13.10: ¹³C NMR Spectroscopy - Signal Averaging and FT-NMR
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Objectives
After completing this section, you should be able to
- Understand why the nuclei C NMR looks at is 13 C and not 12 C.
- Know why more scans are needed in 13 C NMR than 1 H NMR.
Most of what we have learned about 1 H-NMR spectroscopy also applies to 13 C-NMR, although there are several important differences.
Signal strength in 13 C-NMR spectroscopy
The 12 C isotope of carbon - which accounts for more than 98% of the carbons in organic molecules - does not have a nuclear magnetic moment, and thus is NMR-inactive. Fortunately for organic chemists, however, the 13 C isotope, which accounts for 1.1% of the remaining carbon atoms in nature, has a magnetic moment just like protons.
The magnetic moment of a 13 C nucleus is much weaker than that of a proton, meaning that NMR signals from 13 C nuclei are inherently much weaker than proton signals. This, combined with the low natural abundance of 13 C, means that it is much more difficult to observe carbon signals and there is a much lower signal-to-noise ratio than in 1 H NMR. Therefore, more concentrated samples are required to generate a useful spectrum, and often the data from hundreds of scans must be averaged in order to bring the signal-to-noise ratio down to acceptable levels. This type of signal averaging works since background noise in a spectrum is typically random while the signal caused by the 13 C nuclei is not. Therefore if the spectra from multiple scans is averaged, the noise gets closer to 0 while the signal stays the same, increasing the signal-to-noise ratio.
Fourier Transform (FT) NMR
Earlier versions of NMR instruments used variable magnetic fields which required recording of each signal in the spectrum sequentially which is called continuous wave (CW) NMR. The Fourier Transform allows recording of all signals simultaneously by introducing a short pulse of all RF frequencies that cause resonance for a given nucleus. This complex signal is then manipulated through complex mathematical processing that creates an NMR spectrum that looks similar to a CW spectrum, but in much less time.