Calculation of conformations and vibrational spectra of a monomeric fragment of the 2,6-carboxymethyl cellulose molecule

Abstract

A theoretical conformational analysis has been carried out for the side substituents of a fragment of the molecule for 2,6-carboxymethyl cellulose (a water-soluble cellulose ether), and the frequencies and the potential energy distribution of the normal vibrations have been calculated for the most stable conformers of the ether groups of this fragment in the approximation of the molecular mechanics method. It has been shown that the most stable conformers are those that have the conformations analysis of the calculated frequencies and the potential energy distribution of the normal vibrations for 13 of the most stable conformers showed, as in the case of the 2,6-hydroxyethyl cellulose molecule, that the frequencies and modes of the normal vibrations are highly sensitive to conformational transitions in the analyzed spectral region (800-1500 cm -1 ). The characteristic patterns for the change in the frequencies and modes of the normal vibrations have been established in connection with conformational transitions within both side substituents. The observed conformational lability of the bulky substituents in the cellulose ether molecules and its manifestations in the vibrational spectrum provide a basis for hypothesizing that one of the major mechanisms for the process of their thermal gelation in aqueous solutions is conformational transitions within these substituents.Introduction. Water-soluble cellulose ethers have great potential for important practical applications because of their capacity for thermal gelation in aqueous solutions. IR [1-3] and Raman [4] spectroscopy are promising methods for understanding and monitoring these processes. The effectiveness of these methods depends on a proper interpretation of the recorded spectra, a necessary condition for which in the case of cellulose ethers is taking into account the probable conformational states of their bulky substituents, the high conformational flexibility of which we have shown for the example of a fragment of the 2,6-hydroxyethyl cellulose macromolecule (2,6-HEC) in [5]. In [6], it was concluded that an important role is played by conformational conversions of the side groups of cellulose macromolecules and cellulose derivatives during structural transitions, esterification reactions, and forming of final articles based on them. The results of spectroscopic studies have made it possible to conclude that in cellulose ethers, amylose ethers, and other polysaccharide ethers, despite introduction of bulky groups into the structure, the chair conformation of the pyranose ring does not change but rather we observe redistribution of the conformations of the side groups, depending on the degree of esterification and the type of polysaccharide [7]. We may hypothesize that during thermal gelation in aqueous solutions of cellulose ethers, an important role is played by conformational transitions of the ether groups, as a result of which the three-dimensional structure of these polymers changes. We cannot rule out the possibility that one of the major reasons ...

Adsorption of Carboxymethyl Cellulose onto Titania Particle Films Studied with in Situ IR Spectroscopic Analysis

“…The dominant contribution of these modes most likely originated from the symmetric stretching of the deprotonated carboxyl group. 16,34 The intensity of the peak at 1325 cm −1 also increases.…”

“…26−29 Andrianov calculated the vibrational modes of a monomeric fragment of the 2,6-carboxymethyl cellulose molecule for different conformations. 16 The pH dependence of the vibrational spectra of CMC in aqueous solution is discussed in other published work. 30 The carboxymethyl substitution of the hydroxyl groups is not homogeneous along the polymer.…”

“…The relative number of unsubstituted, mono-, di-, and trisubstituted sites vary with the average number of substitution. 31−33 Peaks in the 1000−1500 cm −1 region of the vibrational spectra of pristine and carboxymethyl cellulose arise mostly from overlapping and mixed vibrations, 16,26 ensure that peaks used for semiquantitative analysis are distinct and assignable. The most notable changes in the spectrum of solution CMC upon pH change occur to vibrations of the carboxylic group.…”

“…These modes originate from mixed vibrations located on the carboxymethyl side groups involving the O6 and O2 oxygens and are predicted to be sensitive to conformational changes of these groups. 16 Conformational changes of the carboxyl side groups are connected to changes of the intramolecular hydrogenbonding pattern.…”

“…(Left) Schematic drawing of a fragment of CMC for identification of carbon atoms . (Right) Scheme of carboxymethyl cellulose, showing the hydrogen bonds between O3H3···O5 and O2H2···O6.…”