8.18: 1H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first. However, the splittings can be applied in any order, and the same result is obtained.
In propyl bromide, the protons M are coupled to three methyl and two methylene protons, with the coupling constants JAM and JMX. The splitting diagram predicts a quartet of triplets with 12 lines, which would all be visible in the spectrum if JAM >> JMX. However, if JAM ≅ 2 JMX, some peaks overlap, their intensities are added, and 9 peaks are seen. When JAM ≅ JMX, as is the case in propyl bromide, 6 peaks are observed. As a result, the relative magnitudes of the J values can cause deviations from the number of peaks and relative intensities predicted by the n+1 rule and Pascal's triangle.
