Splitting Patterns The Ethyl Group

There are eight possible combinations of the nuclear spins of the three methyl protons in CH3CHCI2. 

These eight combinations cause the signal of the CHCI2 proton to be split into a quartet, in which the intensities of the peaks are in the ratio 1 3 3 1. 

The methyl protons of 1,1-dichloroethane split the signal of the methine proton into a quartet. 

Its effect is greatest when the number of bonds is small. Vicinal protons are separated by three bonds, and coupling between vicinal protons, as in 1,1-dichloroethane, is called three-bond coupling, or vicinal coupling. Four-bond couplings are weaker and not normally observable. 

A very important characteristic of spin-spin splitting is that protons that have the same chemical shift do not split each other s signal. Ethane, for example, shows only a single sharp peak in its NMR spectrum. Even though there is a vicinal relationship between the protons of one methyl group and those of the other, they do not split each other s signal because they are equivalent. 

Coupling of nuclear spins requires that the nuclei split each other s signal equally. The separation between the two halves of the methyl doublet in 1,1-dichloroethane is equal to the separation between any two adjacent peaks of the methine quartet. The extent to which two nuclei are coupled is known as the coupling constant J and in simple cases is equal to the separation between adjacent lines of the signal of a particular proton. The three-bond coupling constant Jab in 1,1-dichloroethane has a value of 7 Hz. The size of the coupling constant is independent of the field strength the separation between adjacent peaks in 1,1-dichloroethane is 7 Hz, irrespective of whether the spec-tram is recorded at 200 MHz or 500 MHz. 

FIGURE 13.13 The 200-MHz NMR spectrum of ethyl bromide, showing the characteristic triplet-quartet pattern of an ethyl group. 

There are four possible combinations of tiie nuclear spins of tile two metiiylene protons in CH3CH2Br. 

These two protons spht — the methyl signal into a triplet. 

These three protons split the methylene signal into a quartet. 

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Spin-Spin Splitting in NMR Spectroscopy 555 Splitting Patterns The Ethyl Group 557 Splitting Patterns The Isopropyl Group 559 Splitting Patterns Pairs of Doublets 559 Complex Splitting Patterns 561 NMR Spectra of Alcohols 563... 

At first glance splitting may seem to complicate the interpretation of NMR spectra In fact It makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal With practice we learn to pick out characteristic patterns of peaks associating them with particular structural types One of the most common of these patterns is that of the ethyl group represented m the NMR spectrum of ethyl bromide m Figure 13 15... 

The c/s-fused diaziridines (31a) and (31b) are also an equilibrium system, interchanging exo and endo positions of methyl and ethyl groups. The NMR spectrum shows two methyl peaks at 0 °C, coalescing to a single sharp peak at 75 °C. The ethyl group shows the sharp characteristic quartet-triplet splitting pattern at 75 °C (74JOC3187). 

The next question is how can we understand and predict what spin-spin splitting patterns will be observed And how do they give us structural information The important point is that the multiplicity of lines for protons of a given chemical shift often is seen to be (n + 1), in which n is the number of protons on the contiguous carbons. For example, the CH2 resonance of the ethyl group of ethyl iodide is a quartet of lines because of the spin-spin inter-... 

The characteristic splitting patterns for ethyl, isopropyl, and tert-butyl groups. 

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