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 J_{¹⁹F –¹⁹F} coupling constant. By assuming that the sign of the one bond J_{¹³C –¹⁹F} coupling is negative, the signs of the remaining J_{¹⁹F –¹⁹F} and of three of the five J_{¹³C –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, T_ij, corrected for the effect of molecular vibrations, were used to obtained the r_α 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=[σ_aa–½(σ_bb+σ_cc)] has been evaluated and compared with the value from solid-state n.m.r. measurements.