Why is carbon NMR harder to do than proton NMR? A) The mechanism of interaction of the isotopes of carbon, number 13, with a magnetic field is dependent on how many hydrogens are attached to it. B) It has a higher atomic mass which makes it slower to move and relax on the NMR time scale. C) The actual atoms of carbon observed, the isotope we can detect, is of lower relative abundance than for hydrogen. Or D) the gyromagnetic ratio, the NMR sensitivity factor, is over 100 times less for carbon-13 than for hydrogen-one.
Let’s have a look at each of these possible answers in turn. We’ll begin with A, the mechanism of interaction between carbon and a magnetic field. Carbon NMR is performed in a similar way to proton NMR. The sample that we’re interested in is placed inside a magnetic field, which I’ve labeled here as B nought. The spin of our carbon-13 nucleus will then either align with the magnetic field or against the magnetic field.
Remember that NMR only works on nuclei which have a non-zero spin. In the case of carbon, the only isotope which we are interested in is carbon-13, since it has a spin. Whether there are hydrogens attached to our carbon or not doesn’t affect its spin, or whether the spin aligns with or against the magnetic field. So, answer A, that the mechanism of interaction of our carbon-13 with the magnetic field depends on how many hydrogens are attached, is just not true.
Hydrogens can, however, make the spectra much more complicated. So, traditionally, we perform proton decoupled carbon NMR. This allows us to almost ignore the hydrogen effect in our spectrum and makes life much simpler when it comes to interpreting the spectrum. If a carbon NMR is performed under proton decoupled conditions, you may see it denoted like this, with curly braces around the symbol for proton.
Proton decoupling means that we ignore any coupling effects between carbon and hydrogen nuclei. But it doesn’t really affect the interaction mechanism of carbon with the magnetic field. So, answer A is not correct.
Now let’s look at B. It has a higher atomic mass, which makes it slower to move and relax on the NMR time scale. So, we already know that we’ve put our carbon-13 nucleus inside a magnetic field. But what happens next? The next step is to hit the nucleus with a pulse of radiofrequency. This gives the nucleus more energy and forces it to align its nuclear spin against the magnetic field. This is, of course, a more high energy state.
Eventually, the nucleus will relax and switch its spin back to align along with the magnetic field. As the nucleus relaxes, this releases energy. And it’s this energy release that we detect. So, answer B is talking about the time it takes for this relaxation to occur.
Generally speaking, relaxation times are very short. This could be between 0.1 and may be 10 seconds. Carbon nuclei do take slightly longer to relax than protons. But this is still in the order of seconds. In reality, carbon NMR spectra do take slightly longer than proton NMR spectra to record. But the reason is not due to the carbon taking longer to relax. We’ll look at this more in a minute. For now, we can rule out answer B.
C says, for the actual atoms of carbon observed, the isotope we can detect is of lower relative abundance than for hydrogen. We’ve already established that the carbon isotope we can detect by NMR is carbon-13. This is because it has a non-zero spin. So, let’s compare abundances.
Carbon-13 is approximately 1.1 percent abundant. This means that in a sample only 1.1 percent of the carbon atoms in our sample are actually visible by NMR. Conversely, hydrogen-one is approximately 99.985 percent abundant. Which means that in a sample put in an NMR spectrometer 99.985 percent of all the hydrogen atoms will be NMR active. This is clearly a considerable difference. So, answer C is true that the isotope in carbon we detect is of lower relative abundance. But does this make carbon NMR more difficult than proton NMR?
Imagine that you’re the detector in an NMR spectrometer. As you’re looking at the sample, only the yellow carbon-13 is detectable. All of the rest are invisible. You can see that, therefore, it is much more difficult to detect signals in carbon NMR. The opposite is true in proton NMR, where almost all of the protons are detectable. So, the low relative abundance of carbon-13 does cause problems when trying to record NMR spectra. In the laboratory, this means that recording a proton NMR spectrum may only take 15 to 30 minutes. Whereas a carbon NMR spectrum, could take several hours. This lower relative abundance also has several other effects.
Due to the rarity of carbon-13 nuclei, we don’t see carbon–carbon coupling in these spectra. Where carbon coupling does occur, it’s very rare. And the satellite peaks, which we see, are very small. This makes it difficult to measure the distance between the peaks. This also means that there is minimal coupling seen to other nuclei as well.
So, the lower relative abundance of carbon-13 in comparison to hydrogen-one definitely is a correct answer to reason why carbon NMR is more difficult than proton NMR. But let’s consider the last one, just in case.
The gyromagnetic ratio gamma arises from the spin on the nucleus. This ratio determines the direction of precession within a magnetic field. It is also responsible for how sensitive a nucleus is to NMR. The gyromagnetic ratio for a carbon-13 nucleus is lower than that for hydrogen-one, but not by 100 times. The gyromagnetic ratio for carbon-13 is roughly a quarter of that for hydrogen-one.
While this will make carbon-13 less sensitive to NMR than hydrogen-one, it is not the main reason that carbon NMR is more difficult to carry out than proton NMR. So, D is not true. The gyromagnetic ratio is not over 100 times less for carbon-13. It’s actually only about a quarter of that for hydrogen-one.
So, we’re left with the correct answer of C. The reason that carbon NMR is more difficult to do than proton NMR is because the isotope of carbon that we can detect, carbon-13, is of a significantly lower relative abundance than for hydrogen-one.