In the previous NMR post, we looked at a small bit of NMR theory and also discussed chemical shift. This post will focus on proton NMR and include a discussion of integration (proton counting) and spin-spin splitting (coupling). Subsequent posts will discuss chemical equivalency, coupling constants, and carbon 13 NMR. One thing to mention before getting started. You will often see a TMS peak in many NMR spectra. This stands for tetramethylsilane and is used for our reference peak at zero.
Integration
We can actually determine how many protons are causing a peak to occur in an NMR spectrum by integrating the peak area. So for a specific peak, we can see how many protons are causing that peak. Take a look at the following example.
In the proton NMR spectrum above, we can clearly see two peaks. The peak at 2.1 is due to the H's on the CH3 next to the carbonyl. The other peak, is due to all three CH3 groups on the other carbon next to the carbonyl. remember this isn't going to show up in a proton NMR since there are no H's on that carbon, but there are methyl groups that have H's and that is what we see. Now, we can see that there is a 1:3 ratio on our spectrum. This allows us to determine the number of H's by integration of the area under each peak.
This part of the discussion brings us to another important question that we should be asking ourselves. Why were all of those methyl groups only showing up as one peak? It turns out that the methyls are all the same, or as we say, chemically equivalent. This means that they will only show up as one peak. Chemically equivalent?? This basically means that there is no discernible difference in any of the H's. For instance, all the methyl groups on our molecule are the same, none of them are different. Here is another example.
Above, we see the proton NMR spectrum for 1,4-dimethyoxybenzene. What we notice is that there 10 total protons but we only see two peaks. This has to do again with chemical equivalency. If you look at the molecule above, you will see that it is symmetrical, each half is exactly the same. Therefore, all of the ring H's are the same and all of the methoxy H's are the same.
Spin-Spin Splitting
Now for the fun part! It turns out that nonequivalent H's that are adjacent actually split each other. Another name for this is called coupling. What happens is that multiple absorptions of a nucleus are caused by the interaction of other close nuclei. Basically, the little magnetic fields that are produced by each nucleus affects other neighboring nuclei. We can gain a great deal of structural information from splitting patterns. Let's look at an example of splitting.
Above, is the proton NMR spectrum for iodoethane. We can see that there are two peaks but they are not the same as we saw earlier. We can justify the positions of the chemical shifts because the 2 H's on the carbon with the I are more deshielded since iodine is more electronegative than carbon. Now, we need to justify how the peaks are split. The rule that is used is the n + 1 rule to determine splitting. Notice that the CH2 portion of the molecule is next to a CH3 group. The CH2 group is split into 4 pieces (called a quartet). How does this happen?
Think of each proton as its own mini magnet. Each magnet can either be aligned with or against the applied magnetic field. So, if we had only one proton, then the average would be one against the field and one with the field. Consider the following example with two adjacent protons, Hx and Hy.
When considering the splitting pattern for the Hy proton, we note that it is adjacent to one Hx proton. Our rule for splitting would be n + 1, so we should expect Hy to be a doublet. Let's now look at why this is the case. The adjacent Hx proton can align either with or against the applied magnetic field. Because of this, Hy is split into a doublet. The same goes for the Hx proton, it would also be split into a doublet. Also, note that the two possibilities are of equal probability. Therefore, the doublet should have two peaks of the same height. Let's now look at what happens when there are two adjacent protons.
Looking at the above picture, you can see that Hx can be split by Hy due to the three possible environments created by Hy. Hy can align either all with the applied magnetic field, all against, or in a mixture of the two possibilities. Note that the statistical probability would be higher for the two that are a mixture of both alignments and also note that the shape of a triplet fits that probability. Now, you can see where the n + 1 rule comes in and how splitting occurs. What about a quartet?
In the above picture, you can see that Hx can be split in four separate ways depending on the alignment of Hy. This then explains the nature of the n + 1 rule and why we see the quartet.
That does it for this post concerning splitting and integration. I hope you find this helpful. Stay tuned for future posts regarding chemical equivalency.