Strongly coupled nuclei
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص531-532
2025-12-11
59
Strongly coupled nuclei
NMR spectra are usually much more complex than the foregoing simple analysis suggests. We have described the extreme case in which the differences in chemical shifts are much greater than the spin–spin coupling constants. In such cases it is simple to identify groups of magnetically equivalent nuclei and to think of the groups of nuclear spins as reorientating relative to each other. The spectra that result are called first order spectra.
Transitions cannot be allocated to definite groups when the differences in their chem ical shifts are comparable to their spin–spin coupling interactions. The complicated spectra that are then obtained are called strongly coupled spectra (or ‘second-order spectra’) and are much more difficult to analyse (Fig. 15.25). Because the difference in resonance frequencies increases with field, but spin–spin coupling constants are independent of it, a second-order spectrum may become simpler (and first-order) at high fields because individual groups of nuclei become identifiable again. A clue to the type of analysis that is appropriate is given by the notation for the types of spins involved. Thus, an AX spin system (which consists of two nuclei with a large chemical shift difference) has a first-order spectrum. An AB system, on the other hand (with two nuclei of similar chemical shifts), gives a spectrum typical of a strongly coupled system. An AX system may have widely different Larmor frequencies because A and X are nuclei of different elements (such as 13C and 1H), in which case they form a heteronuclear spin system. AX may also denote a homonuclear spin system in which the nuclei are of the same element but in markedly different environments.
Fig. 15.25 The NMR spectra of an A2 system (top) and an AX system (bottom) are simple ‘first-order’ spectra. At intermediate relative values of the chemical shift difference and the spin–spin coupling, complex ‘strongly coupled’ spectra are obtained. Note how the inner two lines of the bottom spectrum move together, grow in intensity, and form the single central line of the top spectrum. The two outer lines diminish in intensity and are absent in the top spectrum.
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