Analysis of two peaks in a proton NMR spectrum gives a three to two integration. What does this mean? A) There are three hydrogens on one carbon and two on another. B) One of the peaks is a methyl group, and the other is a methylene group. C) The peaks belong to a straight chain aliphatic compound. Or D) the ratio between the two types of hydrogen environment is three to two.
This question is specifically asking us about proton NMR spectroscopy. In particular, we’re looking at two different peaks which have a three to two ratio of their integrations. As an example, these two peaks could look a little something like this. Exactly what they look like and where they appear isn’t important in this case. The important thing is the one piece of information we’ve been given about these peaks, which is their three to two integration ratio.
Remember that integration means the area under the peak. Let’s review the types of information we can get from a proton NMR spectrum. The first piece of information about a peak in an NMR spectrum could be the chemical shift. This is where along the 𝑥-axis the peak appears. This is measured in ppm, or parts per million. And it’s often given the Greek symbol 𝛿. The chemical shift tells us about the type of chemical environment that a proton is in.
For example, protons in an aromatic environment are pulled to the left. We describe this as being pulled downfield. This is because these hydrogens are more deshielded. On the other hand, aliphatic compounds appear more upfield to the right. This is because the protons are less deshielded, or we could say more shielded.
But chemical shift isn’t the only information we get from a proton NMR spectrum. We can also learn interesting things from the peak splitting patterns. These are caused by spin-spin coupling and give us information about the neighboring hydrogen environments. Examples of splitting patterns might be a doublet or a triplet or a quartet. Splitting patterns can get very complicated in molecules with lots of hydrogens.
Finally, we come to the integration of the peaks in our spectrum. The integration of peaks in proton NMR spectra are directly proportional to the number of protons in that particular environment. Remember that comparing peak integrations is unique to proton NMR spectroscopy. It doesn’t work with things like carbon NMR. So, going back to our question, we have two peaks which have a three to two integration ratio. So, it could be that there are three protons in one environment and two in the other.
However, remember that this is a ratio and not exact numbers. So, equally this could mean six hydrogens in one environment and four hydrogens in the other, since this would still give us a three to two ratio. Let’s now look at each of the possible answers and see which one is correct.
A says that there are three hydrogens on one carbon and two on another. As we’ve seen, whilst this might be true, it’s not necessarily true. It could be that there are three hydrogens in one environment and two hydrogens in another. But the three to two ratio could mean that there are six hydrogens in one and four in another, or nine hydrogens in one and six in another. So, whilst A might be true, it’s not definitely true. So, let’s rule this one out.
B says that one of the peaks is a methyl group and the other is a methylene group. Again, whilst this might be true because a CH₃ and a CH₂ group do have a three to two ratio, it’s not necessarily true. So, let’s rule this answer out.
C says that the peaks belong to a straight-chain aliphatic compound. Going back to our notes, we can see that the type of proton environment is actually denoted by the chemical shift, not the integration. If these peaks were in a straight-chain aliphatic compound, they would likely be quite upfield towards the right. However, this question doesn’t give us any information about the chemical shift of these two peaks, so we can’t know what type of hydrogen environment we’re looking at. So, this can be ruled out.
Finally, we come to D. The ratio between the two types of hydrogen environment is three to two. This answer is correct. This is the only accurate deduction we can make from the information that we’ve been given. We would need far more information from the spectrum in order to narrow down the types of proton environments or any further information about the molecule. So, if we have a proton NMR spectrum, and two peaks have a three to two integration ratio, this means that the ratio between the two types of hydrogen environment is also a three to two.