Spin decoupling homonuclear

Take a moment to review the H spectrum of structure 8-2 (Example 8.11). What would be the effect of irradiating the Hb multiplet at 8 1.76 Saturating spin states of Hb by irradiation at that frequency would cause all H-H spin couplings involving that nucleus to disappear. As a result, the doublet centered at 8 0.915 would collapse to a singlet, and the triplet at 8 3.40 would collapse to a doublet. 

There are, however, certain operational problems with this technique. First, it can be quite tedious and time consuming to locate v2 at exactly the desired frequency, then adjust its power to saturate only that signal without disturbing any nearby signals. Second, to confirm all couplings in a molecule with n sets of mutually coupled nuclei, it would be necessary to generate at least n - 1 separate decoupled spectra. Fortunately, there are easier ways to confirm which sets of nuclei are coupled, and we will discuss these in Chapter 13. 

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H. Liu. S. J. Glaser and G. P. Drobny, Development and optimization of multipulse propagators Applications to homonuclear spin decoupling in solids. J. Chem. Phys., 1990, 93, 7543-7560. 

Before the advent of 2D NMR spectroscopy, the classical procedure for determining proton-proton connectivities was by homonuclear proton spin decoupling experiments. Such experiments can still serve to determine some H/ H connectivities in simple molecules. 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

Bloch-Siegert phase shift is usually introduced in homonuclear nuclear decoupling with an inversion pulse that is far off-resonance to the spins in precession. To minimize the disturbance, the strength of the inversion pulse... 

Spin decoupling or nuclear magnetic double resonance (NMDR) is achieved by irradiating an ensemble of nuclei not only with a radio frequency fl, at resonance with the nuclei to be observed, but additionally with a second alternating field B2 at resonance with the nuclei to be decoupled e.g. H). Decoupling experiments can be carried out to convert homonuclear ( H— JH, 19F —19F) or heteronuclear multiplets (19F—1H,... 

Undesired homonuclear spin interactions can be also suppressed using suitable multiple-pulse sequences while still exploiting the information content provided by interactions that are not affected. Using a combination of MAS and pulse decoupling it is even possible to reintroduce parts of an interaction that would be averaged out by one of the manipulation techniques alone ( recoupling ) [11]. This high flexibility of solid-state NMR enables one to fully exploit the rich information content provided by the spin interactions. It becomes particularly powerful if such experiments are combined to multidimensional NMR techniques as discussed in Section 14.3. 

We will discuss this technique again in Chapter 12, but for now we can say that spin decoupling allows us to electronically erase the effect of H/C coupling, greatly simplifying the appearance of the spectrum. Can this same technique be used to decouple all homonuclear coupling H spectra No, not exactly. But in Chapter 13 we will see an indirect way to accomplish this feat ... 

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