Nuclear Magnetic Resonance - What Is It?
Did You Know?
Check it out: Protons have their own magnetic field and it either compliments or subtracts from an external magnetic field.
What is a Proton and What Does it Have to Do With NMR?
Believe it or not, protons are highly relevant to NMR and MRI. Protons are essentially the nuclei of a hydrogen atom. The vast majority of hydrogen nuclei are made up of just one proton and nothing else.
One property of all atomic particles is spin. Protons spin and it can described using opposing terminology - up or down, positive or negative, north and south, clockwise or counter-clockwise... Most in the scientific community choose up-spin and down-spin to describe the spin of protons.
Under normal circumstances, up-spin and down-spin have equal energies. They do not slow down, they just perpetually spin. The population of protons up-spin are roughly equal to down-spin and they swap often due to interactions with outside forces.
However, when an external magnetic field is applied to protons, the up-spin and down-spin no longer have the same energies. The difference in energy is equal in energy to electromagnetic energy in the radio frequencies of the spectrum. Therefore, scientists can see a signal in the spectrum when there are protons in a strong magnetic field.
What Does NMR Have to do With Organic Chemistry?
Actually, there are different types of NMR that apply to more than just Organic Chemistry. The uses in O-Chem are probably the most abundant. Scientists are just now being able to see atoms and molecules with super-high-resolution microscopy, but for the last generation they have not. They had to piece together a puzzle from clues. Scientists already knew that hydrogen binds to carbon a lot in O-Chem but it became a puzzle to identify the atomic structure of various organic molecules. Scientists knew that carbon makes four bonds and it would be helpful to find out how many of those were with hydrogen (protons, don't forget).
Alas! Scientists could put a pure sample in a tiny vial and insert it into a massive superconducting magnet. They pass radio waves through it and study the proton interactions. The integration of the signal will suggest how many hydrogens are bound to a particular carbon. How many peaks the signal has indicates how many hydrogens are on the next adjacent carbon, plus one. Often times, the sample is mixed with a relatively inert chemical, Tetra-Methyl Silane (TMS), with a known signal structure (TMS has one massive peak with area equivalent of twelve protons). If the sample signal has two signal regions, that means two of the carbons have hydrogens bound to them, and the strength of those signals indicates how many. If a signal region has two peaks, that means an adjacent carbon has only one hydrogen bound to it. If a signal region has three peaks, that means an adjacent carbon(s) has two hydrogens bound to it. If a signal region has four, that means an adjacent carbon(s) has three hydrogens bound to it. A hydrogen bound to a carbon sandwiched between multiple carbons can potentially have lots and lots of peaks - ten if I've done the math right for isobutane. TMS is used as kind of a blank signal since all hydrogen signals lack interference from protons on adjacent carbons because there are no adjacent carbons.
This gets a little messy with some chemicals that have symmetry. Take benzene, for example. All the protons have the same position relative to the other protons. Their signals will all overlap. Another thing I haven't mentioned is that temperature plays into this. If your NMR is too warm, you will only see one signal for benzene's protons. If you cool it down significantly, it will take on multiple peaks in a signal region.
Why multiple peaks in an NMR signal? Because all protons have their own magnetic field that is either in line with the external or out of line with the external - and when they're in line they expose other protons to a higher overall magnetic field and when they're out of line they expose other protons to a lower overall magnetic field. That modifies the energies between up-spin and down-spin and therefore, modify which frequency in the radio spectrum the sample absorbs.
The spin properties are what allow scientists to separate up and down and get a signal from it. Note that when a nuclei has an even number of protons and neutrons combined, the overall spin of the nucleus is zero. Therefore, certain types of NMR cannot get a signal from certain elements or certain isotopes. Even if the instrument can get a signal from another element, it will be in a different part of the radio spectrum. This property of isotopes can be useful if scientists study a chemical that was intentionally made with different isotopes - say for example DNA with one strand made entirely with deuterium.
Manipulating the Signal
There are ways to manipulate the magnetic field to get a desired effect. Even the most perfect magnet created by man will still have slight imperfections that will distort a signal. The most common way to deal with this is to rotate the sample inside the NMR device. The result is that the magnetic field the sample is exposed to is homogenized and the results will look cleaner when the instrument translates them into an image.
Another way to deal with the imperfections is to measure the field very precisely so that the operator will know exactly what frequency to expect. This is typically done as part of calibrating and initializing the instrument.
Commonalities of NMR and MRI
Still want more tricks for manipulating the science? How does NMR translate into MRI? First off, the word “nuclear” makes people subconsciously itchy when it comes to medicine. Even though there are no nuclear reactions going on, people still have a subconscious foreboding when working with nuclear technology. So for medicine, the technology was named Magnetic Resonance Imaging. How do those doctors get such high-resolution images of the inside of your body? In MRI, your body or some part of it is the sample. It is exposed to a magnetic field gradient – that is a field that is stronger on one side than the other. This means that the signal from your feet will be shown in a different part of the radio spectrum than the signal from your head. The operator passes a radio wave throughout your body. It excites some of the hydrogen nuclei. When that excited state drops to de-excited, it will emit a radio frequency proportional in energy to the magnetic field at that point in the gradient. This means that the operator can refine which cross-section of your body they are looking at by either tuning into a specific frequency or by just looking at all the signal generated from your body when the radio waves were passed through that specific cross-section. In all reality there are variations in MRI design that have advantages and disadvantages.
Metallurgy and Antennae
One hold-up of a lot of electro-magnetic spectroscopy is what elements absorb the signal, which reflect it, which transmit it. For example, really old refraction telescopes could not see infrared because glass is opaque in the infrared part of the spectrum.
In the older days of NMR, they struggled to get clear signals because the resolution of the wave was not very good. Multiple peaks would overlap with each other. Temperature and magnetic field played big since cold temperatures are expensive and magnetism is still a developing study. Operators got around this by taking hundreds of measurements and kind of averaging them out. The result was that NMR was incredibly time intensive and expensive.
How did they get around this? Metallurgy. They found better materials and alloys to make the antannae out of that allowed very high resolution radio signals. While I was going to college, we got our hands on an NMR that was already obsolete and could get a 3D NMR image in less than 15 minutes. The software would integrate the signal for me, measure an exact location, allow me to choose a signal-to-cutoff ratio... It is truly stunning. The bottleneck of data collection speed was definitely the operator. The computer could even determine a bit of the chemical structure for us, but the professor wouldn't allow that since the purpose of the excersize was for us to solve the chemical structure by just looking at the signal. That was six years ago.