Conformational conversion and exchange processes
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص532-533
2025-12-11
60
Conformational conversion and exchange processes
The appearance of an NMR spectrum is changed if magnetic nuclei can jump rapidly between different environments. Consider a molecule, such as N, N-dimethylform amide, that can jump between conformations; in its case, the methyl shifts depend on whether they are cis or trans to the carbonyl group (Fig. 15.26). When the jumping rate is low, the spectrum shows two sets of lines, one each from molecules in each con formation. When the interconversion is fast, the spectrum shows a single line at the mean of the two chemical shifts. At intermediate inversion rates, the line is very broad. This maximum broadening occurs when the lifetime, τ, of a conformation gives rise to a linewidth that is comparable to the difference of resonance frequencies, δν, and both broadened lines blend together into a very broad line. Coalescence of the two lines occurs when
A similar explanation accounts for the loss of fine structure in solvents able to ex change protons with the sample. For example, hydroxyl protons are able to exchange with water protons. When this chemical exchange occurs, a molecule ROH with an α-spin proton (we write this ROHα) rapidly converts to ROHβ and then perhaps to ROHα again because the protons provided by the solvent molecules in successive exchanges have random spin orientations. Therefore, instead of seeing a spectrum composed of contributions from both ROHα and ROHβ molecules (that is, a spectrum showing a doublet structure due to the OH proton) we see a spectrum that shows no splitting caused by coupling of the OH proton (as in Fig. 15.6). The effect is observed when the lifetime of a molecule due to this chemical exchange is so short that the lifetime broadening is greater than the doublet splitting. Because this splitting is often very small (a few hertz), a proton must remain attached to the same molecule for longer than about 0.1 s for the splitting to be observable. In water, the exchange rate is much faster than that, so alcohols show no splitting from the OH protons. In dry dimethyl sulfoxide (DMSO), the exchange rate may be slow enough for the splitting to be detected.
Fig. 15.26 When a molecule changes from one conformation to another, the positions of its protons are interchanged and jump between magnetically distinct environments.
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