On the 4 J HH long‐range coupling in 2‐bromocyclohexanone: conformational insights

2‐Bromocyclohexanone is a model compound in which a 4 J H2, H6 coupling constant is observed, whereas the corresponding 4 J H2, H4 is absent. The observed long‐range coupling is not only a result of the known W ‐type coupling, in the axial conformation, but also because of the less usual diaxial spin–spin coupling in the equatorial conformer. The carbonyl group plays a determining role in describing the coupling pathway, as concluded by natural bond orbital (NBO) analysis; although the $\sigma _{{\rm C2 - H2}} \rightarrow \sigma \ast_{{\rm C1(O) - C6}}$ and $\sigma_{{\rm C6 - H6}} \rightarrow \sigma \ast_{{\rm C1(O) - C2}}$ interactions in the axial conformer contribute for transmitting the spin information associated with the W ‐type coupling, the strong $\sigma_{{\rm C2 - H2}} \rightarrow \pi \ast_{{\rm C = O}}$ and $\sigma_{{\rm C6 - H6}} \rightarrow \pi \ast_{{\rm C = O}}$ hyperconjugations in the equatorial conformer define an enhanced coupling pathway for 4 J H2, H6 , despite the inhibition of this coupling because of $n_{{\rm O}} \rightarrow \sigma \ast_{{\rm C(O) - C}}$ interaction and the large carbonyl angle. These findings provide the experimental evidence that orbital interactions contribute for the conformational isomerism of 2‐bromocyclohexanone. Copyright © 2008 John Wiley & Sons, Ltd.