A CRITICAL EVALUATION OF DEPENDENCE OF 3JHH COUPLINGS ON BOND ORDER AND BOND LENGTH IN CONJUGATED CARBOCYCLIC MOLECULES . EVIDENCE FOR A STERIC EFFECT ON 3JHH

Previously proposed linear correlations between Pπbond orders and the magnitudes of nmr ortho proton-proton spin-spin coupling constants (3J) in unsaturated systems have been reinvestigated using more accurate values for the coupling constants than were available heretofore. It is shown that, although the relationship is no longer linear over the whole range of bond order, the plot is still approximately linear for aromatic systems. Deviations from linearity are shown to exhibit consistent trends, which may be rationalized in terms of steric interactions. A plot of 3J vs. bond length is shown to be linear over the whole range of bond lengths, and the theoretical back-ground of this correlation is discussed in terms of a simple molecular orbital theory, as well as a previously proposed valence bond treatment. © 1969, American Chemical Society. All rights reserved.