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Application of molecular mechanics to refine and understand the crystal structure of polythiophene and its oligomers
Author(s) -
Ferro Dino R.,
Porzio William,
Destri Silvia,
Ragazzi Massimo,
Brückner Sergio
Publication year - 1997
Publication title -
macromolecular theory and simulations
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.37
H-Index - 56
eISSN - 1521-3919
pISSN - 1022-1344
DOI - 10.1002/mats.1997.040060403
Subject(s) - crystallography , intermolecular force , polythiophene , crystal structure prediction , crystal structure , planarity testing , crystal (programming language) , materials science , chemistry , tetramer , polymer , molecule , conductive polymer , organic chemistry , enzyme , computer science , programming language
A molecular mechanics computational procedure, previously used for the refinement and the analysis of several crystalline polymers, was applied to investigate the crystal structures of the tetramer (T4) and hexamer (T6) of thiophene, as well as the crystal structure of polythiophene (PT). Simultaneous minimization of intra‐ and intermolecular energies of the T4 and T6 structures, obtained by Rietveld analysis of powder X‐ray diffraction profiles, leads to molecular conformations showing smaller deviations from the ring co‐planarity than the original models. For both oligomers the calculations confirm that the molecular centre of inversion is not a crystallographic centre of symmetry, as also revealed by X‐ray diffraction of the T6 single crystal. This surprising effect appears to arise from intermolecular interactions between the terminal residues, hence is not relevant with respect to the PT polymer structure. The small energy cost for constraining the molecules at the crystallographic centre of symmetry is in agreement with experimental findings that reveal the existence of polymorphs for both T4 and T6. The calculations on the T6 single crystal were used to upgrade the MM2‐like force field, which was then used to determine the minimum‐energy model of the monoclinic crystal structure of polythiophene.