Calculation of polarized IR absorption spectra for trans-1,4-polyisoprenes of various conformations

Abstract

535.43Polarized IR spectra for two conformations of trans-1,4-polyisoprene (α-and β-gutta-percha) were calculated. The IR dichroism of the absorption bands was calculated for both conformations. The computed results for polarized IR spectra and IR dichroism agree reasonably well with the respective experiment data.Introduction. Trans-1,4-polyisoprene (TPIP) is of great interest because it not only is used widely in industry but also has several conformational isomers (α-and β-gutta-percha) [1-3]. Namely the latter property is responsible for the interest in the spectral properties of TPIP. Therefore, the experimental and theoretical IR spectra of TPIP have been well studied [4][5][6][7][8].Herein the intensities and dichroism of absorption bands in the IR spectrum of the conformational isomers of TPIP (α-and β-gutta-percha) are calculated based on the method for calculating intensities and dichroism of absorption bands in IR spectra of oriented polymers that was proposed previously [9][10][11]. The calculated IR and polarization absorption spectra of these conformational isomers of TPIP are constructed and compared with the corresponding experimental spectra.Geometrical Structure of TPIP Conformational Isomers. The crystal structure of TPIP was studied several times. Structures for the crystal cells of these polymers that differed from each other were proposed [1-3]. According to one study [1], β-gutta-percha has an orthorhombic cell with constants a = 7.78 Å, b = 11.78 Å, and c = 4.72 Å with four isoprene units in the cell. The monoclinic cell of (-gutta-percha has dimensions a = 7.98 Å, b = 6.29 Å, and c = 8.77 Å, α = 102 o and two isoprene units [1]. These polymers were also studied by conformational analysis [2] and calculation of the packing energy [3]. It was shown that αand β-gutta-percha are conformational isomers that differ from each other only by the angles of internal rotation, the values of which are different for different reports [1][2][3]. The angles of internal rotation (θ 1 , θ 2 , θ 3 , etc.) in the models of the repeating chain unit of TPIP that were used for the calculations correspond with one of the previous reports [3]: 1) θ 1 = 105 o , θ 2 = -105 o , θ 3 = 180 o with a repeat length of 4.75 Å (β-form); 2) θ 1 = 105 o , θ 2 = -105 o , θ 3 = 180 o , θ 4 = -105 o , θ 5 = 105 o , θ 6 = 180 o with a repeat length of 8.75 Å (α-form). The repeating chain unit of TPIP has the form:(for β-gutta-percha θ 4 = θ 1 , θ 5 = θ 2 = θ 6 = θ 3 , two of its repeating units are shown).S. U. Umarov Physico-Technical Institute, Academy of Sciences of the Republic of Tajikistan, 299/1 Aini St.,