6.S: Summary
- Page ID
- 432209
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Concepts & Vocabulary
6.2 C-13 NMR Spectroscopy- Signal Averaging and FT-NMR
- The magnetic moment of a 13C nucleus is much weaker than that of a proton. This means that NMR signals from 13C nuclei are inherently much weaker than proton signals, which makes 13C NMR harder to acquire good data.
- Chemical shift is similar to 1H, where the environment around the carbon changes for each carbon in the molecule.
- Integration is not done in 13C NMR spectroscopy because the signals for some types of carbons are inherently weaker than for other types.
- Because of the low natural abundance of 13C nuclei, it is very unlikely to find two 13C atoms near each other in the same molecule, which means that spin-spin coupling is not observed between neighboring carbons in a 13C NMR spectrum.
- There is heteronuclear coupling between 13C carbons and the hydrogens to which they are bound, proton-coupled 13C spectra show complex overlapping multiplets, which makes for a very difficult interpretation. For clarity, broadband decoupling is used, which essentially 'turns off' C-H coupling, resulting in a spectrum where all carbon signals are singlets.
6.3 Characteristics of C-13 NMR Spectroscopy
- Carbons resonate from 0-220 ppm relative to the TMS standard, as opposed to only 0-12 ppm for protons. Because of this, 13C signals rarely overlap, meaning we can almost always distinguish separate peaks for each carbon.
- The 13C NMR is used for determining functional groups based on characteristic shift values.
- 13C chemical shifts are greatly affected by electronegative effects and magnetic anisotropy.
6.4 DEPT C-13 NMR Spectroscopy
- Distortionless enhancement by polarization transfer, DEPT, is one of these techniques and making it possible to distinguish between methyl (CH3), methylene (CH2), methine (CH), and quaternary carbons.
- In DEPT, it takes advantage of the 13C to 1H coupling that is removed in broadband-decoupled 13C spectra.
6.5 Interpreting C-13 NMR Spectra
- Chemical shift is a big indicator into what type of carbon is at that resonance.
- Different carbons are carbons in distinct chemical environments and each different carbon will appear at a different resonance.
- Tables of chemical shift data can be used to distinguish different types of carbons.
6.6 Uses of 13C NMR Spectroscopy
- 13C NMR spectroscopy derives information that is helpful for structure determination.
- Scientists use 13C as a way to determine how many non-equivalent carbons are in a molecule of interest.
Skills to Master
- Skill 6.1 Distinguish between different types of carbons in a molecule.
- Skill 6.2 Estimate the chemical shift of carbons.
- Skill 6.3 Know which carbons will be more downfield.
- Skill 6.4 Determine which carbons are attached to hydrogens using DEPT
- Skill 6.5 Solve unknown structure determination problems with 1H and 13C NMR spectroscopy.