Analytical chemsitry-13C NMR- spectra

Take a simple molecule such as ethanol. It has two carbon atoms that exist in two different environments, they are shown below. The ¹³C NMR spectrum will therefore have two peaks.

If you recall from ¹H NMR the chemical shift of nuclei is dependent on the nuclear shielding. The nuclear shielding experienced by a nucleus depends on the electronegativity of neighbouring atoms, such as oxygen. Nuclei close to oxygen tend to be shifted further to the left of the reference point (TMS). That means that the peak at about 60 (the larger chemical shift) is due to the CH₂ group because it has a more electronegative atom attached.

¹³C NMR spectra
¹³C is a naturally occurring isotope of carbon that has a nuclear spin. As a consequence it is used in NMR spectroscopy to identify different environments in which carbon atoms are bonded within a molecule. ¹³C NMR spectra will form triplets and singlets just like H NMR. However this gives a complex spectra which is hard to read. To improve the spectra allowing easy counting of the number of carbons, the protons are often irradiated at a frequency that excites prottons and gives singlets. This is known as a "proton decoupled" spectrum. Proton decoupled ¹³ C-NMR is probably the most widely used technique because it clarifies the spectrum making it easier to determine the number of carbon atoms. For simplicity we will only deal with proton decoupled ¹³C NMR spectra. Also keep in mind that unlike H NMR, where the area under the peaks is an indication of the number of hydrogen atoms, this is not the case in ¹³C NMR spectra.
Take a simple molecule such as ethanol. It has two carbon atoms that exist in two different environments, they are shown below. The ¹³C NMR spectrum will therefore have two peaks. If you recall from ¹H NMR the chemical shift of nuclei is dependent on the nuclear shielding. The nuclear shielding experienced by a nucleus depends on the electronegativity of neighbouring atoms, such as oxygen. Nuclei close to oxygen tend to be shifted further to the left of the reference point (TMS). That means that the peak at about 60 (the larger chemical shift) is due to the CH₂ group because it has a more electronegative atom attached.
Isoleucine is shown on the right. How many peaks will be present in its ¹³C NMR spectrum? If you selected 6 then your are correct. The 6 carbons exist in different environments
How many peaks are found in the ¹³C NMR spectrum of leucine? To view the solution pass the cursor over the image on the right.
Predict the number of peaks formed in the ¹³C NMR of the following molecules. Pass the cursor over the image to see the solution.



A compound has the empirical formula C₄H₉Br Name and draw all the possible isomers and give the number of peaks in an ¹³C NMR spectrum of each isomer. Solution
Acetone has the following semi-structural formula CH₃COCH₃ How many peaks will the ¹³C NMR have and identify to which carbon each peak belongs to. Solution
How many peaks will a ¹³C NMR spectrum of hexane have. Solution