On the (4)J(HH) long-range coupling in 2-bromocyclohexanone: conformational insights

2-Bromocyclohexanone is a model compound in which a 4JH2, H6 coupling constant is observed, whereas the corresponding 4JH2,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 C2-H2-->sigma*C1(O)-C6 and sigma C6-H6 -->sigma*C1(O)-C2 interactions in the axial conformer contribute for transmitting the spin information associated with the W-type coupling, the strong sigma C2-H2-->pi*C=O and sigma C6-H6-->pi*C=O hyperconjugations in the equatorial conformer define an enhanced coupling pathway for 4JH2,H6,despite the inhibition of this coupling because of nO-->sigma*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.