The carbonyl group as a reluctant transmitter of hyperconjugative or σ–π spin–spin coupling interactions in derivatives of benzaldehyde, acetophenone, and benzophenone

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Abstract
Unlike their counterparts in anisole or toluene derivatives, the six-bond spin–spin coupling constants between para ring protons or 19F nuclei and protons or 13C nuclei in the sidechain of derivatives of benzaldehyde, acetophenone, and benzophenone can apparently contain components of opposite sign, at least for the fluorine derivatives. The σ–π components are much smaller in magnitude than in toluene derivatives, leading to very small or unobservable coupling constants. Consequently they are of limited use in conformational analysis. INDO MO FPT computations and their modifications are examined as to the reasons for the small σ–π magnitudes. Although the spin polarizability of the 2pz orbital on oxygen appears to play an important role in the transmission of nuclear spin state information, the computations do not account for a 19F coupling mechanism that appears to be significant for planar conformations. On the other hand, spin–spin coupling constants over five formal bonds to meta protons are sizeable and stereospecific.

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