Carbon coupling constants electronegativity

Moreover, INADEQUATE experiments for analysis of carbon-carbon connectivities require careful interpretation when the magnitudes of two-bond and one-bond carbon-carbon coupling constants approach each other. Large two-bond CC couplings, for example, are observed for alkynyl, cyclobutyl, cyclobutenyl substructures and for carbonyl compounds substituted by electronegative groups in an a position. 

Some other values of carbon-carbon coupling constants are given in Table 9. The data in Table 9 show that Jqc substituted cyclopropanes are proportional to in substituted methanes it is also apparent that the coupling constants increase with increasing substituent electronegativity. 

Table 2.6. Structural features (carbon hybridisation, electronegativity, ring size) and typical one-bond CH coupling constants Jch (Hz) ...

As indicated, the magnitudes of three-bond F—F and F—H coupling constants are observed to vary as a function of the sum of the electronegativities of the other substituents that are on the two carbons in question, with the absolute values of these coupling constants... 

The numbers in Table 49 indicate that as the electronegativity of R increases, the chemical shifts of the sp3 and (for the most part) the sp2 carbon atoms a and ft to the nitrogen atom also increase. In all three cases, the pattern is reproduced and similar coupling constants are observed. The only exception appears to be carbon d in 115, which has approximately the same chemical shift as that of 114. 

Carbon-hydrogen spin-spin coupling constants are included in a review by Hansen and discussed in terms of ring strain, steric effects, electronegativity, lone pair effects and electric field effects. Additivity of these effects in a few systems is commented on. 

The presence of electronegative elements directly attached to the same carbon atom as one of the vicinally coupled protons decreases the magnitude of the coupling constant, while the presence of electropositive elements increases it. This effect is small in chains (which are capable of relatively free rotation) but more pronounced in rigid systems such as alkenes. 

F-H coupling constants also decreases as one accumulates additional electronegative substituents on the carbons bearing the coupling nuclei. 

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