13C satellites

Because fluorine is relatively sensitive to its environment and has such a large range of chemical shifts, considerable changes in chemical shift can be observed when a nearby atom is replaced by an isotope. For example, replacement of 12C by 13C for the atom to which the fluorine is attached, gives rise to a quite measurable shift, usually to lower frequency. A consequence of this isotope effect is the observation that the 13C satellites in a fluorine spectrum are not symmetrical about the 12C—F resonance. 

Two final interesting points relating to 13C satellites... Whilst they are generally, evenly spaced on either side of the major peak, they do not have to be exactly symmetrically disposed about it. It is... 

Other even more cunning methods have been devised to suppress the water signal in samples that have a large water content (e.g., bio-fluid samples) such as the WET and the WATERGATE pulse sequences. Other sequences have been devised to cope with signals from carbon-bound hydrogens. Some of these actually collapse the 13C satellites into the main 12C peak prior to suppression. Such a sequence would be useful if you were forced to acquire a spectrum in a nondeuterated solvent. 

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

Using the atomic weights given in Table 4.2, calculate the mass spectral resolution required to separate the molecular ion of atomic composition C35H48N8O11S from the isotopic peak containing one 13C atom and carry out a similar calculation for the ion of composition C284H432N8407gS7 and its single 13C satellite. 

Fig. 1 Esr spectrum of di-t-butyl nitroxide in benzene. Note 13C-satellite lines...

Aside from 13C-H coupling constants (often obtained from 13C satellites in 1H NMR spectra) not very much information is available on the 13C NMR spectra of small heterocycles. A recent review (B-79MI50101) devotes only seven pages to the 13C NMR spectra of all three- and four-membered heterocyclics. 

The l3C H coupling constants (often obtained from 13C satellites in ]H NMR spectra) of small heterocycles have been listed (B-79MI50101). 

The H NMR spectrum of pyridazine shows two symmetrical quartets of an A2X2 or A2B2 type dependent on the solvent and concentration. The 13C satellites have been used to obtain all coupling constants. Spectra of C-substituted pyridazines, methylthio- and methylsulfonyl-pyridazines, both as neutral molecules and as cations, N-l and N-2 quater-nized species, pyridazinones, hydroxypyridazinones, N-oxides and 1,2-dioxides have been reviewed (B-73NMR88) and are summarized in Tables 6, 7 and 8. 

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. 

Selective population or polarization transfer (abbreviation SPT) was first achieved for chloroform [51] by brief irradiation of one of both 13C satellites in the proton NMR spectrum before recording the 13C—1H doublet as FID signal. In this experiment, the decoupling pulse inverts the population of the proton precession states connected by the irradiated satellite transition (Fig. 2.43(b, c)). The two SPT experiments shown for the 13C —H doublet of chloroform in Fig. 2.43 (b, c) give an impression of the signal enhancement achievable by population transfer in comparison to a normal 13C NMR spectrum... 

INADEQUATE involves effective suppression of the strong central signal. The process (d) -> (e) in Fig. 2.48, denoted as double quantum transfer, does not at all enhance the 13C—13C satellites. Their signafnoise ratio remains at 0.5% of the normal carbon-13 NMR sensitivity. 

INADEQUATE spectrum, however, may become difficult, even in the case of smaller molecules At a tertiary carbon for example, three doublets will appear, usually overlapping due to similar coupling constants. Further, isotope shifts and AB effects will remove the 13C — 13C satellites from the shift position of the known 13C 12C signal. Thus, it was not until a second dimension was introduced to the experiment - that INADEQUATE became a practical method of structure elucidation. 

To conclude, the second dimension is introduced if the switching time ti (Fig. 2.48) is incremented in a series of single experiments so as to reach all possible double quantum frequencies vDQ within a sample molecule by the reciprocals l/t1. Again, the acquired FID signals will depend on two variable times t1 and t2, respectively. A first Fourier transformation in the t2 domain generates 13C — 13C satellite spectra. The corresponding AB or AX type doublet pairs, however, are modulated by the individual double quantum frequencies which characterize each AB or AX pair. The second Fourier transformation in the tl domain liberates the double quantum frequency as the second dimension Maximum AB or AX 13C—13C subspectra are observed at the corresponding double quantum frequencies, so that each doublet appears with unique coordinates,... 

A number of other features apparent in the toluene proton spectrum are worthy of note at this stage. Each absorption is accompanied by a number of small satellite peaks equally spaced on either side of the main absorptions. These may be spinning side-bands or 13C satellites (p. 342). The spinning side-bands are caused by inhomogeneities in the magnetic field and in the sample tube. They... 

Figure 1.18 H NMR spectra of two HPLC-grade solvents, ACN (a) and THF (b). The 13C-satellites of the suppressed signals are marked with asterisks...

The commonly used solvents for reversed-phase HPLC separations are methanol and acetonitrile. Both of these solvents give rise to a singlet resonance in the H NMR spectrum which can be suppressed easily. However, the 13C satellite peaks, caused by the one-bond H-13C spin couplings from the 1.1 % of molecules with the naturally abundant 13C isotope at the methyl carbon, remain following suppression of the main peak and are often much larger than the signals for the analytes, and thus these must also be suppressed. 

The non-rotation of the sapphire flow cell, together with the detection volume and the employed flow rate, determine the resulting NMR line width. Whereas with rotation of the NMR tube in a conventional NMR probe, the signal line width of chloroform at the height of the 13C satellites in degassed acetone-d6 is about 3-4 Hz, SFC continuous-flow probes show values of the order of 15-18 Hz in the liquid and in the supercritical state. This hump test also indicates that there is no change in the signal line width in the H NMR spectra in the liquid and in the supercritical state. 

In the upper spectrum taken from a LC-NMR run (Figure 7.2.5(a)), despite solvent suppression there are large remaining solvent signals, i.e. water at 3.7 ppm and acetonitrile with 13C satellites at 2.1 ppm. In cases where the sample peaks lie under the solvent signals, structure elucidation is difficult or... 

Subsequently, Uhrinova et al.29 reconsidered the problem using both proton-and carbon-detected experiments. For example, couplings of anomeric carbons were measured from the 13C satellites in proton NMR spectra. The critical factor in these methods is the suppression of signals from protons bound to, 2C atoms. In the pulse-sequence proposed, these protons were selectively inverted by a BIRD (Bilinear Rotation Decoupling) pulse,30 and the spin-echo method introduced by Bendall et al.31 was used. 

Magic angle spinning (MAS) NMR brings the power of H NMR to solid-phase chemistry [2,25], A simple MAS spectrum, that of the reaction product of succinic anhydride and TentaGel S NH2, is shown in Fig. 2. Presaturation of the PEG resonance at 3.6 ppm is critical to spectral quality. The complex peaks from 3.4 to 3.9 ppm are due to residual PEG, 13C satellites and the terminal CH2 of the PEG chain. The characteristic doubling of the solution-phase CDC1, and TMS resonances can be seen. 

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