Predicting Carbon-13 NMR Spectra
Predicting 13C molecular environments
- The key to carbon-13 NMR spectroscopy is identifying different carbon environments
- It can help to look for symmetry in the molecules
- For example, propanol
- There are 2 isomers of propanol: propan-1-ol and propan-2-ol
- Since both isomers contain 3 carbons, the maximum possible number of 13C NMR peaks is 3
- Propan-1-ol
- There is no symmetry or equivalent carbons in a molecule of propan-1-ol
- Therefore, there will be 3 peaks in the 13C NMR spectrum of propan-1-ol
- Propan-2-ol
- There is a line of symmetry through the second carbon with the hydroxyl / OH group attached
- This means that the CH3 groups on either side are equivalent
- Therefore, there will be 2 peaks in the 13C NMR spectrum of propan-2-ol
Identifying molecular environments in propanol
Propan-1-ol has no symmetry / equivalent carbons in its structure, while propan-2-ol has symmetry and, therefore, 2 equivalent carbons
- For example, pentane-1,4-diol
- Since pentane-1,4-diol contains 5 carbons, the maximum possible number of 13C NMR peaks is 5
- There are 2 carbons with hydroxyl / OH groups attached but these are not equivalent
- Carbon-1 is CH2OH with a neighbouring CH2
- Carbon-4 is CHOH with a neighbouring CH2 and a neighbouring CH3
- This means that there is no symmetry within the molecule and, therefore, no equivalent carbons
- So, there will be 5 peaks in the 13C NMR spectrum of pentane-1,4-diol
Identifying molecular environments in pentane-1,4-diol
Pentane-1,4-diol has no symmetry / equivalent carbons in its structure
- The same principle of carbon environments also applies to aromatic compounds
- For example, benzene
- Since benzene contains 6 carbons, the maximum possible number of 13C NMR peaks is 6
- The benzene molecule has several lines of symmetry leading to the fact that all of the carbons are equivalent
- So, there will be 1 peak in the 13C NMR spectrum of benzene
Identifying molecular environments in benzene
Benzene has symmetry / equivalent carbons in its structure
Examiner Tip
- Counting the number of 13C resonances should be the first step in analysing a spectrum
- For example, it is possible to differentiate the three isomers of dihydroxybenzene quickly be considering the symmetry of the molecules and therefore the number of resonances expected in their spectra.
Worked example
Predict the number of peaks / resonances in the 13C spectrum of 1,3-dihydroxybenzene.
Answer:
- There are 4 chemical environments
- Therefore, there will be four peaks / resonances in the 13C spectrum
