Two-Bond Geminal Coupling Constants

Recall that two nuclei attached to the same atom can be nonequivalent [as required to produce observable coupling between them (Section 8.4)] if they are diastereotopic (Section 4.3). Even if the two nuclei are equivalent, there is a way to measure the coupling constant between them. For example, if two hydrogens are equivalent, one can (in principle, at least) 

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Table 2.7. Structural features and geminal (two-bond) CH coupling constants Jqh (Hz)...

Hz, and, as we have seen already, aliphatic geminal (two-bond, 2J) coupling constants often lie in the range 10-20 Hz and aliphatic... 

As the relative reactivity, log( 1 /r ) becomes larger, the difference between the chemical shifts of the two /3-methylene protons increases linearly. The geminal coupling constant between the two /3-methylene protons, which can be used as a measure of the. v-character of the C—H bond,378 increases mostly with an increase in the reactivity. These results indicate that the contribution of the resonance form is greatest in methyl vinyl ether, which is the least reactive, and is least in /-butyl vinyl ether, which is the most reactive resonance stabilization in alkyl vinyl ether may play some important role in determining the reactivity.377... 

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

Typical values of two-bond spin-spin coupling 7(E-X- F) are reported in Table 4. Geminal coupling constants 7(E-X- F) are sensitive not only to the factors reported for the one-bond coupling, i.e. electronegativity, but also to the nature of the intervening atom X, to the F-X-E bond angle, and finally on the stereochemistry of the compound. 

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