Selective homonuclear decoupling

Fig. 3.4 One pulse experiment with selective homonuclear decoupling...

In a homonuclear decoupling experiment a particular multiplet is irradiated suppressing the coupling interaction between the irradiated nucleus and its coupling partners. A comparison of the standard coupled ID spectrum and the selectively homonuclear decoupled spectrum reveals which nuclei are coupled. Whether a homonuclear decoupling experiment or a 2D homonuclear COSY experiment would be the best solution for multiplet analysis in a one-dimensional spectrum depends very much on the nature of the problem under investigation. If a large number of multiplets need to be irradiated then a two-dimension approach may be preferable. 

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).

Selective saturation experiments were performed with the HX270 employing the homonuclear decoupling mode availcible with the instrument but substituting the usual situation by... 

Pig. 1. Pulse sequence for selective reverse INEPT. The time-shared homonuclear decoupling during acquisition is optional, and a variety of simplifications may be made to the sequence depending on the instrument used and on the spin system under investigation, as discussed in the text. A DANTE sequence is shown as the selective 90° carbon-13 pulse, but this may be replaced by a soft pulse or some other form of selective excitation. Phase cycling for this sequence is summarized in table 1. 

Non-selective ID experiments as well as selective ID experiments including either selective weak pulses (ID ROESY, ID TOCSY) or selective continuous radiofrequency irradiation (ID homonuclear decoupling, ID NOE). 

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). 

Jij/2, in complete analogy to the heteronuclear case (Chingas et al., 1981 Ernst et al., 1991). The concomitant increase of the transfer time by a factor of 2 is characteristic for heteronuclear and doubly selective homonuclear Hartmann-Hahn transfer. During doubly selective irradiation, mismatched spins m are effectively decoupled if c vf and Jim and the three spins i, j, and m approach an effective POO... 

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 5 Band-selective homonuclear H- H WURST decoupling of Si(CH2)CH3 resonances in compound 1 recorded at 400 MHz ( H) frequency on a Varian Unity Plus spectrometer, (a) The ordinary spectrum (b) expansion of the Si-H region of (a) (c) the same as (b) in a band-selectively H- H decoupled spectrum with decoupling band centred on SiCH2 and SiCHj peaks between -0.6 and +0.6 ppm...

The time-consuming 2D experiment can be replaced by ID selective inver-sion-recovery/saturation-transfer experiments [119]. Its application to study the heterogeneity of PS/PVME is mentioned afterwards (see Section 10.3.1.1). Furthermore, Campbell and VanderHart [109] showed that the selective techniques under multiple-pulse homonuclear decoupling are not necessary. They realized that at certain t value in a 2D exchange NMR experiment, polarization gradient necessary for spin diffusion can be achieved by the chemical shift difference. The optimum preparation period for the ID analogue of the 2D exchange NMR experiment was discussed. 

---