Splitting Patterns Pairs of Doublets

Number of protons to which nucleus is equally coupled 

The intensities correspond to the coefficients of a binomial expansion (Pascal s triangle). 

This proton splits the signal for the methyl protons into a doublet. 

These six protons split the methine signal into a septet. 

The 200-MHz NMR spectrum of isopropyl chloride, showing the doublet-septet pattern of an isopropyl group. 

We often see splitting patterns in which the intensities of the individnal peaks do not match those given in Table 13.2, bnt are distorted in that the signals for conpled protons lean toward each other. This leaning is a general phenomenon, bnt is most easily illustrated for the case of two nonequivalent vicinal protons as shown in Fignre 13.16. 

The appearance of the splitting pattern of protons 1 and 2 depends on their conpling constant / and the chemical shift difference Av between them. When the ratio Av/J is large, two symmetrical 1 1 donblets are observed. We refer to this as the AX case, nsing two 

Chemical shift difference much larger than coupling constant 

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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... 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

To show that the idea of diastereotopic groups is quite general, let us examine one final example in which there are diastereotopic fluorine atoms. The H proton spectrum of l-bromo-2-chloro-1,1,2-trifiuoroethane shows two doublets of doublets, one pair of doublets at about 5.7 ppm and the other at about 6.5 ppm. To save space, the spectrum is not included, but Figure 5.65 shows the splitting pattern and the Newman projection of this molecule. Remember that this is a proton spectrum, but the single proton is split by the three fluorines (spin = ). 

The signals near 7 ppm are likely a result of aromatic protons. Notice that the combined integration of these two signals is 4H. This, together with the distinctive splitting pattern (a pair of doublets), suggests a 1,4-disubstituted aromatic ring ... 

In the NMR spectrum, the dipolar interaction represents itself as a line splitting. For isolated pairs of unlike spins, the i and j spin resonances are each split into a doublet with a splitting of 2d , for like spins a doublet with a splitting of 3d is found. For powder samples a so called Pake powder pattern is found instead of the doublet. Resolved splittings are only observable for fairly well isolated spin systems consisting of two or a very few spins (e.g., the proton pair of the crystal water in gypsum [9]). For extended spin systems, a broad and rather featureless resonance line is observed. 

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