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On the 4 J HH long‐range coupling in 2‐bromocyclohexanone: conformational insights
Author(s) -
Coelho Jakelyne V.,
Freitas Matheus P.,
Tormena Cláudio F.,
Rittner Roberto
Publication year - 2009
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2385
Subject(s) - conformational isomerism , coupling (piping) , natural bond orbital , chemistry , crystallography , coupling constant , sigma , spin (aerodynamics) , stereochemistry , physics , computational chemistry , molecule , particle physics , quantum mechanics , density functional theory , thermodynamics , mechanical engineering , organic chemistry , engineering
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.