8.13: Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a spin of plus-half. Because the methyl and methylene protons affect adjacent carbon atoms, the effect becomes mutual through the intervening bonds.
Accordingly, the absorption frequencies of the peaks in coupled multiplets are separated by the same interval, which is 7.2 hertz for the triplet and quartet in bromoethane. This interval is the coupling constant, J, which is a quantitative measure of the coupling interaction. NMR spectra recorded at different operating frequencies reveals that the coupling constant is field-independent.
The chemical shift (in ppm), integral value, signal multiplicity, and coupling constant (in Hz) are listed in the abbreviated notation for reporting NMR spectra.
