3.15 NMR Spectroscopy

Question 1

What does NMR stand for

Answer 1

Nuclear magnetic resonance

Question 2

How to carry out NMR

Answer 2

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

Question 3

Give one use of NMR

Answer 3

MRI scans

Question 4

What kind of nuclei does NMR work with and examples

Answer 4

Those with uneven number of nucleons, meaning they will spin
eg 1H, 13C

Question 5

What percentage of carbon atoms are 13C

Answer 5

1% - but modern instruments are sensitive enough to detect this

Question 6

What defines the resonant frequency of a 13C atom

Answer 6

The chemical environment that it is in; the amount of electron shielding it has

Question 7

What graph is produced by NMR spectroscopy

Answer 7

Energy absorbed against chemical shift

Question 8

What is chemical shift
What is its symbol
What are its units

Answer 8

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

Question 9

What means 13C atoms show a different chemical shift value

Answer 9

Having different chemical environments (but equivalent atoms show the same peak)

Question 10

What kind of environment leads to a greater chemical shift

Answer 10

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

Question 11

Summarise what these mean for 13C NMR
Number of peaks
Chemical shift
Area under peak
Spliting

Answer 11

Peaks: One signal for each carbon environment
Chemical shift: Greater shift from atoms closer to electronegative atoms or C=C
Area under peak: No meaning
Splitting: No splitting for 13C NMR

Question 12

Why is it easier to get a spectrum of 1H NMR than 13C NMR

Answer 12

Most H atoms are 1H- it s much more abundant than 13C. This means almost all H atoms have spin so show up

Question 13

What leads to a lower shift value for H NMR

Answer 13

1H with more electrons around them i.e. further from electronegative groups/atoms

Question 14

On a low res spec, what peaks would you expect to see for H NMR

Answer 14

One peak for each set of inequivalent H atoms
(each chemical environment shows 1 peak)

Question 15

What does the area under the peak represent (H NMR)

Answer 15

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

Question 16

What is the integration trace

Answer 16

A stepped line that makes it easier to measure the area under the curve
Height of line = area under peak

Question 17

What is TMS (name and structure)

Answer 17

Tetrtamethylsilane

Si with 4 methyl groups / google

Question 18

What state is TMS at room temperature

Answer 18

Liquid

Question 19

Why is TMS used

Answer 19

Can be added to sample to calibrate the NMR equipment
It provides a peak at exactly 0ppm
Is is the reference point against which all shifts are measured

Question 20

What are other advantages of using TMS

Answer 20

Inert, non toxic, easy to remove from the sample (as relatively volatile)

Question 21

When does splitting/spin-spin coupling occur

Answer 21

Neighboring hydrogen atoms (3 or fewer bonds away, or on the adjacent atom) affect the magnetic field of 1H atoms and causes their peaks to split

Question 22

What is the n+1 rule

Answer 22

If there are n equivalent 1H atoms on the neighboring carbon then the peak will split into (n+1) smaller peaks

Question 23

Draw the splitting patterns for 0,1,2 and 3 inequivalent H atoms 3 bonds or less away

Answer 23

No coupled protons - singlet - google
One coupled proton - doublet - google
Two coupled proton - triplet
Three coupled proton - quartet

Question 24

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

Answer 24

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

Question 25

Which solvents are used

Answer 25

Deuterated solvents: CDCL3, D2O, C6D6
CCL4 - tetrachloromethane

Question 26

Summarise what these indicate for 1H NMR
Number of signals
Chemical shift
Splitting
Area under peak

Answer 26

Number of signals: One main signal for each set of inequivalent 1H atoms (for each hydrogen environment)
Shift: Larger ppm for 1H atoms closer to electronegative atoms or C=C
Splitting: Number of smaller peaks = 1+ number of inequivalent H atoms 3 bonds away
Area under peak: Proportional to the number of atoms represented by that peak

Question 27

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

Answer 27

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