Non-Equivalent Carbon Atoms

The two equatorial methyl groups are equivalent and appear as single peak at a different place than the two axial methyl groups. There are a total of 6 peaks in the spectrum 4 unique carbon atoms on the ring, and two for the non-equivalent methyl groups. 

FIGURE 6.14 Examples of compounds with non-equivalent methyl groups. 

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The relative configuration of adjacent, constitutionally non-equivalent, carbon atoms can be specified as erythro or threo, as appropriate, by adding the required prefix to the terms diisotactic and disyndiotactic , as necessary (see Section 2.2). 

Look at the structure of methyl -glucopyranoside (Figure 4 29) and satisfy yourself that it has seven non-equivalent carbon atoms. You might also try to assign the two extreme signals. 

Process the data of the basic ID C experiment and determine the number of chemically non-equivalent carbon atoms in your molecule. Find signals representative of a particular type of functional group. To confirm your (tentative) assignments use suitable reference data if available and/or check with standard C correlation charts (see recommended reading). 

A mathematical analysis of all four isomeric thiadiazoles by the simple molecular orbital method has provided molecular diagrams of the free base and conjugate acid of each thiadiazole, with electron densities, bond orders, and free valencies. On this basis, predictions have been made concerning the reactivities of the six non-equivalent carbon atoms, the basicities of the nitrogen atoms, and the delocalization energies in these molecules. The 5-position in free 1,2,4-thiadiazole should possess maximum reactivity in nucleophilic substitution reactions. The treatment also accounts for the order of the polarographic half-wave potentials and the position of the absorption maxima in the ultraviolet region of the spectra of 1,2,4- and 1,3,4-thiadiazoles.4... 

In other words, the secular determinant in eq. (7.1.37) can be factored into two 2x2 and two 3x3 blocks. To obtain the explicit forms of these ten combinations, we need the results of each of the eight symmetry operations. Also, since the system now has three types of (structurally non-equivalent) carbon atoms, we need the operation results on the 2p orbitals of these three kinds of atoms ... 

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

Hence we see that the two non-equivalent carbon-atoms in butadiene have rather different total bond-numbers the end-carbon-atoms are nothing like so deeply involved in bonding (total bond-number 3-894) as are the middle atoms (total bond-number 4-341). In an intuitive and heuristic way, this may be taken to imply that the end-atoms have more potential bonding... 

In the first row of Table 7-1 are listed the charges on the six non-equivalent carbon-atoms of azulene, calculated by the simple, unrefined Hiickel-method, as described in Chapter Two and 6.5. The second row of the Table contains the corresponding quantities obtained after carrying through the co-technique iteration to completion. Notice that the charges are now much more uniform in the simple HMO-calculation the maximum charge-difference between two carbon-atoms is (1-173 — 0-855) 0-32, while the corresponding quantity from the results of the co-technique calculation is (1-116 - 0-907) % 0-21. 

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

The spectroscopic results in solution show clearly that the Pd-diene interactions involve non-equivalent carbon atoms at each of the equivalent double bonds. This is confirmed in the crystal structure where the Pd-C distances, for each C=C, to the two carbons are significantly different. ... 

Table 1 The activation energies (and free energies in parentheses) are given in kcal/mol units for the C-BH2 bond formation for each non-equivalent carbon atom corresponding to the energy difference between the transition state (TS) and the most stable BH3-diene adduct reactant state of the first eight molecules in Fig. 1...

Carbon-13 NMR produces a spectrum with different chemical shifts for non-equivalent carbon atoms in a molecule. Typical carbon-13 NMR shifts are shown in Table 29.4. As in proton NMR, the chemical shifts are measured with reference to the TMS peak at 0 ppm on the spectrum (see page 439). 

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