1. Chemsheets AS006 (Electron arrangement)
19/04/2017
NMR
SPECTROSCOPY
© A Jul-12

2. 1H NMR
SPECTROSCOPY
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3. 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) the nucleus has an overall spin.
A nucleus that spins generates a magnetic field.
The direction of the magnetic field depends which way the nucleus spins.
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4. 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
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5. butanone
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6. EQUIVALENT H's
In a spectrum, there is one signal for each set of equivalent H atoms.
The intensity of each signal being proportional to the number of equivalent H atoms it represents.
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7. 2 sets of equivalent H’s: ratio 6:2 (3:1)
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8. dimethylethyl propanoate but-2-ene
For each of the following compounds, predict the number of signals and the relative intensity of the signals.
a) methylpropene
propene
2-chloropropane
propanone
methylamine
ethyl propanoate
1,2-dibromopropane
dimethylethyl propanoate
but-2-ene
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9. 2 signals: ratio 6:2 (3:1) 1 signal 3 signals: ratio 2:1:3
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10. 3 signals: ratio 2:1:3 3 signals: ratio 3:2:9
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11. 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
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12. RELATIVE INTENSITY
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19. CHEMICAL SHIFT
The spectra are recorded on a scale known as the chemical shift (d), which is how much the field is shifted from the field for TMS compared to the field for TMS.
d = Field for TMS - Field measured x 106 Field for TMS
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.
© A Jul-12

20. Data from AQA datasheet

21. f) ethyl methylpropanoate
For each of the following compounds, predict the number of signals and the relative intensity of the signals.
a) but-1-ene
1,2-dichloropropane
propanal
d) 2-methylpropan-2-ol
e) pentane
f) ethyl methylpropanoate
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22. ethyl methylpropanoate
a) but-1-ene
1,2-dichloropropane
propanal
d) 2-methylpropan-2-ol
Pentane
ethyl methylpropanoate
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23. Data from AQA datasheet
Number of signals
Relative “size”
Chemical shift
a) propanal
b) ethyl methylpropanoate
Data from AQA datasheet

24. Number of H’s next door +1
SPIN-SPIN COUPLING
Coupling / Splitting / Multiplicity
Number of H’s next door +1
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25. n+1 0 H next door singlet (s) 1 H next door doublet (d) 2 H next door
triplet (t)
3 H next door
quartet (q)
more H next door
multiplet (m)
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26. SPIN-SPIN COUPLING signal singlet doublet triplet quartet appearance
number of lines 1 2 3 4
number of H’s next door
relative size
1:1
1:2:1
1:3:3:1
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32. Number of H’s next door +1
But you don’t couple to
H’s that are equivalent
H’s on O’s
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33. SUMMARY Number of signals 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 H atoms for peaks
how many H atoms on adjacent C atoms
© A Jul-12

34. 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 of each signal.
a) methylpropene
propene
2-chloropropane
propanone
methylamine
ethyl propanoate
1,2-dibromopropane
dimethylethyl propanoate
but-2-ene
© A Jul-12

35. 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
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36. 3 signals: ratio 2 : 1 : 3 d m d 3 signals: ratio 3 : 2 : 9 t q s
d q
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38. Number of signals Relative “size” Chemical shift Multiplicity
a) 1,2-dichloropropane
b) but-1-ene
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39. 13C NMR
SPECTROSCOPY
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40. KEY POINTS 13C NMR spectra are often simpler than 1H NMR spectra.
They give a lot of valuable information about the chemical environment of C atoms (e.g. the difference between C atoms in C=O, C-N, CN, C-C, C=C, etc.).
There is one signal for each set of equivalent C atoms.
There is no coupling (unlike 1H NMR).
The size of signal is not relative to the number of equivalent C atoms (unlike H atoms in 1H NMR).
As in 1H NMR, the chemical shift (d) is measured relative to TMS.
Although deuterated solvents are usually used, there will be a signal for any C atoms in the solvent.
© A Jul-12

41. Data from AQA datasheet

42. Data from AQA datasheet

43. Data from AQA datasheet