Chapter 8: Interpreting Nuclear Magnetic Resonance Spectra

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8.18:

¹H NMR: Complex Splitting

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JoVE Central Analytical Chemistry

¹H NMR: Complex Splitting

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8.1: Chemical Shift: Internal References and Solvent Effects

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8.20: ¹H NMR: Interpreting Distorted and Overlapping Signals

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Recall that a proton M coupled to one proton X results in a doublet signal for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When proton M is coupled to protons X and A, such as in styrene oxide, the signal appears as a doublet of doublets. This complex coupling can be depicted by a splitting diagram. For propyl bromide, where protons M are coupled to three methyl and two methylene protons, the splitting diagram predicts a quartet of triplets with twelve lines. If JAM >> JMX, all twelve peaks would be visible. If JAM ≅ 2 JMX, some peaks overlap, their intensities are added, and nine peaks are seen. When JAM ≅ JMX, as observed in propyl bromide, six peaks are observed. So, the relative magnitudes of the J values can cause deviations from the predicted number of peaks and relative intensities.

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 J_AM and J_MX. The splitting diagram predicts a quartet of triplets with 12 lines, which would all be visible in the spectrum if J_AM >> J_MX. However, if J_AM ≅ 2 J_MX, some peaks overlap, their intensities are added, and 9 peaks are seen. When J_AM ≅ J_MX, 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.