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NMR4. 13C NMR and Geometry

Factors in Chemical Shift: Carbon Geometry

As in IR spectroscopy, the frequency at which different carbons absorb in the NMR spectrum is pretty predictable. There are two main factors that control the shift. These two factors are :

  • the geometry around the carbon atom.
  • the electronegativity of the other atoms attached to the carbon.

First we'll look at the influence of geometry.

In all of the hydrocarbon spectra seen above, the only peaks observed are at the right-hand end of the spectrum. In the 13C NMR spectrum of another hydrocarbon, 1-hexene, most of the peaks show up at the right-hand end, but two others show up farther to the left. A comparison of the structures will tell you that these two peaks are from the double-bonded carbons. As a general rule:

  • sp2 or trigonal planar carbons absorb in the left half of the spectrum (100 - 200 ppm).
  • sp3 or tetrahedral carbons absorb in the right half of the spectrum (0 - 100 ppm).

Figure NMR4.13C NMR spectrum of 1-hexene. 

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 15 August 2008)

The alkene, 1-hexene, shows four peaks below 20 ppm and two near 100 ppm. The two carbons that absorb near 100 ppm are the two trigonal carbons that form the alkene functional group; the others are tetrahedral carbons.

Remember, a general rule means that there will be exceptions. You might see sp2 carbons at 210 ppm or 95 ppm, for example. You are not likely to see them at 30 ppm, however.

Another compound that contains both sp2 and sp3 carbons is toluene or methylbenzene. Its 13C NMR spectrum shows one peak near 30 ppm and four between 120 and 130 ppm.

  • sp2 carbons in alkenes show up near the upfield end of the sp2 half of the spectrum (around 100-120 ppm).
  • sp2 carbons in aromatic systems like benzene absorb a little farther downfield than alkenes (around 120 ppm).
  • This difference is due to the special conjugation in aromatic systems (or resonance effects, in Lewis terms).

Figure NMR5.13C NMR spectrum of toluene. 

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 15 August 2008)

Problem NMR.2

Draw an approximate 13C NMR spectrum for each of the following compounds. You will need to take into account symmetry and carbon geometry.

In addition, there are also linear carbons in organic chemistry, although they are much less common as tetrahedral and trigonal carbons.  One example is 1-hexyne.

  • sp or linear carbons absorb around 60-80 ppm.
  • clearly, many sp3 carbons absorb at frequencies similar to sp carbons. Be careful.

Figure NMR6.13C NMR spectrum of 1-hexyne. 

Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 15 August 2008)