Question

Why do the relative areas of C13 NMR resonances do not always reflect the relative number of carbon atoms in a molecule?

Answer 1

Solution:-

The chemical shift of carbons is caused by the same phenomenon as the chemical shift of hydrogens, i.e., the electrons in the molecule generate small magnetic fields that affect the net field experienced by each carbon nucleus. In general, electrons surrounding an atom move in such a way so as to create a field at the atom that tends to counteract the applied magnetic field. The electrons thus "shield" the carbon nucleus from the applied magnetic field and this means that less energy is necessary to excite the carbon nucleus from one spin state to another and therefore its chemical shift comes at a lower frequency than it would otherwise. For example, the carbon atom in a carbonyl group has a relatively low electron density around it, and thus is relatively "deshielded" and consequently has a higher chemical shift than most other types of carbons.

Carbon-12 atoms do not have a nuclear spin, and hence don't show up in the NMR. When we take a carbon NMR we are looking only at carbon-13 atoms. Only 1% of naturally occuring carbon atoms are carbon-13, so the sensitivity of natural abundance carbon NMR is lower than that for proton NMR. Another consequence of this low abundance, is that we don't normally observe coupling between adjacent carbon atoms (like we do between adjacent protons in H-NMR) since 99% of the neighboring carbons are carbon-12 and don't have a nuclear spin. However, protons attached to a carbon atom will cause splitting of the carbon signal. This splitting will lower the signal to noise ratio, so carbon NMR spectra are usually obtained under conditions of proton decoupling. Under these conditions each nonequivalent carbon atom in a molecule will appear as a single peak in the carbon NMR.

Another difference between proton and carbon NMR is that carbon NMR spectra are not normally integrated. This is due to the fact that unless a long delay is introduced between acquisitions the carbon intensities don't accurately reflect the relative numbers of carbon atoms. For example, what you will usually observe is that carbon atoms with no hydrogens attached to them (e.g., carbonyl carbons) will be less intense than those that do have hydrogens attached. Nevertheless, for carbon atoms that do have hydrogens attached the relative height of the NMR peak usually can be used to estimate the relative number of carbon atoms. For example, in the carbon NMR of isopropanol, the 2 methyl carbons are equivalent and will show up as a peak that is approximately twice as high as the methine (one H-attached) carbon peak.