Coupling between chemically equivalent protons

The AA XX notation may be interpreted as follows. The chemical shifts of the A and X nuclei are very far from each other (at opposite ends of the alphabet). The A and A nuclei are chemically equivalent, but magnetically nonequivalent, as are the X and X nuclei. Figure 4-2 illustrates the proton AA part of the spectrum of 1,1-difluoroethene, in which 10 peaks are visible. This appearance is quite different from the simple 1 2 1 triplet expected in the first-order case. The multiplicity of peaks in Figure 4-2 in fact permits the measurement of 7aa Ihe coupling between the equivalent protons. 

Sheppard and Turner (1959) have obtained proton-proton coupling constants between chemically equivalent protons by observing C13 side band proton resonances. Molecules such as CH Cl.C H Cl have magnetically non-equivalent protons, and coupling constants Jhccr can be measured. By varying the dielectric constant of the solvent these workers were able, with the aid of additional information from the infrared spectra, to estimate coupling constants in individual rotamer forms. 

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). 

Spectral analysis can sometimes be facilitated by taking advantage of dilute spins present in the molecule. Earlier in this chapter, cyclopropene (4-5) was mentioned as an example of an A2X2 spectrum, and in Section 4-5 the vicinal coupling between the protons on the double bond (/aa) was quoted as being 1.3 Hz. How was such a coupling constant between two chemically equivalent protons measured Its small value prohibits the use of deuterium, as /hd would be only 0.2 Hz. For 1.1% of the molecules, the double-bond spin system is... 

For example, two protons of a methylene in the mmm configuration are in different environments. These two protons are non-equivalent, show different chemical shifts, and split due to the geminal J-coupling between the two protons. There are also vicinal couplings between protons three bonds apart, therefore the spectra are very complex. The vicinal coupling helps us to connect a methylene and the neighboring methine. 

The E and Z configurations were assigned by determining the Jhh coupling constant between the two chemically equivalent olefinic protons from the satellites in the H P NMR spectrum, and has been confirmed by X-ray analysis of both isomers [19],... 

With this information in hand, search the proton NMR spectrum for confirmation and further leads. If the spectrum allows, determine the total proton count and ratios of groups of chemical shift-equivalent protons from the integration. Look for first order coupling patterns and for characteristic chemical shifts. Look at the 13C/DEPT spectra determine the carbon and proton counts and the numbers of CH3, CH2, CH, and C groups. A discrepancy between the proton integration and the... 

We have not yet answered a very basic question just which protons in a molecule can be coupled We may expect to observe spin spin splitting only between non-eqnivalent neighboring protons. By non-equivalent protons we mean protons with dilTerent chemical shifts, as wc have already discussed (Sec. 13.8). By neighboring protons we mean most commonly protons on adjacent carbons, as in the examples we have just looked at (Fig. 13.8, p. 427) sometimes protons further removed from each other may also be coupled, particularly if n bonds intervene. (If protons on the same carbon are non-equivalent—as they sometimes are—they may show coupling.)... 

If C-l is a C13 nucleus, the C13-H side band of the proton at C-l introduces magnetic non-equivalence and the coupling between equivalent chemical protons in the ring appears as sub-structure on these C13... 

What we have in the symmetric molecule dimethyl adipate is an extreme case of strong coupling between the sets labeled A2, for which Av/J is zero. That is, the protons of the A2 sets are chemical-shift equivalent and couple as a conglomerate with the X2 protons. This is another example of virtual coupling. 

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