13C nucleus

Figure 13.3 shows both the H and the l3C NMR spectra of methyl acetate, CH3CO2CH3. The horizontal axis shows the effective field strength felt by the nuclei, and the vertical axis indicates the intensity of absorption of rf energy. Each peak in the NMR spectrum corresponds to a chemically distinct 1H or 13C nucleus in the molecule. (Note that NMR spectra are formatted with the zero absorption line at the bottom, whereas IR spectra are formatted with the zero absorption line at the top Section 12.5.) Note also that 1H and 13C spectra can t be observed simultaneously on the same spectrometer because different amounts of energy are required to spin-flip the different kinds of nuclei. The two spectra must be recorded separately. 

Each electronically distinct 1H or 13C nucleus in a molecule comes into resonance at a slightly different value of the applied field, thereby producing a unique absorption signal. The exact position of each peak is called the chemical shift. Chemical shifts are caused by electrons setting up tiny local magnetic fields that shield a nearby nucleus from the applied field. 

So if this all sounds a bit bleak, what s the good news Well, strangely, there is quite a lot. For a start, let s not forget that had the 13C nucleus been the predominant carbon isotope, the development of the whole NMR technique itself would have been held back massively and possibly even totally overlooked as proton spectra would have been too complex to interpret. Whimsical speculation aside, chemical shift prediction is far more reliable for 13C than it is for proton NMR and there are chemical shift databases available to help you that are actually very useful (see Chapter 14). This is because 13 C shifts are less prone to the effects of molecular anisotropy than proton shifts as carbon atoms are more internal to a molecule than the protons and also because as the carbon chemical shifts are spread across approximately 200 ppm of the field (as opposed to the approx. 13 ppm of the proton spectrum), the effects are proportionately less dramatic. This large range of chemical shifts also means that it is relatively unlikely that two 13C nuclei are exactly coincident, though it does happen. 

Other good news comes in the shape of the 13C nucleus having a spin quantum number of /2. This means that 13C signals are generally sharp as there are no line-broadening quadrupolar relaxation issues to worry about and we don t have to deal with any strange multiplicities. 

Two factors contribute to r K. One is the ratio of the magnetogryric ratios of the two different spins, and the other depends on relaxation mechanisms. Provided that the relaxation mechanism is purely dipole-dipole, other relaxation mechanisms affect spin I, then 4> may approach zero. Assuming that the dipolar mechanism is operational (no quadrupolar nuclei with I > 1/2 are present), r has the value ys/ 2y and is regarded as rimax. In the homonuclear case we have r max = 1/ 2. Usually one chooses nuclei where ys > y/ to ensure that the NOE is significant. For observation of 13C for instance, if the protons in the molecule are double irradiated, the ratio is 1.99 and 1 + r max equals approximately 3. To repeat a statement made above, proton broad-band irradiation enhances the intensity of the 13C nucleus, which otherwise has very low receptivity. 

Because of the low natural abundance of 13C nuclei (1.1%), practically all observed product molecules contain only one 13C nucleus. Accordingly, the polarization signals in the 13C NMR spectrum clearly do not originate from one and... 

It follows that, as used in solid state NMR spectroscopy, 13C observation may have potential advantages in that the 13C nucleus has a wider chemical shift range than 1H, making the spectral resonances of individual molecular species more easily... 

The first part, before the t period, of the experiment is identical to the HN(CO)CA-TROSY scheme. This step chooses solely the sequential pathway in an HN(CO)CA-TROSY manner. The chemical shift of the 13C nucleus is recorded during the t evolution period. The back-transfer route is, however, quite different. We transfer the desired coherence from 13C directly back to the 15N nucleus and remove the second 13C -> 13C INEPT step found in HN(CO)CA-TROSY and replace it with the HNCA like back-transfer step. The antiphase 2./N<> coupling then refocuses simultaneously with VNc during the 13C 15N back-INEPT step. Thus, the HN(CO)CANH-TROSY... 

As can be seen by inspecting Figure 23, the coherence transfer efficiency is overwhelmingly good thanks to the very efficient 15N— -13C INEPT step employed for transferring magnetization exclusively from 15N to the sequential 13C nucleus. In addition, the possibility of an accidental resonance... 

The advantage of 13C NMR over proton NMR is that much greater range of chemical shifts is exhibited by 13C (and most other nuclei), some 200 ppm in contrast to 10 ppm for H. Moreover, there is no carbon-carbon spin coupling because of the low chemical abundance of the 13C nucleus, 1.1 per cent... 

Fig. 38.—13C-N.m.r. Spectrum of A, Partly Depolymerized O-Methylcellulose (d.s. 2.8) in CDC13 at 30° (R, signal due to reducing-end residue Me, O-methyl inset lines represent chemical shifts of corresponding carbon atoms in methyl hepta-O-methyl-jS-cellobioside) and of B, O-Methylcellulose (d.s. 0.7), Partially Degraded by Cellulase, in D20 at 30°. (S represents a 13C nucleus bonded to an OMe group inset lines give the chemical shifts of corresponding carbon atoms in methyl /3-cellobioside.)...

NMR 13C spin-lattice relaxation times are sensitive to the reorientational dynamics of 13C-1H vectors. The motion of the attached proton(s) causes fluctuations in the magnetic field at the 13C nuclei, which results in decay of their magnetization. Although the time scale for the experimentally measured decay of the magnetization of a 13C nucleus in a polymer melt is typically on the order of seconds, the corresponding decay of the 13C-1H vector autocorrelation function is on the order of nanoseconds, and, hence, is amenable to simulation. 

You may have noticed another characteristic of 13C NMR spectra—all of the peaks are singlets. With a spin of j, a 13C nucleus is subject to the same splitting rules that apply to H, and we might expect to see splittings due to 13C—13C and l3C— ll couplings. We don t. Why ... 

The relaxation of the 13C nucleus is dominated by 13C— H dipolar interactions. For slow rotational reorientation (cocrc > 1) assuming a single correlation time, tc, the following equations for l3C spin-lattice and spin-spin relaxation times are valid ... 

The 13C NMR spectrum of a methyl group, representing an AX3 system, is a quartet according to eq. (1.45), since Ix = j and n = 3. The H NMR spectrum of a methyl group, however, is a doublet because one 13C nucleus is adjacent to three equivalent protons, so that /A = 4 for 13C and m = 1. Due to the low natural abundance of 13C, the doublets (13C satellites) arising from coupling of carbon-13 with protons are usually lost in the noise of H NMR spectra. 

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