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Organic Chemistry Year 2 > NMR Soectroscopy > Flashcards

Flashcards in NMR Soectroscopy Deck (26)
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1

What does NMR stand for

Nuclear magnetic resonance

2

What are the basic principles of NMR

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

3

How would you carry out NMR spectroscopy

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

4

Give in use of NMR

MRI scans

5

What kind of nuclei does NMR work with

Uneven number of nucleons
Because they will spin

6

What percentage of C atoms are in 13C

1%

7

What defines the resonant frequency of a 13C atom

The chemical environment that it is in

8

What graph is produced by NMR spectroscopy

Energy absorbed against chemical shift

9

What is chemical shift
What are it’s units

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

10

What means 13C atoms show a different chemical shift value

Having different chemical environments

11

What kind of environment leads to a greater chemical shift

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

12

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

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

13

What leads to a Lowe chemical shift value for H NMR

1H with more electrons around then

14

What does the area under the peak represent for H NMR

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

15

What is the integration trace

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

16

What is TMS

Tetramethylsilane


CH3
|
H3C-Si-CH3
|
CH3

17

What state is TMS at room temp

Liquid

18

Why is TMS used

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

19

Advantages of TMS

Inert
No toxic
Easy to remove from the sample

20

When does splitting occur

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

21

Why is the n+1 rule

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

22

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

Singlet
Doublet
Triplet
Quartet

23

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

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

24

Which solvents are used

CDCl3
D2O
C6D6
CCl4

25

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

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

26

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

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