3.3.15 Nuclear magnetic resonance spectroscopy

Question 1

What are the two types of NMR?

Answer 1

• ¹³C NMR - which gives information about how the carbon atoms in a molecule are arranged
• ¹H or proton NMR - which give information about how the hydrogen atoms in a molecule are arranged

Question 2

What are nucleons?

Answer 2

Protons and neutrons

Question 3

Why does the atomic nucleus need to have an odd number of nucleons

Answer 3

So it has nuclear spin, therefore will have a weak magnetic field, that reacts in an external magnetic field

Question 4

Explain why Hydrogen nuclei spin but carbon doesn’t and how carbon NMR still works?

Answer 4

Hydrogen nuclei are single protons, so they have spin. Carbon usually has six protons and six neutrons, so it doesn’t have spin. However 1% of carbon atoms are the isotope ¹³C (six protons and seven neutrons), they do have spin so can be used for NMR

Question 5

Explain how nuclei align themselves in an external magnetic field?

Answer 5

1. Normally nuceli spin in random directions, so their magnetic fields cancel out
2. When a strong external magnetic field is applied the nuclei will either align with the field or opposed to it
3. The nuclei algigned with the external field are at a slightly lower energy level than the opposed nuclei
4. Radio waves of the right frequency can give the nuclei that are aligned with the external magnetic field enough energy to flip up to the higher energy level. The nuclei opposed to the external field can emit the radio waves and flip down to the lower energy level
5. To start with, there are more nuclei aligned with the external field, so there will be a higher overall absorption of energy. NMR spectroscopy measure this absorbtion

Question 6

Explain how nuclei in different enviroments absorb different amounts of energy

Answer 6

1. A nucleus is partly shielded from the effects of external magnetic fields by it’s surrounding electrons
2. Any other atoms and groups of atoms that are around a nucleus will also effect the it’s amount of electron shielding - e.g. if a carbon atom bonds to a more electronegative atom (like oxygen) the amount of electron shielding around it’s carbons nucleus will decrease
3. This means that the nuclei in a molecule feel different magnetic fields depending on their enviroments Nuclei in different enviroments will absorb different amount of energy at different frequencies
4. Its the differences in energy absorbtion between enviroments that you’re looking for in NMR spectroscopy

Question 7

What effects an atoms enviroment?

Answer 7

All the group it’s connected to, going right along the molecule - not just the atoms that it’s actually bonded to. To be in the same enviroment, two atoms must be joined to exactly the same things

Question 8

What is used as a standard for chemical shift in NMR?

Why?

Answer 8

Nuclei in different enviroments absorb energy of different frequencies. NMR spectroscopy measures these differeneces relative to a standard substance - the difference is call the chemical shift (δ), The standard substance used is called tetramethylsilane (TMS), Si(CH₃)₄. This molecule has 12 hydrogen atoms all in identical enviroments, so it produces a single absorbtion peak, well away from most other absorbtion peaks

Question 9

What is chemical shift measured in?

Answer 9

Chemical shift is measured in parts per million (or ppm) relative to TMS.
So the single peak produced by TMS is given a chemical shift value of 0. You’ll often see a peak at δ = 0 on spectra because TMS is added to test compound for calibration purposes

Question 10

What is another reason that TMS is used other than having 12 hydrogen atoms all in identical enviroments?

Answer 10

Tetramethylsilane is inert, non toxic, and volitile so it’s easy to remove from the sample

Question 11

How does ¹³C NMR spectrum identify how many different carbon enviroments there are?

Answer 11

The number of peaks on a ¹³C spectrum tells you how many different carbon enviroments are present in a particular molecule. The spectrum will have one peak on it for each carbon enviroment in the molecule.

For example ethanol has two peaks due to different amount of electron shielding, the carbon bonded to the alchohol group is less sheilded so has a higher chemical shift

Question 12

How does ¹³C NMR spectrum identify how many different carbon enviroments there are in aromatic compounds?

Answer 12

Molecules containing an aromatic ring are more complicated, they can still be used to predict what their spectra will look like by looking at the number of different carbon environments. Be aware of line of symmetry.

For example:
Cyclohexane-1,3-diol’s ¹³C NMR spectrum will have 4 peaks

• One for the carbons bonded to the OH group
• One for the carbons adjacent to carbon bonded to the OH group
• And then peaks for the carbons opposite eachother in the line of symmetry

Question 13

Why can it be hard to identify what peaks represent in a ¹³C NMR spectrum?

Answer 13

The chemical shifts can overlap - e.g. a peak at ~ δ = 30 might be caused by, C-C, C-Cl or C-Br and a peak a ~ δ = 210 is due to C=O for an aldehyde or ketone, but you don’t know which

Question 14

How do you interpret ¹³C NMR spectrums

Answer 14

• Use the question to work out groups what the peak can’t be - e.g. if the question says the molecular formula only has C,H and O then there wont be an amine group
• Identify how many different carbon enviroments there are from the number of peaks
• Compare peaks to number of carbons, if there are three peaks and four carbons there must be carbons that have the same enviroment

Question 15

How can you use the peaks on a ¹H spectrum to tell you the number of different enviorments and the number of hydrogens in each enviroment ?

Answer 15

• Each peak on a proton NMR spectrum is due to one or more hydrogen nuclei in a particular enviroment
• The numbers above the peaks tells you the ratio of the areas under the peak. This relative area under each peak also tells you the relative number of H atoms in each enviroment

For example if there are two peaks there are two enviroments, if the area ratio is 1:3 there is one H atom in one enviroment and 3 H atoms in the other

An intergration trace may be drawn on the proton NMR spectrum to show the ratios of the areas under the peak more clearly

Question 16

What do splitting patterns on a proton NMR spectrum tell you about the structure of the compound?

Answer 16

The peaks may be split into smaller peaks. Peaks always split into the number of hydrogens on the neighboring carbon, plus one. This is called the n+1 rule.

• Singlet peak (not split) = 0 hydrogens on the adjacent carbon
• Doublet peak (split into two) = 1 hydrogen on the adjacent carbon
• Triplet peak (split into three) = 2 hydrogens on the adjacent carbon
• Quadruplet peak (split into four) = 3 hydrogens on the adjacent carbon

Question 17

How do you interpret a proton NMR spectrum?

Answer 17

Use the information give by the chemical shift, the ratio of the area’s under the peak and the splitting patterns

Question 18

What is an intergration trace?

Answer 18

When the peaks are split, it’s more diffucult to see the ratio of the area’s under the peak. So an intergration trace is shown instead of peak ratios. The intergration traces is a line on the spectrum that’s height is proportional to the to area’s under the peak.

A ruler can be used to measure the height of the peaks to determine the ratio

Question 19

Why do samples for proton NMR need to be dissolved in hydrogen-free solvents?

Answer 19

• If a sample has to be dissolved, then the solvent cannot contain any ¹H atoms, as they would show up on the spectrum
• For this reason deuterated solvents may be used instead - the hydrogen atoms on the solvent are replaced with deuterium (D or ²H)
• Deuterium is a isotope of hydrogen that has two nucleons therefore it has an even number of nucleons and doen’t have spin
• CCl₄ can also be used as a solvent - as it doent contain any ¹H atoms either