Carbon Chemical Shifts and Structure

The third factor in eq. 3-10,, is related to charge densities and bond orders and can 

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The information on carbon chemical shifts and multiplicities is invaluable for structure determination. It would be ideal if we also had a method for obtaining information directly on carbon-carbon bonding in the compound under study, since this would allow us to draw on paper at least parts of the carbon framework of the molecule. 

Proton chemical shifts are very valuable for the determination of structures, but to use the shifts in this way we must know something about the correlations that exist between chemical shift and structural environment of protons in organic compounds. The most important effects arise from differences in electronegativity, types of carbon bonding, hydrogen bonding, and chemical exchange. 

With a better understanding of the FI axis and the diagonal, we can proceed with interpretation of the spectrum. Table 5.1 lists carbon chemical shifts and carbon numbers based on the structure given earlier we refer to these numbers in the present discussion. From Figure 5.19, we can make the high-frequency connections quite easily. The carbon at highest frequency is C-8 at 151.0 ppm by tracing vertically down from this peak on the F2 axis, we intersect three cross peak doublets. These cross peaks connect horizontally with C-7 at 29.2 ppm, C-9 at 48.0 ppm, and C-13 at 112.0 ppm. Toward lower frequencies, C-13 at 112.0 ppm comes next, and it has only one cross peak, namely the reciprocal connection to C-8 at 151.0 ppm. 

When all possible proton and carbon assignments have been made, one should have a reasonable picture of the actual structure. Recall that any nuclei other than protons and carbons have not been directly investigated, so keep in mind other possible heteroatoms that may be present. Certainly proton and carbon chemical shifts and correlations (or lack thereof) will provide clues to the presence of other nuclei. Complementary data from other techniques, such as MS, may provide additional insights into the presence of heteroatoms. Also, as previously discussed, NMR is limited to identifying... 

The XH NMR spectrum shows bands in the region 4.0-6.0 ppm (See Table 6). Following the discussion of priority of paths of delocalization of aceheptylene dianion 232 and acenaphthylene dianion 82 also 332 and 342 show that specific paths of delocalization are favoured. While in the neutral structure 33 and 34 the competition is between aromatic and nonaromatic structures, in the respective dianions the competition is between nonaromatic and antiaromatic structures (Fig. 9). From the spectroscopic parameters, i.e., chemical shifts and coupling constants of the bridge protons it can be concluded that the neutral systems are best represented by structures with an aromatic skeleton connected to a virtually isolated double bond. In the charged systems, viz. 332 and 342 it seems that a nonaromatic path of conjugation is preferred to an antiaromatic path (Fig. 9). These considerations are also reflected in the carbon chemical shifts and in their HOMO-LUMO gap (AE) (vide infra) 122). It can be concluded from all these observations that there is a tendency of aromatic systems to remain so and to avoid as much as possible paratropic antiaromatic contributions. 

Urry, D. W., Mitchell, L. W., and Ohnishi, T. (1974). Biochem. Biophys. Res. Comm. 59, 62. Solvent Dependence of Peptide Carbonyl Carbon Chemical Shifts and Polypeptide Secondary Structure The Repeat Tetrapeptide of Elastin. 

The HMBC experiment correlates long-range (two to three bond) H C pairs (Fig. 12.11) three is used to determine the C chemical shifts and structural connectivity of quaternary and carbonyl carbons. Quaternary and carbonyl carbons do not have directly bonded hydrogens and as a result do not have a cross peak in the 2D H- C HSQC spectrum. Fig. 12.11 shows cross peaks for the correlation between hydrogens 10 and 13 with carbonyl carbon 12, and hydrogen 20 with carbonyl carbon 19. Also shown in the HMBC spectrum is the correlation between hydrogens 23 and 25 with quaternary carbon 21. Despite a lack of correlations to spiro-carbon 14, the overwhelming body of evidence from interpretations of multiple NMR... 

The correlations between chemical shift and structure are shown in Fig. 23. It can be seen that 8h s follow the same general trends as carbon reso-... 

The two axes (dimensions) in our 2D spectra are thus both frequency axes. We shall see as we continue that we can adjust our experiment so as to choose different types of frequency information. An early experiment, known as the J-resolved experiment, was designed in such a way that one axis was the (proton or carbon) chemical shift axis and the other the one-bond proton-carbon coupling constant. Flowever, this experiment is not generally very useful for structural determination, so that we shall not discuss it here. 

Usually, a careful analysis of the combination of fluorine, proton, and carbon NMR chemical shifts and spin-spin coupling constants will provide definitive information regarding the structure of disubstituted fluoroaromatics. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

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