortions of conformation C1 take place only on the introduction into the glucopyranose cycle of additional closed bonds (for example, in compounds with a-oxide cycles, 2,3- and 3,6-anhydride derivatives of cellulose). Our theoretical studies of model systems point to a strong simplification of the chair conformation of the pyranose cycle (Fig. 2). In our works [17 – 24], we discovered and systematized a strong dependence of vibrational energy distribution over C –C and C – O bonds of the skeletal base of carbohydrate molecules on the steric factors which leads to specific changes in band intensities in IR and Raman spectra. This explained the so-called ‘phenomenon’ of vibrational spectra of carbohydrates—their strong dependence on the change in
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Fig. 3. Conformational chart of the dimer fragment of a cellulose chain. The position of the global minimum is asterisked; (- - -) curves of equal distances O4· · ·O10 . The numbers indicate energies in kcal/mol of glucopyranose residues.
the spatial arrangement of C – O and C – H bonds in individual carbon atoms. A characteristic feature of spectra of carbohydrates containing C – O bonds only in the equatorial position relative to the plane of pyranose cycles (accordingly, of C –H bonds in the axial position) is the presence in the structuresensitive range 750– 950 cm21 of only one band at 900 ^ 20 cm21. Equatorial disposition of C –O bonds in all carbon atoms of molecules of b-D -glucopyranose and their derivatives is only possible in the conformation C1 (4C1) of pyranose cycles. At fixed conformations of pyranose cycles a change in the form of macromolecules of cellulose and other polysaccharides can only take place due to
Fig. 4. IR spectra of celluloses I (1) and II (2).
Fig. 5. Raman spectra of celluloses I (1), II (2), and III (3) and cellulose I, reconstructed from cellulose III (4).
rotary isomerism of the spatial arrangement as a result of rotations around the glycoside bonds C1 – O – C0 4 (C1, C2; Fig. 1). The results of the investigations point to a limited freedom of rotation of glucopyranose residues. The most probable conformation features a relatively strong intramolecular hydrogen bond O0 3 –H· · ·O5 (Fig. 1). Hydrogen bonds and positions of lateral groups have a considerable influence on stability of spatial forms of cellulose macromolecules. However, the question of the degree of this influence had to be elucidated. In our calculations for the first time all kinds of rotamers of OH and CH2OH groups and intramolecular hydrogen bonds were taken into account. The calculated values of potential energy depending on the values of angles of rotations around the glycoside bond C1 –O – C0 4 C1 and C2 are given in Fig. 3 [13]. The rotary isomer of pyranoside cycles with C1 ¼ 22108; C2 ¼ 21508 is characterized by the most stable state. At excess energy DU # 2 kcal=mol variations of angles C1 and C2 by 208
120
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Fig. 6. Intramolecular and intermolecular hydrogen bonds in ordered regions of natural cellulose.
are possible. Intramolecular hydrogen bonds considerably limit the mobility of elementary links. In this connection, of great interest was experimental detection of rotary isomerism of elementary links of cellulose by an example of celluloses of various structural modifications. As found earlier, celluloses I and II differ considerably in IR spectra in the region of stretching vibrations of OH groups included in hydrogen bonds, especially in the frequency range of intramolecular bonds [19] (Fig. 4). Fig. 5 shows for the first time the Fourier transform Raman spectra of microcrystalline celluloses of various structural modifications in the 200 – 700 cm21 range. The spectrum of natural cellulose I is distinguished from the spectra of celluloses II and III by the opposite intensity ratio of bonds at 380 and 350 cm21. When cellulose I is obtained back from cellulose III, the initial intensity ratio of these bands is also reconstructed. Such a kind of interrelated change in the intensity ratio of closely spaced bands is a typical manifestation of the phenomenon of rotary isomerism. According to the results of theoretical calculations, the 330 – 350 cm21 spectral range contains vibrational frequencies with prevailing contribution to the potential energy distribution (PED) of C1C2, C3C4 bonds which are directly connected to the glycoside bridges between the elementary links [20]. The earlier-mentioned spectral changes should be definitively attributed to the rotary isomerism of
cellulose pyranose links—the change in their relative position caused by rotations around the glycoside bond C1 –O –C0 4 (angles C1 and C2). Fourier Raman spectroscopy opens up unique possibilities of analyzing rotary isomers of elementary links of cellulose and other polysaccharides. As mentioned earlier, in cellulose and its derivatives only one most stable chair conformation C1 (4C1) of pyranose cycles is realized and their mutual mobility is strongly limited. In specific structural modifications specific types of rotary isomers of elementary links are realized, which determine the form of macromolecules. From this a conclusion of fundamental importance follows: the formation in cellulose of regions with a different degree of ordering, most ordered (the so called ‘crystalline’) and least ordered (the so-called ‘amorphous’) ones, the processes of orientation and crystallization in products on the basis of cellulose and its derivatives are largely determined by the rotamers of hydroxyl groups (their substitutes) and by the system of intra- and intermolecular interactions formed by them. The study of the specificity of these rotamers in various cellulose materials is of great scientific and practical importance [25]. Of particular interest is analysis of most voluminous CH2OH groups whose hydroxyl groups are capable of entering simultaneously into intra- and intermolecular hydrogen bonds (Fig. 6). It was
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Fig. 7. Raman spectra of celluloses I (1), III (2) and II (3).
mentioned earlier that celluloses I, II and III differ in the form of macromolecular chains, the system of intra- and intermolecular hydrogen bonds. Naturally, they should also differ in the specificity of rotamers of oxymethyl groups which can be of two types: rotations around C5 –C6 and C6 – O6 bonds. From Figs. 7 and 8 it is readily seen that Raman and IR spectra of celluloses I, II and III differ considerably in the range of vibration frequencies with prevailing contribution to PED of atoms and bonds of oxymethyl CH2OH groups. According to the data of theoretical calculations, the greatest contribution to the band intensity of vibrational spectra of carbohydrates in the 1400 – 1500 cm21 range is made by deformation vibrations of HC6H and COH groups. The bands caused by vibrations of methylene groups are highly characteristic (up to 80% of the contribution to PED) and lie in a sufficiently narrow spectral range 1460– 1490 cm21 [23,24]. The band of internal deformation vibrations of methylene groups in the Raman spectrum of cellulose
Fig. 8. IR spectra of celluloses I (1), III (2) and II (3).
I with a maximum at 1479 cm21 has a complicated origin—an asymmetric contour with a projection near 1465 cm21. In the spectra of celluloses III and II, instead of this projection sharply defined bands appear at 1465 and 1460 cm21, respectively, while the band at 1479 cm21 disappears. As mentioned earlier, such a kind of interrelated change in intensities of closely spaced bands in vibrational spectra of compounds identical in chemical structure should be attributed to the phenomenon of rotary isomerism, in our case, the change in the spatial arrangement of methylene groups caused by rotations around the C5 – C6 bond. Another highly characteristic vibration band of groups of atoms in C5 and C6 elementary links of cellulose is the band at 900 cm21. The contribution of C5C6H and O6C6H groups to the PED of vibrations with this frequency is 45 and 35%, respectively. The increase in this band intensity on passing from cellulose I to celluloses II and III points to a change in the surrounding of the above groups of atoms due to the
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Fig. 9. IR spectra of cellulose I (1), cellulose I after treatment with 13% NaOH (2), cellulose II (3) (A) and the results of their deconvolution (B).
rotary isomerism of oxymethyl groups. This also explains the gradual disappearance of the sharp band at 1120 cm21 in the region of stretching vibrations of C5C6 and C6O6 bonds, the disappearance of the band at 1290 cm21 and the appearance of a new band at 1266 cm21 in the region of frequencies of non-plane deformation vibrations of methylene groups. In the IR spectrum of cellulose I, as opposed to the Raman spectrum, the main maximum of the band in the 1400 –1500 cm21 range lies at 1433 cm21 (Fig. 8). According to the results of calculations the lowfrequency range in this region has bands caused by vibrations with prevailing contribution to the PED of deformations of angles of C6O6H and C3O3H. The shift of this band maximum from 1433 cm21 (cellulose I) to 1425 cm 21 (cellulose III) and 1420 cm21 (cellulose II) should be attributed to the change in the form of rotamers of hydroxyls in positions ‘3’ and ‘6’ (rotations round bonds C3 –O3
and C6 – O6). As mentioned earlier, exactly these hydroxyls form intramolecular hydrogen bonds between the elementary links of cellulose I stabilizing the form of its macromolecular (O0 3 – H· · ·O5 and O2 –H· · ·O0 6). Since modifications II and III differ from modification I in the form of macromolecules (rotary isomers of elementary links), such a transition presupposes breakage of the former system of intramolecular H-bonds and the formation of a new one with a different spatial arrangement of the corresponding hydroxyl groups. As in the case of Raman spectra, the transition of cellulose I to celluloses II and III leads, in their spectra, to a decrease in the 1115 cm21 band intensity and an increase in the intensity of the 900 cm21 band. In this case, however, the main contribution to the intensities of these bands is made by the C5 –C6 bonds and C5C6H groups. As a result of mathematical treatment (deconvolution) of IR spectra of celluloses I and II, components of the complex band in the 1400 –1500 cm21 range have been revealed. As seen from Fig. 9, by analogy with Raman spectra, at transition cellulose I ! cellulose II the disappearance of the band at 1480 cm21 and the appearance of a new band at 1468 cm21 is observed. The shift of the main maximum of the band at 1430 cm21 is due to the decrease in intensity of the bands at 1430 and 1425 cm21 and the appearance of a new band at 1415 cm21. The components of the complex band at 900 cm21 (900, 895 and 885 cm21) retain their position, but there occurs a considerable increase in intensity of the band at 895 cm21. As seen from the experimental data obtained, cellulose III is intermediate between celluloses I and II as to spectral changes caused by rotary isomerism of oxymethyl groups. The spectral changes caused by rotary isomerism of elementary links in the case of celluloses II and III are identical. It should be concluded that cellulose III is a transitional structural modification in passing from cellulose I to II. To additionally substantiate this basic conclusion, we investigated the kinetics of cellulose II transition from cellulose I by an example of celluloses regenerated after treatment with water solutions of NaOH with a gradual increase in the alkali concentration. From Figs. 9 and 10 it is seen that within the range of
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Fig. 10. Raman spectra of cellulose I (1), cellulose, regenerated after treatment with water solution of NaOH of 13 (2), 16 (3) and 17.5% concentration (4).
Fig. 11. IR spectra of celluloses I (1), and III (2) and cellulose I, reconstructed from cellulose III (3).
concentrations from 13 to 17.5% there occurs a gradual low-frequency shift of 1432 and 1480 cm21 bands in IR and Raman spectra and other spectral changes observed in the cellulose III spectrum, which are typical of the structural transition cellulose I ! cellulose II take place. Convincing evidence that the above differences between the spectra of celluloses I, II and III are due to conformational changes in the structural fragments of cellulose macromolecules is the clearly defined tendency of cellulose I spectrum reconstruction on obtaining cellulose I back from cellulose III (Fig. 11). It should be noted, however, that the 900 cm21 band intensity decreases only partially and the band of internal deformation vibrations of methylene groups (HC6H) in the Raman spectrum retains its position (Fig. 12). It should be concluded that when cellulose I is reconstructed from cellulose III the former system of rotamers of CH2OH groups caused by rotations around C5 – C6 bonds is not reconstructed. Further development and improvement of our
knowledge of rotary isomers of cellulose macromolecules which determine its structural modifications, intra- and intermolecular interactions, the system of molecular and supermolecular ordering is of great importance for the development of structural physicochemistry of cellulose macromolecules and, consequently, for improving products on its basis and their quality. As mentioned earlier, an important role in this process is played by non-destructive spectroscopic methods whose theoretical and experimental base has accumulated an enormous analytical potential and is improving steadily. The combination of Raman and IR spectroscopy and theoretical studies of vibrational spectra open up unique possibilities of studying the specificity of rotary isomers of macromolecules of cellulose and its derivatives. As shown earlier, this phenomenon can manifest itself as a shift of individual bands, a decrease or increase in their intensities. Bands, which are due to vibrations of rotating groups of atoms, change their position at rotation isomerism,
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Fig. 12. Raman spectra of celluloses I (1), and III (2) and cellulose I, reconstructed from cellulose III (3).
and the change in intensities of bands caused by stretching and deformation vibrations has an opposite direction. As we see it, this dependence can be of general importance for spectroscopy of rotary isomers.
3. Conclusion The results obtained enable us to draw the following important conclusions: 1. Celluloses of various structural modifications I, II and III differ in rotary isomerism of elementary links (form of macromolecules), rotamers of C6H2OH groups as a result of rotations around C5C6 and C6O6 bonds. 2. As to the specificity of these differences, the structural modification cellulose III is intermediate between the main modifications, celluloses I and II. 3. The most probable sequence of conformational
transformations of cellulose I to other modifications is as follows: (a) break of intramolecular hydrogen bonds O0 3 – H 0 · · ·O5 and O2 – H· · ·O 0 6 which stabilize the form of cellulose macromolecules and, therefore, of interrelated intermolecular hydrogen bonds O0 6 – H· · ·O00 3 (Fig. 6); (b) change in the form of macromolecules caused by the formation of a new system of rotary isomers of elementary links; (c) the formation of a new system of hydroxyl rotamers; (d) a change in the spatial arrangement of oxymethyl groups (as a result of rotations around C5C6) caused by the change in the form of macromolecules and rotamers of hydroxyl groups; (e) the formation of a new system of intra- and intermolecular hydrogen bonds which stabilize the new structural modification of cellulose. 4. At structural transitions of cellulose I (natural modification) complete reduction of its original structure is impossible. The natural structural modification of cellulose cannot be recognized to be the most stable one. The results obtained are of great importance for the development of a new scientific direction—structural physico-chemistry of cellulose macromolecules which consists in the establishment of interrelationship between the physical structure and conformational properties of macromolecules determined by their chemical structure and intra- and intermolecular interactions.
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