Coupling over more than three bonds

5 Coupling over more than three bonds 

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The bottom line is that observable long-range coupling (over more than three bonds) is possible, but only under limited circumstances. And even then, it usually involves relatively small coupling constants. 

Couplings over More than Three Bonds and Through Space... 

Couplings over more than three bonds and through space 209... 

Coupling Over More than Three Bonds in Alkenes (Long-Range Coupling) (J in Hz)... 

The aromatic quaternary carbon atoms can be divided into three different groups (a) oxygen-bearing carbon atoms [C( 1), C(3), C(6), C(8) 5158-165] (b) carbon atoms ortho to aryloxy substituents [C(2), C(12), C(13) 5108.2, 5108.6, 5115.8] and (c) carbon atoms C(ll) and C(14), which resonate at 5132.9 and 5134.6. To assign these carbon atoms, extensive use was made of (C,H) couplings over more than one bond (Ernst et aL, 1977) and of selective proton decoupling experiments. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

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