General Principles. Anisochrony

Nuclei which are diastereotopic will, in principle, differ in chemical shift, i.e. they will be anisochronous 5).13 It must be pointed out, however, that the chemical shift differences are often small, sometimes so small that the signals can be resolved only at quite high fields, and not infrequently altogether unobservable even at the highest fields available. In the latter situation one may speak of accidental isoehrony , meaning that while the nuclei are in principle anisochronous, they are not, in fact, resolved. 

13 This term was coined by G. Binsch following A. Abragam s use of isochronous for chemical-shift equivalent cf. Ref. 5, p. 23. 

The immediate cause for anisochrony is, of course, the unequal magnetic field sensed by the diastereotopic nuclei. It follows that as the source of the diastereotopic environment is removed further and further from the test nuclei, the anisochrony is expected to diminish. This prediction has been tested 43 with the results shown in Table 1. 

In Table 2 are summarized the shift differences (both H and 13C) between the diastereotopic methyl groups of the compounds46) shown in Fig. 28. (Arguments are adduced in the paper 461 that the conformation shown is by far the preferred one, at least for R = COX.) It is immediately obvious that these differences in shift between diastereotopic protons are much larger for the phenyl than for the cyclohexyl compound presumably because of the much larger differential shielding of the 

R C02HC02CH3 C02Ph COC1 COCHj CONH2 CH2NH2 CH2OH CHjOAc cn  

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