1. The basis of MRI scanning
17/04/2017
NMR SPECTROSCOPY
The basis of MRI scanning

2. NUCLEAR SPIN
Protons and neutrons can be regarded as spinning about their axis.
In many atoms these spins are paired against each other and so the nucleus has no overall spin (e.g. 12C).
In some atoms (e.g. 1H and 13C – an odd number of subatomic particles) the nucleus has an overall spin.
A nucleus that spins generates a magnetic field i.e. behaves like a small magnet.
The direction of the magnetic field depends which way the nucleus spins.

3. Applied magnetic field
NUCLEAR SPIN
Usually the two possible spin states of the nucleus have the same amount of energy.
However, in a magnetic field, the two spin states have different energies.
Energy
Magnetic field opposed to applied field
Applied magnetic field
Energy gap corresponds to frequency of radiowaves
Magnetic field in same direction as applied field

4. Different “types of hydrogens”
The frequency absorbed is determined by the environment of the atoms giving the signal
Atoms in different environments are sometimes said to be of different types
To be of the same type hydrogen atoms must be indistinguishable
Example ethane – all hydrogens identical, so one type
Propane (2 types)
Propan-1-ol (4 types)
Notes

5. Example
Draw a displayed formula of butanone and decide how many types of hydrogen there are
This determines the number of peaks seen in the spectrum

6. butanone 3 3 2

7. EQUIVALENT H ATOMS
In a spectrum, there is one signal for each set of equivalent H atoms. i.e. H atoms in an identical situation/environment
The intensity of each signal being proportional to the number of equivalent H atoms it represents, so the hydrogen atoms are counted (area under peak)
In a real spectrum the relative peak areas is given rather than actual numbers, so
2:1 = 4:2 = 6:3 etc – beware of this!

8. 2 sets of equivalent H’s: ratio 6:2 (3:1)

9. For each of the following compounds, predict the number of signals and the relative intensity/area of the signals.
a) methylpropene
propene
2-chloropropane
propanone
methylamine
ethyl propanoate
1,2-dibromopropane
dimethylethyl propanoate
but-2-ene

10. 2 signals: ratio 6:2 (3:1)
1 signal
3 signals: ratio 2:1:3
2 signals: ratio 3:2
2 signals: ratio 6:1
4 signals: ratio 3:2:2:3

11. 3 signals: ratio 2:1:3
3 signals: ratio 3:2:9
2 signals: ratio 6:2 (3:1)

12. SOLVENTS & CALIBRATION
Samples are dissolved in solvents free of 1H atoms, e.g. CCl4, CDCl3.
A small amount of TMS (tetramethylsilane) is added to calibrate the spectrum.
It is used because:
its signal is away from all the others
it only gives one signal
it is non-toxic
it is inert
it has a low boiling point so is easy to remove

13. CHEMICAL SHIFT
The d is a measure in parts per million (ppm) of how far the magnetic field required for absorption is shifted away from that for TMS.
The d depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift.

14. Butanone again – look at the chemical shift this time. See data sheet

15. Note that the signal from each type of hydrogen is seen in this case as a group of lines (still called a “peak”)

16. SPIN-SPIN COUPLING
Each signal in the spectrum can be split into further lines due to inequivalent H’s on neighbouring C atoms.
NOTE This splitting effect is not observed if adjacent H atoms are in an identical situation/environment
Number of lines that the peak is split into =
number of inequivalent H’s on neighbouring C atoms + 1
The n+1 rule

17. number of neighbouring inequivalent H atoms
SPIN-SPIN COUPLING
signal
singlet
doublet
triplet
quartet
appearance
number of lines 1 2 3 4
number of neighbouring inequivalent H atoms
relative size
1:1
1:2:1
1:3:3:1

18. butanone shift (d) assignment relative intensity coupling coupled to
1.0
CH3CH2 3
triplet
CH2
2.0
CH3CO
singlet
2.4 2
quartet
CH3

19. butane shift (d) Assignment relative intensity coupling coupled to 1.3
CH2 2
quartet
CH3
0.8 3
triplet

20. SUMMARY Number of signals -radiowave region Position of signals
Relative intensities
Splitting
how many different sets of equivalent H atoms there are
information about chemical environment of H atom
gives ratio of numbers of H atoms for the peaks
how many H atoms on adjacent C atoms
(alcohol OH, [aldehyde H] no splitting)
IDENTICAL PROTONS DO NOT CAUSE SPLITTING

21. dimethylethyl propanoate but-2-ene
For each of the following compounds, predict the number of signals, the relative intensity of the signals, and the multiplicity (singlet, doublet etc) of each signal.
a) methylpropene
propene
2-chloropropane
propanone
methylamine
ethyl propanoate
1,2-dibromopropane
dimethylethyl propanoate
but-2-ene

22. 2 signals: ratio 6 : 2 (3 :1)
s s
1 signal
3 signals: ratio 2 : 1 : 3
d m d
2 signals: ratio 3 : 2
t q
2 signals: ratio 6 : 1
d m
4 signals: ratio 3 : 2 : 2 : 3
t q q t

23. 3 signals: ratio 2 : 1 : 3
t m d
3 signals: ratio 3 : 2 : 9
t q s
2 signals: ratio 6 : 2 (3 :1)
d q

24. Use of deuterium based compounds in NMR
Deuterium is 12H 1.e. 1 proton and 1 neutron in the nucleus
Deuterium does not give nmr signal (even number of sub-atomic particles)
CDCl3 is one of the standard nmr solvents (the solvent does not interfere with the spectrum)
D2O removes peaks due to O-H and N-H from the spectrum
As a result of rapid H/D exchange

25. Past paper example

26. Mark scheme
(i) carboxylic acid / COOH (protons) NOT just “OH protons” [1]
(ii) D replaces protons on OH groups / OH protons are labile 􀀹
peak for (CO)OH protons disappears 􀀹

27. Further example

28. Further example continued

29. Protons on a benzene ring
Need to identify aromatic protons from chemical shift patterns
No need to worry about their splitting patterns
Signals can appear as one or two complex peaks

30. Example (past paper)

31. Mass spec mol. Ion peak at 86
Pentan-3-one
Mass spec mol. Ion peak at 86