NMR Soectroscopy

Question 1

What does NMR stand for

Answer 1

Nuclear magnetic resonance

Question 2

What are the basic principles of NMR

Answer 2

You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them. If radio waves of the right frequency are absorbed the nuclei flips from parallel to applied magnetic to field to anti-parallel. This energy change can be monitored and recorded. Uses the resonance of nuclei with spin

Question 3

How would you carry out NMR spectroscopy

Answer 3

Dissolve the liquid sample in suitable solvent, put in a robe along with a small amount of TMS and put the tube in a NMR machine. The sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against TMS. Radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions are detected

Question 4

Give in use of NMR

Answer 4

MRI scans

Question 5

What kind of nuclei does NMR work with

Answer 5

Uneven number of nucleons

Because they will spin

Question 6

What percentage of C atoms are in 13C

Answer 6

1%

Question 7

What defines the resonant frequency of a 13C atom

Answer 7

The chemical environment that it is in

Question 8

What graph is produced by NMR spectroscopy

Answer 8

Energy absorbed against chemical shift

Question 9

What is chemical shift

What are it’s units

Answer 9

The resonant frequency of the nuclei compared to that of a 1H atom in TMS
Parts per million (ppm)

Question 10

What means 13C atoms show a different chemical shift value

Answer 10

Having different chemical environments

Question 11

What kind of environment leads to a greater chemical shift

Answer 11

A C atom next to more electronegative atom had a greater chemical shift

Question 12

13C 
Number of signals?
Chemical shift?
Area under peak?
Splitting?

Answer 12

One signal for each carbon environment
Greater shift from atoms closer to electronegative atoms or C=C
No meaning
There is no splitting for 13C NMR

Question 13

What leads to a Lowe chemical shift value for H NMR

Answer 13

1H with more electrons around then

Question 14

What does the area under the peak represent for H NMR

Answer 14

Proportional to the number of 1H atoms represent by the peak

Question 15

What is the integration trace

Answer 15

A stepped pine that makes it easier to measure the area under the curve

Question 16

What is TMS

Answer 16

Tetramethylsilane

              CH3
                 |
      H3C-Si-CH3
                 |
               CH3

Question 17

What state is TMS at room temp

Answer 17

Liquid

Question 18

Why is TMS used

Answer 18

Can be added to calibrate the NMR equipment. It provides a peak at 0
Reference point against which all are measured

Question 19

Advantages of TMS

Answer 19

Inert
No toxic
Easy to remove from the sample

Question 20

When does splitting occur

Answer 20

Neighbouring hydrogen atoms affect the magnetic field of 1H atoms and causes their peaks to split

Question 21

Why is the n+1 rule

Answer 21

If there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into n+1

Question 22

No coupled protons
One coupled protons
Two coupled protons
Three coupled protons

Answer 22

Singlet
Doublet
Triplet
Quartet

Question 23

Why must solvents used for 1H NMR not contain any hydrogen

Answer 23

Signals from the solvent would swamp signals from the sample as there is much more solvent than sample

Question 24

Which solvents are used

Answer 24

CDCl3
D2O
C6D6
CCl4

Question 25

1H NMR
Number of signals 
Chemical shifts
Splitting
Area under peak

Answer 25

One main signal for each set of inequivalent 1H atoms
Larger shift for 1H atoms closer to electronegative atoms or C=C
Number of smaller peaks =1 + number of inequivalent hydrogen atoms 3 bonds away
Proportional to the number of atoms represented by that peak

Question 26

Why does the peak from O-H bonds disappear if D2O is used as a solvent

Answer 26

O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another