13C spectra

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. 

To define the position of an absorption, the NMR chart is calibrated and a reference point is used. In practice, a small amount of tetramethylsilane [TMS (CH )4Si] Is added to the sample so that a reference absorption peak is produced when the spectrum is run. TMS is used as reference for both l H and 13C measurements because it produces in both a single peak that occurs upfield of other absorptions normally found in organic compounds. The ]H and 13C spectra of methyl acetate in Figure 13.3 have the l MS reference peak indicated. 

Everything we ve said thus far about NMR spectroscopy applies to both H and 13C spectra. Now, though, let s focus only on I3C spectroscopy because it s much easier to interpret. What we learn now about interpreting l3C spectra will simplify the subsequent discussion of 1H spectra. 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

How many signals would you expect each of the following molecules to have in its lH and 13C spectra ... 

C Nuclear Magnetic Resonance Spectra. The solid-state 13CNMR spectra of 6,7-dimethyl-2,3-di(pyridin-2-yl)quinoxaline (130) and its salts have been used to complement X-ray information (see above) on fine structure.908 A study of the 13C spectra of mono- to tetramethylquinoxalines has made possible the analysis of mixtures of such methylated quinoxalines obtained from ambiguous primary syntheses.526... 

The most exciting research is yet to be performed on 13C NMR of the corrin enzymes. This could be accomplished by biosynthesis of C-13 enriched samples of biologically active B12 derivatives followed by their incorporation into enzymes. Since it has been shown that 13C-spectra of corrinoids are well resolved, and sensitive to small changes in the molecular conformation, then one could hope to get quite detailed information pertinent to the binding of B12 and to the mechnism of enzyme catalysis. 

MHz, solvent CDC13 (degassed) T ] determination of the carbon atoms by inversion recovery experiment selected 13C spectra... 

As is the case for 11 spectra, but not for 13C spectra, the intensities of individual signals in 19F NMR spectra constitute an accurate measure of the relative number of fluorine atoms responsible for such signals. 

A comment about the carbon NMR spectrum of t-butyl fluoride is appropriate. Because of the signal weakness of carbons such as the tertiary carbon of r-butyl fluoride, which bear fluorine but no hydrogens, many published tabulations of 13C spectra of compounds that contain such structural features fail to report these crucial signals. They can easily be missed, especially if you do not know what you are looking... 

The 13C NMR spectra of the three isomeric difluorobenzenes are distinctive and interpretable upon inspection, with the 1,2-isomer exhibiting three signals, the 1,3-isomer four, and the 1,4-isomer two. (Figs. 3.23-3.25). The signal observed in each of these 13C spectra at -128.3 ppm derives from the solvent, benzene-d6. 

Data related to the effect of P-halogen on the proton and carbon chemical shifts of CF2H or CF2 groups are scarce (Scheme 4.13), although there is a recent review of 13C spectra of chlorofluorocyclo-propanes.3... 

For a start, we must be mindful of the fact that 13C is only present as 1.1 % of the total carbon content of any organic compound. This, in combination with an inherently less sensitive nucleus, means that signal to noise issues will always be a major consideration in the acquisition of 13C spectra - particularly 1-D13 C spectra which we will restrict the discussion to for the moment. (Note that the overall sensitivity of 13C, probe issues aside, is only about 0.28 % that of proton because the nucleus absorbs at a far lower frequency - in a 400 MHz instrument, 13C nuclei resonate at around 100 MHz.). So it takes a great deal longer to acquire 13 C spectra than it does proton spectra. More material is obviously an advantage but can in no way make up for a 350-fold inherent signal to noise deficiency ... 

Q4. The proton spectra of these two compounds are very similar and so it would be unwise to try to discriminate between them in this way. The 13C spectra however, would show differences in the alkyl chain which would be both significant and predictable. 

Broadband decoupling Decoupling applied across a wide range of frequencies, e.g., the decoupling of all proton signals during the acquisition of 1-D 13C spectra. 

A number of attempts to transfer the spin polarization directly from the triplet electrons to the 13C spins that are present in the sample have been unsuccessful. Nevertheless, it is possible to polarize the 13C spins indirectly, by polarizing the 1H spins first, and then transporting the enhanced 1H polarization to the I3C spins by the conventional crosspolarization technique. Figure 18B shows the enhanced 13C spectra obtained in this way using the same crystal of pentacene-doped naphthalene. 

AO.9 ppm appears as an overlapping doublet. The spectra of these samples in DMF are of similar appearance. Several parameters from the 13C spectra which we can use to elucidate molecular dynamics are given in Table II. The widths of the ortho, meta... 

No 13C spectral data are available for 4-aminoimidazoles (179), but the 13C spectra of four 5-aminoimidazoles (180) have been reported. Typical chemical shifts for the C5 and C4 atoms are recorded in Table VIII. 

Adamantane (C10H16) External referencing for 13C spectra, optimizing Hartmann-Hahn match, linewidth measurement, sensitivity measurement [30]... 

For NMR spectroscopic experiments, a thin film of pTrMPTrA was prepared by reacting a quantity of monomer and photoinitiator confined between glass plates with 1 mm separation. The polymerization conditions were the same as those for the photocalorimetry experiments. After 1 hour of UV exposure, the film was removed from the plates and ground to a fine powder using a mortar and pestle. A solid-state 13C NMR spectrum of the powder was obtained immediately, as described below. The remaining polymer powder was divided into two portions, one of which was stored under atmospheric conditions. The other portion was stored under N2. After one week, 13c spectra were again obtained for each of these polymer samples. Both samples were then heated to 280 °C in a vacuum oven and analyzed once more by 13C NMR spectroscopy. 

TABLE 14.1. Important Diagnostic Absorptions of Major Organic Functionalities in Mid-Infrared, XH NMR, and 13C Spectra... 

Direct heteronuclear chemical shift correlation Conceptually, the 2D J-resolved experiments lay the groundwork for heteronuclear chemical shift correlation experiments. For molecules with highly congested 13C spectra, 13C rather than XH detection is desirable due to high resolution in the F% dimension [40]. Otherwise, much more sensitive and time-efficient proton or so-called inverse -detected heteronuclear chemical shift correlation experiments are preferable [41]. 

The pH dependence of the 13C spectra for the rhenium(V) system was studied in the pH range 0-8, and showed only one signal, resulting from the fast proton exchange between the dioxo, hydroxo oxo and aqua oxo species. Thus 13C NMR provides an excellent means for... 

Even though the 13C spectra for the Tc(V) system span a large range of chemical shift there was a rapid formation of the dinuclear [Tc203(CN)g]4 species whenever there were appreciable amounts of the [TcO(OH)(CN)4]2 ion present (13). This excluded the determination of the pKa values for the Tc(V) system by 13C NMR. 

Figure 16 shows the 13C spectra of N-acetylglycine obtained from the traces of the indirectly detected dimension of the HSQC experiment. The spectrum... 

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