Three-bond vicinal coupling

Spm-spm splitting of NMR signals results from coupling of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds vicinal coupling)... 

No unusual couplings nor coupling constants are observed in the NMR spectra of the Ketones. The aliphatic three bond vicinal coupling JHC-CH is 6-8 Hz, the aromatic ortho coupling constant varies from 8-9 Hz, and the olefinic coupling constants display the values listed. 

The most fruitful method, making use of (8), has been the treatment of three-bond vicinal coupling constants ethane-like fragments... 

That our current picture of three-bond vicinal coupling is substantially correct can be seen best in the effect of the dihedral angle between adjacent C—H bonds on the magnitude of the spin interaction. Recall that two nonequivalent adjacent protons give rise to a pair of doublets, each proton splitting the other. 

The mechanism of cis and trans couphng in alkenes is no different from that of any other three-bond vicinal coupling, and that of the terminal methylene protons is just a case of two-bond geminal coupling. All three types have been discussed already and are illustrated in Figure 5.19. 

Valeraldehyde (pentanal). The aldehyde peak on carbon 1 appears at 9.8 ppm. It is split into a triplet by the two methylene protons on carbon 2 ( J =1.9 Hz). Aldehyde protons often have smaller three-bond (vicinal) coupling constants than typically found. The pattern at 2.4 ppm (triplet of doublets) is formed from coupling with the two protons on carbon 3 ( J = 7.4 Hz) and with the single aldehyde proton on carbon 1 ( J= 1.9Hz). 

Two- (geminal) and three-bond (vicinal) coupling constants, J( Si,X) and... 

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