Decoupling

Decoupling

  Most ¹³C NMR spectra are very complex. The methyl carbon of an ethoxy group will appear as a large quartet, with each line further split into triplets. Even in fairly simple molecules each carbon may be coupled to a number of different protons. In complicated molecules, these multiplets overlap badly, and may be impossible to analyze.

    ¹J_CH = 100-250 Hz
    ^2,3J_CH = 2-10 Hz

  To simplify ¹³C spectra, we usually use some form of broadband decoupling (noise decoupling) to remove the effect of proton couplings. This also dramatically increases signal intensity, since now all carbons appear as singlets (assuming absence of other spin 1/2 nuclei like ³¹P or ¹⁹F). The increase is actually greater by a factor of 2-3 than would be predicted on the basis of simply combining the ¹³C multiplet intensities because the Nuclear Overhauser Effect causes additional increases in signal intensity.

  

Figure  75.4 MHz ¹³C NMR spectrum of isobutyrophenone in CDCl₃. Identify all of the C-H couplings in the expansions of the coupled spectrum. Assume the coupling within the aromatic ring follow those given for benzene itself. Note that the very closely spaced ortho and meta carbons can be assigned based on the ³J_CH couiplings observed.

  

Figure  25 MHz ¹³C NMR spectrum of diphenyl selenide in CDCl₃.

  Figure  shows the fully coupled and decoupled ¹³C NMR spectra of diphenyl selenide. Although the large ¹J_C-H splittings are easy to identify, the fine structure of the individual multiplets is not first order (e.g., only the para carbon has an approximately centrosymmetric pattern, the others do not). This is because we are looking at the X part of an AA'BB'CXpattern (^JABC^j are protons, X is carbon). Since the AA' and BB' parts are strongly coupled, we see the usual complex effects of "virtual coupling" on the X resonance. When noise {¹H} decoupling is applied, the spectrum becomes much more intense, and only 4 lines remain, one for each carbon.

  In this compound we have a second magnetically active nucleus (⁷7Se, natural abundance 7.5%, I = 1/2), so each of the ¹³C peaks has ⁷7Se satellites, although coupling between C-4 and the selenium is too small to detect (the satellites are under the main peak).

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Gizo, on Ghizo Island, is the capital of the Solomon Islands’ far-flung Western Province, a paradise of coral cays, atolls, lagoons and volcanic islands east of Papua New Guinea where, on a rainy day in late July, crowds flocked to the local netball court for the opening of the inaugural Akuila Talasasa Arts Festival.

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DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO
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