Accidentally equivalent protons

As we have already seen, accidental equivalence could be responsible for the theoretically nonequivalent protons of an AB system presenting as a singlet and for the more complex ABX system presenting as a simple doublet and triplet. But occasionally, even more interesting manifestation of accidental equivalence can be observed. Consider the molecule below (Structure 6.10) and its spectrum (Spectrum 6.6) which shows only the regions of interest to us - expanded and with the intervening region removed. 

The complex multiplet centred at 5.04 ppm results from the overlap of the methine and -OH protons (i.e., they are accidentally equivalent ) whilst the equally complex methyl signal is centred at 1.48 ppm. Because of this overlap, their lines are indistinguishable and so the -OH is said to be virtually coupled to the methyl group. Virtual coupling is another potential consequence of non-first order behaviour. 

The N-benzyl protons are accidentally equivalent, presenting as a singlet at 3.59 ppm and overlap with the two protons alpha to the chlorine atom which present as the heavily roofed AB part of an ABX system (i.e., eight lines) centred at 3.55 ppm. 

Proton NMR spectrum of o-xylene. There are three types of protons in o-xylene, but only two absorptions are seen in the spectrum. The aromatic protons Hb and Hc are accidentally equivalent, producing a broadened peak at 8 7.1. 

Note the three types of aromatic protons above are accidentally equivalent because the isopropyl group has little effect on their chemical shifts. 

In Table 4-17 some NMR data of typical norbornadiene complexes are listed. On the basis of these data C7H8Mn(CO)2(7i-C5H5) is formulated as having a norbornadiene bonding mode. The iron derivative displays an accidental equivalence of two types of protons. This also has been observed in other complex organometallic systems and is supported by the relative intensity values. 

The three types of aromatic protons are accidentally equivalent. 

Nuclei are said to be chemically equivalent when they have the same chemical shift, usually as a result of molecular symmetry (e.g., the 2 and 6 protons or the 3 and 5 protons in phenol) but occasionally as a result of an accidental coincidence of shielding effects. Nuclei in a set are magnetically equivalent when they all possess the same chemical shift and all nuclei in the set are coupled equally with any other single nucleus in the molecule. Thus, in the tetrahedral molecule difluoromethane (I) H and H(, are magnetically equivalent because by symmetry they must have... 

From the above discussion, diastereotopic protons cannot be placed in the same set since they are not chemical-shift equivalent. However it is not uncommon for diastereotopic protons to appear to be chemical-shift equivalent in a given magnetic field in a particular solvent. Such accidental chemical shift equivalence can usually be detected by using an instrument with a higher magnetic field or by changing solvents. 

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