Three-Bond Vicinal Coupling Constants

TABLE 9.3 Some Representative Two-Bond Coupling Constants  

To get the value of (a geminal coupling constant), refer back to Table 9.3 the value of 7hh for cyclohexane is -13 Hz. Note Cyclohexane is another example of a molecule in which two hydrogens attached to the same carbon (H and H ) are nonequivalent.  

Vicinal coupling across double bonds (a + ti) shows a similar variation with stereochemistry. But the presence of the double bond has two important consequences. First, the dou- 

Second, because of the extra intervening (n) bond, there are more electrons communicating spin information between the nuclei, and Vhh is about 20% greater than in the case of all single bonds. Thus, cis Vhh values across a double bond are typically 10-12 Hz (8 Hz for ortho hydrogens on an aromatic ling), while the corresponding trans values are 16-18 Hz. 

It may seem somewhat anomalous that the value of Vhh in the linear molecule acetylene (H-C=C-H) is only 9 Hz, because both the presence of the triple bond (a -e 2 7t) and j-character effects might have led us to predict a larger value. The problem here is that the two C-H molecular orbitals are collinear (and face opposite directions), so there is no dihedral angle. 

So far in this section we have dealt only with homonuclear (HH) vicinal coupling. But it should come as no surprise that 

---

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

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... 

Table 2.8. Structural features and vicinal (three-bond) CH coupling constants Jch (Hz)...

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. 

One very useful tool for studying conformations in solution is a relationship between dihedral angles and three-bond NMR coupling constants ( 7) of vicinal protons. The relationship was derived theoretically by Karplus, and one form of the resulting equation is shown in equation 3.10, where is the H-C-C-H dihedral angle ... 

---