Coupling Constants on Electronegativity of Substituents

Vicinal coupling constants ( J) decrease if an electronegative substituent is attached to the H—C—C—H moiety. The relationship between the electronegativity of a substituent X and the coupling constant between vicinal 

The coupling among the equivalent geminal protons in methane or methyl chloride is not visible but the coupling constants can be calculated from data obtained after isotopic substitution. Replacement of one of the protons by deuterium results in H—D coupling being observed and the proton-proton coupling constant may then be obtained from the equation 

Neighboring n bonds make a negative contribution to the coupling constants, i.e., their absolute value increases. If the methylene system (or methyl group) is freely rotating and can adopt any of the possible conformations with respect to the adjacent n bond, then the negative contribution is about 2 Hz for each adjacent n system. Thus, as shown in Table 2.5, the value for the geminal protons in methane is —12.4 Hz with no adjacent n bonds, —14.5 Hz in toluene with one adjacent bond, —16.9 Hz in acetonitrile 

2 Spin-Spin Coupling in H-NMR Spectroscopy Table 2.4. Effect of / -substitution on Jgem 

Vicinal coupling constants ( J) are markedly dependent on the angles 6 and 6 between the C—H and C—C bonds. This effect is best illustrated in cyclic olefins in which the dihedral angle is zero, and 6 = 6 With decreasing ring size, the angles 9 and 6 increase, and the coupling constants between the vicinal protons decrease. This is illustrated in Table 2.6. 

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