Homonuclear decoupling experiment

The results of a homonuclear decoupling experiment on sucrose are shown in Fig. 5.18. The experiment is set up by acquiring a normal H spectrum and determining the exact RF frequency of each peak (each resonance) we wish to test by CW irradiation during the acquisition of the FID. The desired frequencies are actually offsets from a fundamental decoupler frequency for example, an offset (Bruker o2 for channel 2 offset Varian dof for decoupler offset) of 132.6 Hz is added to the fundamental frequency (Bruker BF2... 

All of the protons in each of 12 thermo- and photochromic BIPS were assigned through a combination of homonuclear decoupling experiments and correlation spectroscopy. The relative stereochemistry of the gem-dimethyl groups could be assigned on the basis of Nuclear Overhauser Effect (NOE) experiments.131... 

Although we might be able to get all the H-H coupling information through a long series of homonuclear decoupling experiments, there is a much simpler way the 2D NMR technique known as homonuclear shift correlation spectroscopy, or COSY. 

If one is truly pushing the limits of NMR detection or has only an extremely time-sensitive sample, yet another strategy is preferred. Here a 1-D proton spectrum may be all that is realistically attainable. Possible additional experiments include a 1-D version of TOCSY and a 1-D version of COSY, also known as a homonuclear decoupling experiment. This applies to sample quantities down to hundreds of nanograms, which is currently the practical limit of NMR detection using commercially available equipment. 

Figure 1.8 illustrates homonuclear decoupling experiments with the CH protons of 3-aminoacrolein. These give rise to an AMX system (Fig. 1.8a). Decoupling of the aldehyde proton X (Fig. 1.8b) simplifies the NMR spectrum to an AM system Hz)-, decoupling of the M proton (Fig. 1.8c) simplifies to an AX system...

Figure 4.8. The aesthetically unappealing decoupler frequency spike sometimes observed in homonuclear decoupling experiments as in (a) can be readily removed by setting the transmitter and decoupler frequencies to be the same, as in (b).

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. 

Figure 3.24 Homonuclear decoupling experiment. A 250 MHz NMR spectrum of ethylbenzene in deuterated chloroform obtained (a) without decoupling, (b) with irradiation of the methyl resonance, and (c) with irradiation of the methylene resonance. (From Bruch and Dybowski, used with permission.)...

Figure 3.25 Homonuclear decoupling experiments of the 300 MHz proton NMR spectrum of sucrose dissolved in D2O. The fully coupled spectrum is shown in (c). (a) Selective saturation of the triplet at 4.05 ppm collapses the doublet at 4.22 ppm, showing the coupling between the positions of protons a and b marked on the sucrose structure, (b) Saturation of the doublet at 5.41 ppm collapses the doublet of doublets at 3.55 ppm, leaving a doublet. The experiment shows the coupling between the protons marked c and d on the structure. (The spectra are from Petersheim, used with permission. The sucrose structure is that of D-(- -)-sucrose, obtained from the SDBS database, courtesy of National Institute of Industrial Science and Technology, Japan, SDBSWeb http //www. aist.go.jp/RIOBD/SDBS. Accessed 11/05/02.)...

Homonuclear correlation spectroscopy (COSY) offers a way to identify spin-coupled pairs of nuclei, even when structural information for the specific molecule under study is completely lacking. Correlation is established using homonuclear coupling, so the technique essentially shows the same information in one plot as in ID homonuclear decoupling experiments. A potentially useful application for this class of analysis is the spin-spin correlation of C- C, but there are two main problems with this approach. The first one is related to the need of the presence of C nuclei in two adjacent atoms. In natural abundance, this probability is too low, so the experiment is highly insensitive and suitable only for concentrated samples rich in carbon content. The second one relates to the relative weakness of the spin-coupled peaks from C- C pairs when compared with the peaks from isolated C nuclei. 

For APT, COSY and C/ H HETCOR experiments, standard pidses sequence were used. (9-11) For the long-range HETCOR experiment, J(C,C,H) was assumed to be 8 Hz. NOE Difference spectra (homonuclear decoupling experiments ) were recorded using the cyclenoe sequence. 1024 scans were recorded for each spectrum. 

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