Introduction to carbon-13 NMR
Carbon-13 nuclear magnetic resonance spectroscopy is commonly known as carbon-13 NMR, 13C NMR or only carbon NMR. Like 1H NMR, it is a type of NMR spectroscopy but identifies carbon atoms in an organic molecules. As 12C has no spin, so NMR cannot detect it. The natural abundance of 13C is 1.1% and because of its spin, it is detectable by carbon-13 NMR. Thus it is a very important tool to elucidate the chemical structure of any organic compound. Again due to its low natural abundance, unlike 1H NMR, 13C NMR spectra doesn’t show any splitting of signal. So the spectrum of 13C NMR is simpler and less crowded.
Chemical shifts of carbon-13 NMR
Chemical shift of carbon-13 NMR arises in the same way as 1H NMR. The chemical shift range of 13C NMR is 0-250 ppm. Each carbon has different electron environment and feels different magnetic field. Thus each of them absorbs at different applied field strength. Chemical shifts depends on two factors:
- Electronegativity
- Hybridization of carbon atom
The presence of electronegative atom or double bond causes the deshielding of the carbon atom directly attached to it. Deshielding results the downfield shift of that carbon atom in the 13C NMR spectra.
1. Electronegativity
The effect of electronegativity on chemical shift of carbon atom is demonstrated in following table:
| Particular | I | Br | Cl | F |
| Electronegativity | 2.5 | 2.8 | 3.0 | 4.0 |
| Sp3 Carbon | CH3I | CH3Br | CH3Cl | CH3F |
| Chemical shift (ppm) | 9.6 | 25.6 | 49.9 | 71.6 |
2. Hybridization of carbon atom
Let us look at the chemical shift of carbon atoms in octene.
CH3-CH2-CH2-CH2-CH2-CH2-CH=CH2
Here the chemical shifts of the sp2 hybridized carbon atoms contain double bond (=CH2) and (-CH) are 114 and 139 respectively. The chemical shift is larger because of the presence of double bond. The chemical shift of the carbon atoms next to the unsaturated carbon atoms decreases eventually which is demonstrated in following table:
| Carbon atom | Position | Chemical shift |
| =CH2 | C-1 | 114 |
| -CH= | C-2 | 139 |
| -CH2– | C-3 | 34.1 |
| -CH2– | C-4 | 29.1 |
| -CH2– | C-5 | 29.3 |
| -CH2– | C-6 | 32.1 |
| -CH2– | C-7 | 22.9 |
| -CH2– | C-8 | 14.1 |
Thus we can see that each non equivalent carbon atom shows a signal in 13C NMR spectra and the chemical shift decreases as the distance from the double bond increases. We can see an example of a 13C NMR spectra of a organic compound that contains equivalent carbon atoms, such as benzene.
Distortionless enhancement by polarization transfer (DEPT)
DEPT is a method used in NMR to determine the presence of primary, secondary and tertiary carbon atoms. The DEPT can differentiates by varying the selection of the angle parameters: 135° angle gives all CH and CH3 in a phase opposite to CH2; 90° angle gives only CH groups, the others being suppressed; 45° angle gives all carbons with attached protons in phase.
Attached proton test (APT)
APT is another useful way to determine the number of hydrogens attached to a carbon atom in an organic molecule. This can distinguishes between the carbon atoms contain even or odd number of attached protons. It shows methine (CH) and methyl (CH3) signals as positive and quaternary (C) and methylene (CH2) signals as negative. It is less sensitive than DEPT and unlike DEPT it shows all the carbon signals at once.