R. Baughman, B. Kohler, I. Levy
Jun 1, 1985
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Synthetic Metals
Abstract
Abstract The crystal structure of trans,trans -1,3,5,7-octatetraene has been determined using x-ray diffraction techniques. This crystal structure, the first for an unsubstituted polyene, is used to predict the molecular and crystallographic structures of trans -polyacetylene. The structure of the octatetraene crystal is monoclinic, P2 1 /c, and has two centrosymmetric molecules in a unit cell with dimensions a = 10.228(8) A , b = 4.120(1) A , c = 8.313(3) A , and β = 96.53(4)°. The carbon skeleton is planar to within the experimental error and the carbon-carbon bond lengths starting from the chain end are 1.336(4), 1.451(3), 1.327(4), and 1.451(5) A. The average CCC angle is 125.0(3)°. From the intermolecular packing mode of octatetraene, the predicted structure of trans -polyacetylene is monoclinic, P2 1 /n, with a = 7.38 A , b = 4.12 A , c = 2.47 A ( chain axis ), β = 97.8° , and two chains per unit cell (2C 2 H 2 units). These axial lengths deviate at most by 1% from the average of three experimental determinations using x-ray diffraction and suggest that alternate proposed cells are less likely. Likewise, the predicted setting angle of the polymer chain (52.7° relative to the (010) of the present cell) is in good agreement with the results of crystal energy calculations (51.0°) and x-ray diffraction analyses (55°). Using the octatetraene data, the predicted single-bond length, double-bond length and CCC angle in trans -polyacetylene are 1.451(5) A, 1.327(4) A, and 125.3(4)°. These results are in reasonable agreement with bond lengths derived from nutation n.m.r. studies on trans -polyacetylene ( 1.44 ± 0.02 a and 1.36 ± 0.02 A ). The dimerization parameter ( μ 0 ) derived from the octatetraene results is 0.035(1) A, which can be compared with values previously estimated from the analysis of the limited available electron and x-ray diffraction intensities for poorly ordered trans -polyacetylene ( 0.012 ± 0.001 A and 0.03 A , respectively.