NMR Spectroscopy OCR

Each proton in a nucleus has a spin.

This spin can generate a magnetic field which can be detected.

It can only be detected if the nuclei contain an odd number of protons (p+) like in C13. This has an unpaired p+ who's magnetic field can be detected.

If we wrere to take nuclei with an even number of p+ like C12, the magnetic field would be very hard to detect because of the paired protons which have opposite spins which cancel out therefore the spins cannot be detected.

Magnetic resonance imaging:

• non-invasive
• painless procedures

Unlike X-rays and CT scans, it does not ionise radiation which can be harmful to the patient.

• Liquid helium can be used to cool the magnets

There are also upright MRI scanners so that patients don't feel claustrophobic.

We know that proton spins generate their own magnetic field, but we also need to consider that there are other, strong external magnetic fields like the ones generated by an NMR machine. The p+ will either align with the external magentic field or move against it.

They will align with it if the magnetic field of the p+ spins have a lower energy than the external magnetic field.

The p+ magnetic fields that align against the external magnetic field have a higher energy than the external magnetic field, but in order to move against the external magnetic field , the p+ must absorb energy. The energy absorbed is in the radiowave region of the spectrum. When energy is absorbed, this is is known as excitation. The p+ then drops back to it's lower state known as relaxation.

In other words, when energy is absorbed in the radiowave region, the p+ magnetic fields move from "with" to "against" (when radiowaves are applied) and back to "with" external magnetic fields. This is known as RESONANCE. It goes from excited to relaxed to excited...etc.

The amount of energy that each nuclei will absorb depends on its environment.

If it doesn't need a lot of energy , then the energy will be absorbed at a lower frequency in the radiowave region .

We can't really tell what this frequency is so we compare it to a stardard compound. It is then know as a chemical shift (how much it shifts from the standard compound).

This is measure in parts per minute (ppm) found on the x-axis of the spectrum. If this is a low value, that means that the frequency absorbed is also low.

For C13 and H1 , the reference compound is tetramethylsilane.

This has carbons and protons in the same environment which will show only one sharp peak (all in the same environment and this indicated by one peak on the spectrum) at 0 ppm (lowest frequency absorbed).

• chemically unreactive
• volatile (easily removed from the sample)

All compounds must be in a solvent. However they are hydrocarbons so you'd need a DEUTERATED solvent such as CDCl3.

Why?

• It wouldn't give an extra signal peak on the spectrum
• It can be evaporated off after analysis of sample so it doesn't contaminate the sample.

These factors affect resonance:

• The applied strong magnetic field;
• The weaker magnetic field generated from electrons surrounding the nucleus and in nearby atom - also known as nuclear shielding ... electrons shield nucleus from external magnetic fields;

For instance we're using C13 NMR and in our carbon compound one of the carbon atoms is bonded to an oxygen atom. The oxygen atom is more electronegative and will pull the shared pair of electrons closer to itself. This means that the carbon atom will now have less nuclear shielding. If there's less nuclear shielding, then the nucleus will absorb a higher frequency of radiowaves as it will experience a greater magnetic field (which is the external magnetic field).

C-C bond: absorbs less energy from radiowave region due to greater nuclear shielding , experiences less of the external magnetic field, low ppm;

C-O bond: absorbs more energy, less nuclear shielding, higher ppm.

This spectrum tells us:

• The number of carbon atoms in different environments
• The type of environment those carbons are in.

If there's 4 peaks , then there are 4 different carbon environments. The carbons in the sam environment will be shown in one peak. So if there were 2 carbon atoms in the same environment in one compound, this will only be shown once of the spectrum and this will be represented bt one peak.

It is very similar to 13C NMR

• Can identify how many different p+ environments;

• Can look at chemical shift to work out the different proton environments;

• Can identify how many protons are in each environment by the relative height/area of each peak. (The spectrum goes up to 20ppm when its proton NMR).

When the peaks in p+ NMR are looked at in more detail, they show splitting.

They split depending on the protons in the neighbouring environment.

If the neighbouring environment has 2p+, the peaks will be split in 3 and this is called a TRIPLET;

If the neighbouring environment has 1p+, the peaks will be split in 2 = DOUBLET;

If the neighbouring environment has 3+ or more ==> MULTIPLET.