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#1 2008-11-30 23:19:48


What does intensity mean in C13 NMR Spectra?

Yahoo Answers chemistry wrote:

What does intensity mean in C13 NMR Spectra? I cannot figure out what the height (intensity) of the peaks means in terms of its structure. How I could use it to identify a molecules composition from the NMR spectrum?

It is identical to an integral in 1H NMR, however due to the low abundance of 13C and a lower sensitivity, some tricks are performed in 13C NMR. The relaxation times are much longer for carbon than hydrogen and the longest for tertiary carbons. Therefore, a smaller RF pulse is used for 13C NMR and a much larger number of pulses are captured. Consequently, the signals for all of the carbons can be found, but the intensity of the signals is not proportional to their number.

However, there are some generalizations one can make. Carbons with hydrogens attached will show up as a larger peak than a carbon without hydrogens. If you have two similar carbons, their intensity will generally be greater than another carbon. 

A quantitative 13C NMR spectrum can be obtained, however a much longer pulse delay is used so each carbon can contribute equally and the total acquisition time is much longer.



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