NMR Spectroscopy

Question 1

What does NMR stand for?

Answer 1

Nuclear Magnetic Resonance

Question 2

Give one use of NMR?

Answer 2

MRI scans

Question 3

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

Answer 3

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

Question 4

What percentage of carbon atoms are 13C?

Answer 4

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

Question 5

What defines the resonant frequency of a 13C atom?

Answer 5

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

Question 6

What graph is produced by NMR spectroscopy?

Answer 6

Energy absorbed against chemical shift

Question 7

What is chemical shift?

Answer 7

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

Question 8

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

Answer 8

0-200ppm

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

Question 11

What does the number of signals mean? (13C NMR)

Answer 11

One signal for each carbon environment (each set of inequivalent 13C atoms)

Question 12

What does the chemical shift mean? (13C NMR)

Answer 12

Greater shift from atoms closer to electronegative atoms or C=C

Question 13

What does the area under the peak mean? (13C NMR)

Answer 13

no meaning

Question 14

What does splitting mean? (13C NMR)

Answer 14

There is no splitting for 13C NMR

Question 15

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

13C NMR?

Answer 15

Most H atoms are 1H- it is much more abundant than 13C.

This means almost all H atoms have spin so show up

Question 16

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

Answer 16

0-10ppm

Question 17

What leads to a lower chemical shift value for H

NMR?

Answer 17

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

Question 18

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

Answer 18

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

Question 19

What does the area under the peak represent (for H

NMR)?

Answer 19

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

Question 20

What is the integration trace?

Answer 20

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

Question 21

What is TMS?

Answer 21

Tetramethylsilane

Question 22

What state is TMS at room temperature?

Answer 22

liquid

Question 23

Why is TMS used?

Answer 23

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

Question 24

What are other advantages of using TMS?

Answer 24

Inert, non-toxic, easy to remove from the sample

as relatively volatile

Question 25

When does splitting/spin-spin coupling occur?

Answer 25

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 26

What is the n+1 rule?

Answer 26

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

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 solvents are used?

Answer 28

Deuterated solvents: CDCl3, D2O, C6D6

CCl4 - tetrachloromethane

Question 29

What does the number of signals mean? (H NMR)

Answer 29

One main signal for each set of inequivalent 1H atoms (for each hydrogen environment)

Question 30

What does the chemical shift mean? (H NMR)

Answer 30

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

Question 31

What does the area under the peak mean? (H NMR)

Answer 31

Proportional to the number of atoms represented by that peak

Question 32

What does splitting mean? (H NMR)

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What are the units and symbol for chemical shift?

Answer 34

Parts per million (ppm)

Symbol: δ