Selective heteronuclear decoupling

By analogy with homonuclear decoupling above, selective heteronuclear decoupling is also possible. The decoupling of a single proton resonance could in... 

Prior to the advent of 2D methods, selective spin decoupling was used extensively in both proton NMR and in heteronuclear (especially 13C) NMR to ascertain which sets of nuclei contribute to observed spin coupling. Such information is critical to assignment of resonances and to the elucidation of the structure of an unknown molecule. 2D methods now largely supply this information much more efficiently, by correlations that depend on the existence of spin coupling. The homonuclear version of one such experiment is called COSY (correlation spectroscopy), and the heteronuclear version is known by several acronyms, most commonly HETCOR (lieferonuclear correlation). 

Broadband Hartmann-Hahn sequences, such as DIPSI-2 or WALTZ-16, can be made band-selective by reducing the rf amplitude of the sequences (Brown and Sanctuary, 1991). Richardson et al. (1993) used a low-amplitude WALTZ-17 sequence for band-selective heteronuclear Hartmann-Hahn transfer between N and in order to minimize simultaneous homonuclear Hartmann-Hahn transfer between and The DIPSI-2 sequence was successfully used by Gardner and Coleman (1994) for band-selective Hartmann-Hahn transfer between C d and H spins. So far, no crafted multiple-pulse sequences have been reported that were optimized specifically for band-selective heteronuclear Hartmann-Hahn transfer. Based on the results of Section X, it is expected that such sequences with well defined regions for coherence transfer and effective homonuclear decoupling will result in increased sensitivity of band-selective heteronuclear Hartmann-Hahn experiments. 

Figure 4.11. The application of selective proton decoupling in the measurement of heteronuclear long-range proton-carbon coupling constants. Lower traces are from the fully proton-coupled carbon-13 spectrum and the upper traces from that in which the methyl ester protons of 4.3 were selectively decoupled to reveal the three-bond coupling of the carbonyl carbon across the alkene.

It is also possible through selective heteronuclear singlefrequency decoupling to correlate bonded carbons and protons (HETCOR, Heteronuclear Correlated Spectroscopy) as an alternative to identify all directly bonded and longdistance carbon-proton pairs in a molecule. 

In recent publications, the 1H and 13C NMR spectra of lovastatin were fuily assigned by the use of selective homonuclear and heteronuclear decoupling and two dimensional techniques (14,15). 

Fig. 7.5 Solution-state H NMR spectrum of is the simple, coupled H NMR spectrum 1-chloroethene and 1-fluoroethene. The results (or the double negative F undecoupled") of of selective homonuclear decoupling are shown 1-fluoroethene, which has twice as many H in (b) and (c). The results of heteronuclear de- transitions as shown in (d) and (e) (each line is coupling are shown in (d) and (e). Not shown then a doublet from /( H— F), n = 2, 3).

Other potential uses of isotopic labelling techniques which we plan to explore in the future include measuring signal assignments from heteronuclear decoupling and NOE studies and the use of selective deuteriatlon to isolate dipolar relaxation contributions. 

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