Decoupling in NMR

Molecules labeled with deuterium and/or tritium are useful for fundamental investigations in a variety of chemical fields, particularly spin decoupling in NMR,1 the measurement of coupling constants in ESR,2 mass spectrometry,3-4 and general reaction mechanism studies. Such tagged compounds are also valuable as tracers in biochemical and medical research. 

Decoupling In NMR spectroscopy, any process that destroys the coupling of nuclear spins between two nuclei. Two types of decoupling are employed in NMR spectroscopy. Broadband decoupling removes all the couplings off-resonance decoupling removes... 

Their effects are illustrated in figure B1.11.4 which is a H-decoupled C NMR spectrum of the same sample of paracetamol, obtained without such precautions. The main peak integrals are displayed both as steps and... 

Figure 9.1. 395 MHz M NMR spectra of 2-iiorbornyl 50 MHz proton decoupled C NMR spectra cation in SbF5/S02CIF/S02F2 solution of 2-norbornyl cation ( C enriched) in SbF5/S02ClF/S02F2 solution.

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

In the //broadband decoupled C NMR spectrum, 15 carbon signals can be identified, in agreement with the molecular formula which indicates a sesquiterpene. The DEPT experiments show that the compound contains four quaternary C atoms, three CH units, seven CH units and a CH3... 

The sample prepared is not particularly pure, so instead of the 30 signals expected, 33 signals are observed in the // broadband decoupled C NMR spectrum. Only by pooling information from the DEPT experiment and from the reliable analysis be obtained, as shown in Table 51.1. Here the AB systems of the geminal CH2 protons are assigned. 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

A convenient way to understand the modern 2D NMR experiment is in terms of magnetization vectors. Figure 3.2 presents a pulse sequence and the corresponding vector diagram of a 2D NMR experiment of a single-line C spectrum (e.g., the deuterium decoupled C-NMR spectrum of CDCl,). 

The nOe experiment is one of the most powerful and widely exploited methods for structure determination. nOe difference (NOED) or the two-dimensional experiment, NOESY, is used extensively for stereochemical assignments. It provides an indirect way to extract information about internuclear distances. The other use of nOe is in signal intensification in certain NMR experiments, such as the broad-band decoupled C-NMR experiment. 

Heteronuclear two-dimensional /-resolved spectra contain the chemical shift information of one nuclear species (e.g., C) along one axis, and its coupling information with another type of nucleus (say, H) along the other axis. 2D /-resolved spectra are therefore often referred to as /,8-spectra. The heteronuclear 2D /-resolved spectrum of stricticine, a new alkaloid isolated by one of the authors from Rhazya stricta, is shown in Fig. 5.1. On the extreme left is the broadband H-decoupled C-NMR spectrum, in the center is the 2D /-resolved spectrum recorded as a stacked plot, and on the right is the con tour plot, the most common way to present such spectra. The multiplicity of each carbon can be seen clearly in the contour plot. 

Figure 5.5 shows the heteronuclear 2Dy-resolved spectrum of camphor. The broad-band decoupled C-NMR spectrum is plotted alongside it. This allows the multiplicity of each carbon to be read without difficulty, the F dimension containing only the coupling information and the dimension only the chemical shift information. If, however, proton broad-band decoupling is applied in the evolution period tx, then the 2D spectrum obtained again contains only the coupling information in the F domain, but the F domain now contains both the chemical shift and the coupling information (Fig. 5.6). Projection of the peaks onto the Fx axis therefore gives the Id-decoupled C spectrum projection onto the F axis produces the fully proton-coupled C spectrum.

Figure 5.36 Effect of decoupling in v, dimension on the COSY spectrum of 9-hydroxytricyclodecan-2,5-dione. The ID H-NMR spectrum on Vj is fully coupled, while along v, it is fully decoupled. The region between the dashed lines represents a band of signals associated with proton Hj and its connectivities with other protons by projection onto the Vj axis. (Reprinted from J. Magn. Reson. 44, A. Bax, el ai, 542, copyright (1981), with permission from Academic Press, Inc.)...

It is by no means unusual to come across compounds which contain more than one phosphorus atom Fig. 23 shows the proton decoupled coupled phosphorus spectrum of compound 6, which contains three chemically different phosphorus nuclei. Phosphorus behaves in NMR just like the proton, so we shall expect to see three signals, split into multiplets if there is an observable coupling between the phosphorus nuclei. 

Fig. 8. Amide region of XH NMR spectra for XAO peptide. Each NH peak in the spectrum of XAO is cleanly resolved and allows accurate determination of the coupling constants. The values of 3/hn for A2, A3, A4, and A5 can be determined from unlabeled sample or 15N-labeled sample (with 15N decoupling in the pulse sequence), as seen from the lower panel those for A6 and A7 can be obtained from the sample XA026 (15N-labeled at 2 and 6 positions) without 15N decoupling in the pulse sequence, as seen from upper panel. From Shi et al. (2002). Proc. Natl Acad. Sci. USA 99, 9190-9195, 2002 National Academy of Sciences, USA.

C NMR of linear cross-linked PS. The proton decoupled 13C NMR spectra of linear and 1% cross-linked PS at 75 MHz in chloroform are illustrated in Figure 3. These spectra are similar to those for linear and cross-linked chloromethylated PS previously reported at lower field (14), although we have been able to resolve more structure in tHe" aliphatic and aromatic regions here. The quarternary and methylene carbon resonances at about 146 ppm and between 40 and 50 ppm respectively, are the most strongly affected by stereochemistry (20). The ortho and meta resonances at 128.4 ppm show partially resolved structure in the linear PS, as does the para carbon at 126.1 ppm. The methine resonance at... 

Figure 3. Protein decoupled 13C NMR spectrum at 75 MHz of 17,500 MW polystyrene at 20% w/v in CDCU/CHCU (lower trace). One percent cross-linked polystyrene (Biobeads SX-1) in the same solvent (upper trace).

A proton decoupled 13C NMR spectrum of the resin-peptide sample swollen in methylene chloride is shown in Figure A. The spectrum obtained in DMF is quite similar. The strong line at 175 ppm is from the lie carbonyl, and the six intense resonances between 60 and 10 ppm are due, in order of increasing field to the... 

Figure 4. Proton decoupled 13C NMR spectrum at 75 MHz of H2NlleAlaGlu(0-BzlHPAM)-(s-DVB) in CH2Cl2/CD2Cl2. Shifts relative to TMS.

The noise-modulated broadband decoupled 13C NMR spectrum of (/)/.)-penicilla-mine is shown in Fig. 5. Two methyl carbons were found to resonate at chemical shifts of 27.7 and 30.2 ppm, in addition to a CH carbon at 64.6 ppm. Two quaternary... 

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