Complex Splitting in ¹H NMR Spectra

– In the last topic we talk about Spin-Spin Splitting in ^¹H NMR Spectra, but In this topic, we will discuss The Complex Splitting.

Complex Splitting in ¹H NMR Spectra

– There are many cases of complex splitting, where signals are split by adjacent protons of more than one type, with different coupling constants.

– The proton NMR spectrum of styrene (Figure) includes several absorptions that show the results of complex splitting.

– Consider the vinyl proton H^a, adjacent to the phenyl ring of styrene.

– The chemical shift of H^a is δ 6.6 because it is deshielded by both the vinyl group and the aromatic ring.

– H^a is coupled to H^b with a typical trans coupling constant, J_ab = 17 Hz.

– It is also coupled to proton with The signal is therefore split into a doublet of spacing 17 Hz, and each of those peaks is further split into a doublet of spacing 11 Hz, for a total of four peaks.

– This complex splitting, called a doublet of doublets, can be analyzed by a diagram called a splitting tree, shown in the following Figure

– Proton H^b is farther from the deshielding influence of the phenyl group, giving rise to the multiplet centered at δ 5.65 in the styrene NMR spectrum.

– H^b is also split by two nonequivalent protons: It is split by H^a with a trans coupling constant J_ac = 17 Hz and further split by H^c with a geminal coupling constant J_bc = 1.4 Hz.

– The splitting tree for showing a doublet of narrow doublets, is shown in the following Figure.

– Sometimes a signal is split by two or more different kinds of protons with similar coupling constants.

– Consider 1-iodopropane (the following Figure), where the b protons on the middle carbon atom are split by two types of protons: the methyl protons (H^c) and the CH₂I protons (H^a).

– The coupling constants for these two interactions are similar: J_ab = 6.8 Hz, and J_bc = 7.3 Hz.

– The spectrum shows the H^b signal as a sextet, almost as though there were five equivalent protons coupled with H^b.

– The trace in the insert box, enlarged and  offset, shows that the pattern is not a perfect sextet.

– The analysis of the splitting pattern serves as a reminder that the N+1 rule gives a perfect multiplet only when the signal is split by N protons with exactly the same coupling constant.

References:

• Organic chemistry / L.G. Wade, Jr / 8th ed, 2013 / Pearson Education, Inc. USA.
• Fundamental of Organic Chemistry / John McMurry, Cornell University/ 8th ed, 2016 / Cengage Learningm, Inc. USA.
• Organic Chemistry / T.W. Graham Solomons, Craig B. Fryhle , Scott A. Snyder / 11 ed, 2014/ John Wiley & Sons, Inc. USA.