Nonequivalent carbon atoms

The information derived from 13C NMR spectroscopy is extraordinarily useful foT structure determination. Not only can we count the number of nonequivalent carbon atoms in a molecule, we can also get information about the electronic environment of each carbon and can even find how many protons each is attached to. As a result, we can answer many structural questions that go unanswered by TR spectroscopy or mass spectrometry. 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

In contrast with 1, for which only four threefold degenerate modes are active, 30 and 33 exhibit 86 and 45 IR active modes, respectively, 58 and 31 of which are twofold degenerate. The lower symmetry of 30 and 33 as compared to 1 also results in a higher number of nonequivalent carbon atoms producing more expected signals... 

Although phenanthridine was discovered in the late nineteenth century1, 2 neither the parent base nor its derivatives attracted attention until useful therapeutic activity was established in certain quaternary phenanthridinium compounds.3 A substantial number of substituted phenanthridines (and many phenanthridinium salts) have now been described and lists of compounds appearing in the literature from 1884 until 1955 are available.4, 6 Phenanthridines have attracted surprisingly little systematic attention, although the system is of considerable theoretical interest and, with its nine nonequivalent carbon atoms, may be expected to provide a rigorous test of molecular orbital reactivity correlations. Naturally occurring derivatives include several Amaryllidaceae and Papaveraceae alkaloids, notably, lycorine, haemanthamine, and chelidonine the chemistry of the phenanthridine alkaloids has been reviewed.6... 

What are the symmetry elements in ortho, meta, para-diethyl phthalates How many nonequivalent carbon atoms and hydrogen atoms are there for each compound Draw the proton decoupled l3C spectrum and DEPT spectra for each compound. 

Our approach applied to benzene gives a unique binary code 111111, since all six carbon atoms and all six C-C bonds are equivalent. Because in naphthalene there are three nonequivalent carbon atoms and two nonequivalent directions of circling around, there are several possible binary codes. The form of the code depends on the selection of the starting atom. For a clockwise circling we obtain the following codes ... 

Predict the number of peaks that you would expect in the proton-decoupled spectrum of each of the following compounds. Problems 2a and 2b are provided as examples. Dots are used to show the nonequivalent carbon atoms in these two examples. 

Figure 3.40 Heteronuclear decoupling of the 75 MHz spectrum of sucrose. The fully coupled spectrum is at the top. The bottom spectrum is the broadband decoupled spectrum. The structure of sucrose was given in Fig. 3.25. The molecule contains 12 nonequivalent carbon atoms the decoupled spectrum clearly shows 12 single peaks, one for each nonequivalent C atom. (From Petersheim, used with permission.)...

Identification of unknown compounds NMR spectroscopy provides the forensic analyst with one of the most powerful techniques for identification of unknown compounds. The full range of structural elucidation techniques of modern spectrometers is available. First, the analyst obtains a high-resolution proton (NMR) spectrum in an appropriate deuterated solvent. The chemical shifts and integration in the spectrum give an indication of the types (aliphatic, olefinic, aromatic, etc.) and relative numbers of protons present in the molecule. The appearance of the coupling patterns in the molecule often provides very useful structural information. If the identity of the unknown cannot be determined from the results of the NMR study alone, the analyst next obtains information. The NMR spectrum gives a count of the number of nonequivalent carbon atoms, as well as the types of carbon (aliphatic, aromatic, carbonyl, etc.) present in the unknown. The number of protons attached to each carbon may... 

H] NMR spectrum of liquid TBP contains four resolved signals assigned to four nonequivalent carbon atoms in TBP molecule. H] NMR spectrum of both... 

The number of different signals in a C-NMR spectrum tell you how many nonequivalent carbon atoms are in a molecule. 

FIGURE 2.22 Count of paths for three symmetry nonequivalent carbon atoms of norbomane. 

Determination of the chemical-state distribution from XPS data is often straightforward provided that the energy axis has been calibrated properly and that differential charging does not produce distortion of the peak shapes. An excellent example is the XPS spectrum shown in Fig. 3 in which the BEs of the different species contributing to the C l.v feature are well separated. There are four chemically nonequivalent carbon atoms in the molecule that can be associated unambiguously and respectively with the four well-defined C l.v peaks in the XPS spectrum. Examples such as that are frequently given in expositions on XPS. but unfortunately they are not representative of those encountered in practice. 

The proton-decoupled spectrum of 2-butanol consists of four signals corresponding to the four nonequivalent carbon atoms, as we saw in Figure 14.18. We can ehminate the isomers 2-methyl-1-propanol (Figure 14.19) and 2-methyl-2-propanol (Figure 14.20) as possible structures because the spectra of these compounds would show three and two signals, respectively. 

Answer 4-Heptanol, which is highly symmetrical, has only four nonequivalent carbon atoms, and therefore only four NMR resonances. In contrast, 3-heptanol has seven NMR resonances. 

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