Molecular structure and ring distortions of p-dichlorobenzene as determined by electron diffraction |
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Authors: | György Schultz István Hargittai Aldo Domenicano |
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Institution: | Hungarian Academy of Sciences, Research Laboratory for Inorganic Chemistry, Department of Structural Studies, H-1088 Budapest, Puskin utca 11-13, P.O. Box: Budapest. Pf. 117. H-1431 Hungary;Istituto di Chimica Farmaceutica e Tossicologica, Universitá di Roma, 1-00185 Roma; and Laboratorio di Strutturistica Chimica “Giordano Giacomello”, 1-00016 Monterotondo StazioneItaly |
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Abstract: | The molecular structure of p-dichlorobenzene in the vapour phase has been studied by electron diffraction. Least-squares refinement of a model with D2h symmetry has led to the accurate determination of the small deviations of the benzene ring from D6h symmetry caused by the chlorine substituents. The most appreciable effect is an increase from 120° to 121.6 ± 0.2° of the internal angle at the ipso carbon, associated with a shortening of the distance between the two ipso atoms. A less pronounced effect is a shortening of the C-C bonds that originate from the ipso carbons as compared to the central C-C bonds (rg = 139.0 ± 0.3 pm vs. 139.5 ± 0.4 pm). Other bond distances are rg (C-Cl) - 173.0 ± 0.4 pm and rg (C-H) = 109.4 ± 1.0 pm. The observed ring distortions are in agreement with those obtained by low-temperature X-ray crystallography on three different crystal phases of p-dichlorobenzene. They are also consistent with those obtained for chloro-benzene by gas-phase electron diffraction and by NMR spectroscopy in a nematic solvent. The rs structure of chlorobenzene obtained in a recent study by micro-wave spectroscopy is shown to need revision, as far as the ipso region of the ring is concerned. |
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