Organic - Nuclear Magnetic Resonance Spectroscopy

Question 1

What are the two types of NMR?

Answer 1

13C NMR and 1H NMR

Question 2

What does NMR stand for?

Answer 2

Nuclear Magnetic Resonance Spectroscopy

Question 3

What is carbon NMR?

Answer 3

Shows information about how the carbon atoms in a molecule are arranged

Question 4

What is hydrogen NMR?

Answer 4

Shows information about how the hydrogen atoms in a molecule are arranged

Question 5

How is an NMR spectrum produced?

Answer 5

• Nuclei in different environments absorb different amounts of energy
• Nuclei in a molecule feel different magnetic fields depending on their environments
• Nuclei in different environments will absorb different amounts of energy at different frequencies.

Question 6

What does 13C NMR show?

Answer 6

• Number of peaks shows how many different carbon environments are present
• The spectrum will have one peak on it for each carbon environment in the molecule

Question 7

What does 1H NMR show?

Answer 7

• How hydrogen nuclei react to a magnetic field
• Each peak on a 1H NMR spectrum is due to one or more hydrogen nuclei in a particular environment
• The numbers above the peaks on a 1H spectrum tell you the ratio of the areas under the peaks

Question 8

What is the splitting pattern rule for 1H NMR?

Answer 8

n+1 rule

Question 9

What is a singlet?

Answer 9

A peak that is not split and has 0 hydrogens on the adjacent carbon

Question 10

What is a doublet?

Answer 10

A peak that is split into two and has 1 hydrogen on the adjacent carbon

Question 11

What is a triplet?

Answer 11

A peak that is split into three and has 2 hydrogens on the adjacent carbon

Question 12

What is a quartet?

Answer 12

A peak that is split into four and has 3 hydrogens on the adjacent carbon

Question 13

What solvents are used for 1H NMR?

Answer 13

• If a sample has to be dissolved, then a solvent is needed that doesn’t contain any 1H atoms
• Deuterated solvents are often used - hydrogen atoms are replaced by deutrium (D or 2H)
• Because deutrium has an even number of nucleons, it doesn’t have a spin (no magnetic field)
• CCl4 can also be used as a solvent as it has no 1H atoms

Question 14

What is the chemical shift measured relative to?

Answer 14

Tetramethylsilane (TMS)

Question 15

Why is TMS used to measure chemical shift against?

Answer 15

TMS has 12 hydrogen atoms, all in identical environments, so it produces a single absorption peak, well away from most other absorption peaks.

Question 16

What is chemical shift measured in?

Answer 16

Chemical shift is measured in ppm relative to TMS, so the singlet peak produced by TMS is given a chemical shift value of 0.

Question 17

What properties make TMS useful?

Answer 17

• Inert (doesn’t react with the sample)
• Non-toxic
• Volatile (so it is easy to remove from a sample)