High-resolution nuclear magnetic resonance spectra of phenanthrenes. Part III. Benzo[c]phenanthrene

Abstract

Complete analyses of the ¹H n.m.r. spectra of benzo[c]phenanthrene (I) have been carried out for solutions in carbon disulphide, acetone, [²H₆]benzene, and mixtures of carbon disulphide and [²H₆]benzene. Spin-decoupling and variable temperature experiments have shown that broadening of the H(1,12) resonance must be attributed to unresolved long-range coupling to H(5) and H(8) and not to intramolecular relaxation. Interaction of (I) with [²H₆]benzene has been investigated by dilution and variable temperature experiments. It is concluded that there is, at most, a structuring of an indefinite number of solvent molecules around each solute molecule, rather than the formation of a specific solute–solvent complex. The distinct effect of [²H₆] benzene on the chemical shift of the sterically-hindered H(1), relative to the shifts of other protons, is best explained by restrictions on the close approach of solvent molecules. Intramolecular chemical shifts correspond well with those calculated on the basis of ring currents in a planar molecular model; for H(1), steric deshielding is apparently compensated by a reduced ring-current deshielding.