6.3.2: 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

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

Question 5

Summarise what these mean for 13C NMR:
Number of signals
Chemical shift
Area under peak
Splitting

Answer 5

Number of signals: One signal for each carbon environment (each set of inequivalent 13C atoms)
Chemical shift: Greater from atoms closer to electronegative atoms or C=C
Area under peak: no meaning
Splitting: there is no splitting for 13C NM

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

What does the area under the peak represent (for H

NMR)?

Answer 8

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

Question 9

What is the integration trace?

Answer 9

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

Question 10

What is TMS name?

Answer 10

Tetramethylsilane

Question 11

Why is TMS used?

Answer 11

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 12

What are other advantages of using TMS?

Answer 12

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

Question 13

When does splitting/spin-spin coupling occur?

Answer 13

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 14

What is the n+1 rule?

Answer 14

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

Question 15

Why must solvents used for 1H NMR not contain any

hydrogen atoms?

Answer 15

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

Question 16

Which solvents are used?

Answer 16

Deuterated solvents: CDCl3, D2O, C6D6

CCl4 - tetrachloromethane

Question 17

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

Answer 17

Number of signals: One main signal for each set of inequivalent 1H atoms (for each hydrogen environment)
Chemical shift: Larger 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 18

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

Answer 18

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