Calculation of infrared spectra for conformers of a dimeric fragment of the 2,6-hydroxyethylcellulose macromolecule in aqueous solution

Based on both a complete calculation of normal vibration frequencies and absolute IR absorption band inten- sities of a dimeric fragment of the 2,6-hydroxyethylcellulose macromolecule and a comparison of the results with the corresponding experimental data, a detailed interpretation of the IR spectrum (1200-960 cm -1 ) of this polymer is given for the first time. Its conformational characteristics in aqueous solution at 20 and 70 o C were determined by the molecular dynamics method. It is shown that the changes observed in IR spectra of hydroxyethylcellulose in aqueous solutions upon heating may be connected with conformational transitions of the ether side groups. analysis, absolute intensities, spectrum interpretation. Introduction. Water-soluble ethers of cellulose (hydroxyethyl-, carboxymethyl-, methyl-derivatives) are widely used in various industrial sectors. Structural features, in particular, the number and location of ether groups on the pyranose ring and their rotational isomerization are responsible for the valuable properties of these compounds. Vibra- tional spectra (IR and Raman) can reveal even the most subtle structural changes in these macromolcules. Therefore, they are exceedingly effective for studying the structural organization of water-soluble ethers of cellulose (1-4). The reversible changes in the spectra that occur on changing the temperature of the polymers are not related to any chemi- cal transformations in the structure of these compounds and can be explained (5) by a change of the rotational iso- meric (conformational) structure of the macromolecules (6, 7). Studies of the conformational features of the side groups in water-soluble ethers of polysaccharides should establish structureproperty relationships for these practically important compounds. The goal of the present work was to determine the structure of side groups in a dimeric fragment of the water-soluble cellulose ether macromolecule 2,6-hydroxyethylcellulose (2,6-HEC) in aqueous solution at various tem- peratures, to calculate the frequencies and potential energy distribution (PED) of normal vibrations and absolute inten- sities of IR absorption bands, and to model the optical density spectrum of the most stable conformers of the ether groups of this fragment. Experimental Method. The stated problem was solved using a combined approach (8) to modeling IR spec- tra of organic compounds that used both molecular mechanics and quantum chemistry methods to calculate the fre- quencies of normal vibrations and absolute intensities of IR absorption bands, respectively, of polyatomic molecules. The algorithm for calculating the absolute intensities within the framework of the applied approach (as for using the valence force field approximation (8, 9)) consisted of calculating the derivatives of the electron-density P matrix over Decartes shifts of atoms and the matrix of derivatives ∂r ⁄ ∂Qi from Decartes shifts of atoms over normal coordinates and multiplying the resulting matrices. Shifts of atoms in Decartes coordinates in each normal vibration were calcu- lated using molecular mechanics (solution of the direct spectral problem). Matrices of charges on atomic nuclei, elec- tron densities, and bond orders were calculated using the CNDO/2 quantum-chemical method. The derivatives of these