6.3.2 - Spectrometry

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 the integration
trace?

Answer 10

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

Question 11

What is TMS (name and
structure)?

Answer 11

Tetramethylsilane

Question 12

Why is TMS used?

Answer 12

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 measure

Question 13

What are other advantages
of using TMS?

Answer 13

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

Question 14

When does
splitting/spin-spin
coupling occur?

Answer 14

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 15

What is the n+1 rule?

Answer 15

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

Question 16

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

Answer 16

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

Question 17

Which solvents are used?

Answer 17

Deuterated solvents: CDCl3
, D2O, C6D6
CCl4
- tetrachloromethane

Question 18

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

Answer 18

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 19

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

Answer 19

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