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

We have used the molecular mechanics method for conformational analysis of a fragment of the molecule of a water-soluble cellulose ether: 2,6-hydroxyethyl cellulose (2,6-HEC). As a result of rotation about the C-C and C-O bonds within the bulky side substituents on the C5 and C2 backbone atoms in the fragment of the 2,6-HEC molecule, we have obtained 12 of the most stable conformers for this fragment with strain energies differing within the range 0–2.3 kcal/mol. We have shown that the most stable conformers have the following conformations for the groups of atoms on the bonds C5-C6, C6-O6, C13-O6, C10-C13: gt, g⁻, g⁺, g⁺ and gg, t, g⁺, g⁺ respectively; and on the bonds C2-O2, C11-O2, C7-C11: g⁺g⁻, g⁻, g⁺ respectively. For the conformers obtained, using the same method we calculated the frequencies and the potential energy distribution (PED) for their normal vibrations (NV). Comparative analysis showed high sensitivity of both the frequency and the PED of the normal vibrations to conformational changes within the indicated bulky substituents over the entire analyzed spectral region (800–1500 cm⁻¹ for the methylene groups). We introduce the concept of “conformational characteristicity” of both the frequency and the mode of vibration. We show that it is possible to analyze the conformations of the bulky substituents of the fragment of the 2,6-HEC molecule using its vibrational spectrum. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 42–53, January–February, 2006.     

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

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 gg, t, g⁻, g⁻, g⁻-for the groups of atoms on the bonds C5-C6, C6-O6, C13-O6, C10-C13, C10-O9 and the conformations g⁺g⁻, g⁺, g⁻, g⁻; g⁺g⁻, g⁻, g⁻, g⁻; g⁺g⁻, g⁻, g⁺, t for the groups of atoms on the bonds C2-O2, C11-O2, C7-C11, C7-O8. Comparative 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. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 5–15, January–February, 2007.     

Low-frequency vibrational spectra of crystals of tutton salts

IR absorption spectra and polarized Raman spectra of crystals of Tutton salts K₂M(SO₄)₂6H₂O and (NH₄)₂M(SO₄)₂·6H₂O, where M=Co, Ni, Zn, have been obtained by experiment at 93 K and at room temperature. The frequencies and forms of normal modes of the [Zn(H₂O)₆]²⁺ octahedral complex have been calculated. The observed lines are assigned to the internal modes of the [M(H₂O)₆]²⁺ complex and external modes of the crystal lattice in accordance with the results of the calculations and factor-group analysis. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 445–449, July–August, 2000.     

Calculation and interpretation of vibrational spectra of halogen-substituted ethanes

The frequencies, forms of the normal vibrations, and the potential energy constants of the CH₃CHF₂, CH₃CHCl₂, and CH₃CHBr₂ molecules were calculated. The interpretation of the vibrational spectra existing in the literature was refined, and the characteristics of the force field in these molecules were established.     

Calculation and interpretation of vibrational spectra of monomethyl-substituted pyridines

A calculation of the fundamental frequencies of antisymmetric vibrations of -, -, and -picolines and -picoline-D₇ is made. The calculated values show good agreement with experimental vibrational-spectral data. The effect of methyl substitution on the nonplanar-vibration frequency of pyridine is explained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 114–119, February, 1972.The authors thank N. K. Siderov and M. L. Kats for their interest.