Have you ever heard of an MRI? Magnetic Resonance Imaging, to more detailed. If you have ever had an injury to body tissue, I am willing to bet you have possibly had a scan. Well, it turns out that chemists use a very similar instrument to look at molecules. I'll come back to the MRI a bit later.
The instrument that chemists use is called an NMR spectrometer, or a Nuclear Magnetic Resonance spectrometer. So how does this instrument work? Well, to understand the principles behind NMR, we need to look at the structure of an atom. Atoms contain a nucleus that consists of protons and neutrons and an electron cloud that is outside of the nucleus. Now imagine, that each of the nuclei that have positive charges are spinning around an axis. This creates a tiny magnetic field. This tiny magnetic field can now interact with an external magnetic field.
This brings us to an important point. In order for a nuclei to have a magnetic field, it must posses spin. All nuclei do not posses spin and those that do not are NMR inactive. The best nuclei to image are those that have a spin = 1/2. This is because the direction of the spin is 2 x's the number of spin. plus 1. So, if a nucleus has a spin of 1/2 it will have two possible orientations. Without an external magnetic field applied, these two orientations are equal in energy.
As you can see from the picture, once a magnetic field is applied, the spin energies are different in every. The nuclei can align in two separate ways in the magnetic field; either parallel or anti-parallel to the applied external magnetic field (note that this is specifically discussing the 1H and 13C nuclei, this is not discussing others in reference to only having a spin of 1/2). It turns out that the parallel orientation is slightly lower in energy by an amount that depends on the strength of the external magnetic field. The nuclei are then irradiated with electromagnetic radiation and energy absorption occurs. Once the energy is absorbed, the lower energy state flips up to the higher energy state. When the flip occurs, the magnetic nuclei are considered to be in resonance with the applied electromagnetic radiation. Ok, so why am I going into all of this? The answer is that you need to know a little about how NMR works to understand the nature of NMR absorptions.
When looking at an NMR spectrum, one will notice that there are peaks in different areas. This is due to a phenomena called shielding. Since the structure of an atom contains an outer area that is full of electrons, a small magnetic field is generated by the movement of the electrons around the nucleus. This small local field acts in direct opposition to the applied magnetic field (remember, we are applying a magnetic field, then irradiating the sample with rf energy to spin-flip to be in resonance). These electrons then in a sense "shield" the nucleus from the effects of the applied magnetic field. Since each nucleus is in a slightly different electronic environment, each is then shielded to a different amount. These differences allow us to determine specific structural information about the molecules under study.
Alright, let's put all this into a bit of practice.
The NMR Spectrum
Below is a proton NMR spectrum of methyl acetate.
The y-axis is the intensity of each peak and the x-axis is the chemical shift value. The chemical shift value is what is affected by shielding. Groups that are more shielded feel the effect of the applied magnetic field less than those that are deshielded. We refer to peaks that are shielded as those that are upfield or to the right in the typical NMR spectrum. Those that are deshielded (those attached to more electronegative elements) are considered to be downfield or to the left on the spectrum. Ok, so what is the deal with the delta scale for chemical shift?
The delta scale is an arbitrary scale where one delta equals 1 part per million of the spectrometer operating frequency. The reason this is done is to negate the differences in frequency from one spectrometer to another. This way, it doesn't matter if your NMR spectrum was collected on a 600 MHz spectrometer or a 90 MHz spectrometer. So, if a spectrum was ever given to you in hertz, all you have to do is divide by the operating frequency of the NMR. For example, suppose a peak was given at 1460 Hz. What is the chemical shift in delta units for a spectrometer operating at 200 MHz? Delta = Hz/operating frequency. So, our ppm or delta value for chemical shift would be 1460Hz/200MHz = 7.3 delta. Pretty simple right?
Now, let's look at our proton NMR spectrum and see if anything we have discussed makes reasonable sense. We see two peaks in the spectrum. The peaks are at different locations, or chemical shift values. We can clearly see one peak is upfield or is more shielded and another peak is downfield or more deshielded. Which is which? Well, the peak that is attached to the more electronegative element is going to be more deshielded since the electronegative element is pulling its electron density and it follows that the electrons that would be swirling around creating a magnetic field are there less. Wait, electronegativity? Remember, this increases as we move up and to the right on the periodic table with F being the most electronegative element. Given this information, can you pick out which peak is which in the NMR?
If we look at the NMR spectrum of methyl acetate, we can see that there are two peaks that have different chemical shift values. These values are different because of the different electronic environment of the protons. First, we see a peak that is at roughly 2.1 ppm. Next, further downfield, we see a peak that is at 3.8 ppm. This should make sense! Oxygen is more electronegative than carbon and therefore is should reason that the nucleus here should be more deshielded and move downfield. The other peak is attached to a carbon and it should then be more shielded because there is nothing to take away the electron density. See if you can reason through this!
I hope this post is helpful to those trying to understand NMR. That is all for now. Take some time to digest the material. The next post will dive into more information about NMR interpretation. Until then!