3.15 NMR Spectroscopy

Question 1

What is NMR spectroscopy?

Answer 1

A method used to determine the molecular structure of a compound using an external magnetic field.

Question 2

How is a nucleus shielded from the external magnetic field?

Answer 2

Shielded by the electrons surrounding the nucleus.

Question 3

Why do nuclei absorb different amounts of energy?

Answer 3

They all have different environments.

Question 4

What affects electron shielding?

Answer 4

The atom/s adjacent to nucleus.
i.e. If an electronegative element is near the carbon atom it will pull electrons toward itself and reduce shielding.

Question 5

How do different environments affect nucleus in NMR?

Answer 5

Nuclei feels magnetic field differently depending on environment. Different amounts of energy and frequencies picked up by NMR.

Question 6

What is considered as same environment in NMR?

Answer 6

Bonded to atom/atom group which are identical.

Question 7

Name the reference compound used in NMR. What is the structure?

Answer 7

Tetramethylsilane (TMS)
Si(CH3)4

Question 8

Why is TMS used as reference?

Answer 8

It has identical environments which results in one singlet peak.

Question 9

Advantages of using TMS.

Answer 9

Inert, non-toxic & volatile so easy to remove from sample.

Question 10

How is a sample for NMR made?

Answer 10

1. Add TMS to sample
2. Put in NMR machine
3. Measure the chemical shift against TMS (delta = 0 as reference)
4. Remove TMS

Question 11

Name the difference between TMS peak and peaks produced by sample being tested?

Answer 11

Chemical shift (Delta symbol)

Question 12

Where is the reference point?

Answer 12

Delta = 0

Question 13

How do you know the number of carbon environments on a 13C spectrum?

Answer 13

No. of different carbon environments = no. of peaks

Question 14

What direction are the peaks which are less shielded ?

Answer 14

left

Question 15

What direction are the peaks which are more shielded?

Answer 15

Right

Question 16

In CH2ClCH3, which carbon atom would have a higher chemical shift?

Answer 16

The carbon that is directly bonded to the chlorine atom will have a higher chemical shift as it will be closer to the chlorine atom and therefore it’s less shielded.

Question 17

In CH2ClCH3, which carbon atom will have a lower chemical shift?

Answer 17

The carbon atom in CH3 will have a lower chemical shift as it’s further away from chlorine atom and more shielded and thus has a lower chemical shift.

Question 18

What does 1H NMR show?

Answer 18

No. of different hydrogen environments and how many hydrogen atoms are in each environment.

Question 19

How can you identify the number of hydrogen environments from a 1H NMR Spectrum

Answer 19

No. of different hydrogen environments = no. of peaks

Question 20

What do the numbers above each peak indicate?

Answer 20

The ratio of hydrogen atoms in each environment

Question 21

How would you calculate the number of hydrogen neighbours on an adjacent carbon molecule from a 1H NMR spectrum?

Answer 21

-Look at the no. of smaller peaks on each peak
-n-1 = no. of hydrogens on neighbouring adjacent carbon atom

Question 22

Names of different number of peaks.

Answer 22

Singlet = 1 peak
Doublet = 2 peaks
Triplet = 3 peaks
Quartet = 4 peaks

Question 23

How many hydrogen neighbours does a singlet, doublet, triplet and quartet have?

Answer 23

Singlet = 0 neighbouring hydrogens
Doublet = 1 neighbouring hydrogens
Triplet = 2 neighbouring hydrogens
Quartet = 3 neighbouring hydrogens

Question 24

What are samples for 1H NMR dissolved in and why?

Answer 24

Non-hydrogen based elements like deuterated solvents or CCl4.

Used to avoid confusing the spectra.