Selective proton decoupling

The 13C signal of a particular carbon can be assigned unequivocally by selective decoupling of the protons attached to it, provided the proton resonance is unequivocally assigned and well separated in the 1H NMR spectrum of the same compound in the same solvent. For selective proton decoupling, the NMR spectrum of the compound is 

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The determination of these two coupling constants can be carried out using a selective proton decoupling experiment. The middle trace in Fig. 18 shows the results of such an experiment. 

Outdated sections have been condensed or deleted, including off-resonance decoupling and selective proton decoupling. A section on the very useful DEPT procedure has been added. Several sections have been revised, and Problems (at 75.5 MHz) have been added. 

The stacked series of selectively proton-decoupled 13C NMR spectra with varying decoupling frequency is conveniently recorded fully computer-controlled, as can be seen for nicotine in Fig. 2.25 (b). The series clearly shows that 13C signals, e.g. those of C-2, C-6, C-4, C-5 and C-4, are unequivocally assigned if the protons attached to these carbons do not overlap with others in the H NMR spectrum. For overlapping proton resonances (e.g. the pairs 2 -H, 5 -H and 3 -H, CH3) more than one carbon is affected by decoupling, of course, and other assignment aids such as two-dimensional CH correlation (Section 2.10) have to be taken into account (e.g. for the pairs C-2, C-5 and C-3, CH3 of nicotine in Fig. 2.25 (b)). 

The offsets necessary for selective proton decoupling do not have to be measured, provided the decoupling frequency of TMS protons is known and the proton shifts of the compound are available from the literature. Tn this case, the decoupling frequency offsets are calculated from the proton shifts. This is performed for the protons of 6-methoxy-a-tetralone. The result of decoupling experiments with these values is shown in F ig. 2.25 (c). A complete assignment of all protonated carbons is achieved. 

Fig. 2.54 presents a two-dimensional carbon-proton shift correlation of D-lactose after mutarotational equilibration (40% a-, 60% / -D-lactose in deuterium oxide), demonstrating the good resolution of overlapping proton resonances between 3.6 and 4 ppm by means of the larger frequency dispersion of carbon-13 shifts in the second dimension. The assignment known for one nucleus - carbon-13 in this case - can be used to analyze the crowded resonances of the other nucleus. This is the significance of the two-dimensional CH shift correlation, in addition to the identification of CH bonds. For practical evaluation, the contour plot shown in Fig. 2.54(b) proves to be more useful than the stacked representation (Fig. 2.54(a)). In the case of D-lactose, selective proton decoupling between 3.6 and 4 ppm would not afford results of similiar quality. 

Based on three-bond carbon-hydrogen coupling constants, specific selective proton decoupling experiments and investigations of specifically deuterated compounds, the resonances of C-6 and C-8 in 5,7-dihydroxyflavonoids appear in the range of 90 to 100 ppm and C-8 is always more shielded compared to C-6. The chemical shift differences found are small for flavanones ( 1 ppm) and larger for flavones and flavonols ( 5 ppm). 

FIGURE 54. 29Si NMR spectra (IGD) of the 3-deuterio derivative 39 under conditions of proton noise decoupling (a) and with selective proton decoupling of (CH3)3Si protons (b). The line due to silicon from the 2HC(3)—O—Si fragment is the line that does not exhibit any splitting in spectmm b. Reproduced by permission of John Wiley Sons, Ltd from Reference 167... 

A variety of techniques including specific deuteration, selective proton decoupling, and lanthanide shift reagents (LSR), Pr(dpm)3, was used by Haines et al. (138) to assign completely the H and Si NMR spectra of methyl-2,3,4,6-tetra-O-trimethylsilyl-oc-D-glucopyranoside. This paper also presents the first LSR study in Si NMR. It shows that the magnitudes of the LSR induced shifts in the spectra are of the same order as those found for the Si resonances. (138)... 

T. Nishida, G. Widmalm, and P. Sandor, Hadamard long-range proton-carbon coupling constant measurements with band-selective proton decoupling, Magn. Reson. Chem., 33 (1995) 596-599. 

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.

K. Bock and C. Pedersen, Assignment of long-range carbon-proton couplings through selective proton decoupling, J. Mag. Reson., 25 (1977) 227-230. 

Application of long-range selective proton decoupling using low-power irradiation has enabled the assignment of indirect coupling in the... 

Seto, H., T. Sasaki, H. Yonehara, and J. Uzawa Studies on the Biosynthesis of Pentalenolactone. Part I. Apphcation of Longrange Selective Proton Decoupling (LSPD) and Selective C- H NOE in the Structure Elucidation of Pentalenolactone G. Tetrahedron Letters 923 (1978). 

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