An investigation into the structure, chemical-shift anisotropies and J couplings of tetrafluoro-1,4-benzoquinone partially oriented in liquid-crystalline solutions

Abstract

The 13C n.m.r. spectrum and the 13C satellites in the 19F n.m.r. spectrum of tetrafluoro-1,4-benzoquinone (TFBQ) dissolved in CDCl3 solution have been recorded and analysed. The partially oriented spectra determine the sign of at least one J19F –19F coupling constant. By assuming that the sign of the one bond J13C –19F coupling is negative, the signs of the remaining J19F –19F and of three of the five J13C –19F have been determined from the isotropic spectra. 19F n.m.r. spectra of TFBQ have been successively obtained in various liquid-crystal solvents: (I)trans-4-heptyl-(4-cyanophenyl) cyclohexane; (II) Nematic Phase IV (Merck); (III) ZLI Nematic Phase (Merck); (IV) 4,4′-bis(heptyl) azoxybenzene; (V) a lyotropic mesophase formed by poly(γ-benzyl-L-glutamate) in CD2Cl2 solution. Some of these solvents [(II) and (IV)] have been shown to form a charge-transfer complex with TFBQ. For solutions (I) and (II) the 13C satellites of the oriented spectra have also been obtained. The dipolar direct couplings, Tij, corrected for the effect of molecular vibrations, were used to obtained the rα structures which have been compared with the electron-diffraction structure and with the solid-state X-ray structures of both complexed and uncomplexed TFBQ. Finally, the chemical shielding anisotropy has been investigated in solutions (II), (IV) and (V). The quantity Δσ=[σaa–½(σbb+σcc)] has been evaluated and compared with the value from solid-state n.m.r. measurements.