CNMR or C-13 NMR

Overview:

CNMR or C-13 NMR covers comparison to the HNMR, number of peaks, chemical shifts, splitting of signals, and interpretation of spectra. The manner in which the C-13 spectrum is derived using the tiny amounts of C-13 isotopes present is explained.

Proton NMR and C-13 NMR are compared with respect to isotopic percent, sensitivity of the instrument, the time required to run the spectra, extent of expected chemical shifts, the radio frequency required, and the relationship among peak areas. The manner in which it is necessary to boost the C-13 signal because the signal is so much weaker than that expected for the proton signal is addressed.

The huge chemical shifts experienced with C-13, when compared to the very small chemical shifts from proton NMR, are explained. The signal being driven downfield to a greater extent in C-13 than in proton NMR because of proximity to the deshielding group is also discussed.
Because the gyromagnetic ratio for C-13 is small, only about a quarter of that of the proton, it is noted that C-13 NMR only requires about one quarter the frequency needed for proton spectroscopy within an instrument of the same magnetic strength. At this point the student begins to realize and appreciate the significant differences between the two forms of NMR.

The number of signals reflects the number of different carbon atoms producing the signals, so the student must learn to determine identity and non-identity of carbon atoms. Since Fourier transform, the method used most often, does not use a technique that relates the area under the curve and the frequency of a specific carbon’s occurrence, the student must look for different information from the height of the peak or area under the curve.

Chemical shift patterns are diagrammed so students can determine where a particular carbon would be expected to produce a signal. The impact of proximity to electronegative groups is noted.

Splitting of signals for C-13 NMR differs markedly from splitting of signals in proton NMR. C-13 spectra are usually recorded using proton spin decoupling, a method that produces a C-13 spectrum with a series of singlets, one for each different carbon environment. The student is introduced both to the proton spin decoupling and the method of off-resonance decoupling which gives information about the number of protons (hydrogens) bonded directly to the carbon in question.

C-13 NMR is often used in conjunction with other spectroscopic techniques to take advantage of the strengths of each, as in the case of C-13 and proton NMR. However, that combination is not addressed in this unit.