3.15 NMR Spectroscopy

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 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 (due to resonance) are detected

Question 4

Give one use of NMR?

Answer 4

MRI scans

Question 5

What kind of nuclei does NMR work with (and examples)?

Answer 5

Those with an uneven number of nucleons, meaning they will spin e,g. 1H, 13C

Question 6

What percentage of carbon atoms are 13C?

Answer 6

1% but modern instruments are sensitive enough to detect this

Question 7

What defines the resonant frequency of a 13C atom

Answer 7

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

Question 8

What graph is produced by NMR spectroscopy

Answer 8

Energy absorbed against chemical shift

Question 9

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

Answer 9

The resonant frequency of the nuclei, compared to that of a 1H atom in TMS.

Symbol δ

Parts per million (ppm)

Question 10

What is the range of chemical shift for 13C NMR?

Answer 10

0-200 ppm

Question 11

What means 13C atoms show a different chemical shift value?

Answer 11

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

Question 12

What kind of environment leads to a greater chemical shift?

Answer 12

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

Question 13

Summarise what these mean for 13C NMR:

number of signals
Chemical shifts
Area under peak
Splitting

Answer 13

number of signals: one signal for each carbon environment (each set of inequivalent 13C atoms)
Chemical shifts: Greater δ from atoms closer to electronegative atoms or C=C
Area under peak: no meaning
Splitting: there is no splitting for 13C NMR

Question 14

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

Answer 14

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

Question 15

What is the range of chemical shift for 1H NMR?

Answer 15

0-10 ppm

Question 16

What leads to a lower chemical shift value for H NMR?

Answer 16

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

Question 17

On a low resolution spectrum, what peaks would you expect to see for H NMR?

Answer 17

one peak for each set of in equivalent H atoms (each chemical environment shows 1 peak)

Question 18

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

Answer 18

The area under the peak is proportional to the number of 1H atoms represented by the peak

Question 19

What is the integration trace

Answer 19

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

Question 20

What is TMS (name and structure)?

Answer 20

Tetramethylsilane

Slide 42

Question 21

What state is TMS at room temperature?

Answer 21

Liquid

Question 22

Why is TMS used

Answer 22

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

Question 23

What are the other advantages of using TMS?

Answer 23

inert, non toxic, easy to remove from the sample

Question 24

When does splitting/spin-spin coupling occur?

Answer 24

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

Question 25

What is the n+1 rule?

Answer 25

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

Question 26

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

Question 27

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

Answer 27

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

Question 28

Which solvent are used?

Answer 28

Deuterated solvents : CDCl3, D2O, C6D6CCl4- tetrachlormethane

Question 29

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

Answer 29

Number of signals: one main signal for each set of inequivalent 1H atoms (for each hydrogen environment)

Chemical shift: Larger δ for 1H atoms closer to electronegative atoms or C=C

Splitting: Number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away

Area under peak: Proportional to the number of atoms represented by that peak

Question 30

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

Answer 30

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