© a2 032 17-jul-12. protons and neutrons can be regarded as spinning about their axis. in many...

• 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.


• 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 Applied magnetic

field

Magnetic field in same direction as

applied field

Magnetic field opposed to applied field

Energy gap corresponds to frequency of radiowaves


butanone


• 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.


CH3 CH CH2 CH3

Br

CH3 CH CH2 CH2 CH3

Br

CH3 CH2 CH3 2 sets of equivalent H’s: ratio 6:2 (3:1)

CH3 C

CH3

CH2

OH

CH3

4 sets of equivalent H’s: ratio 3:1:2:3

5 sets of equivalent H’s: ratio 3:1:2:2:3

4 sets of equivalent H’s: ratio 6:1:2:3


For each of the following compounds, predict the number of signals and the relative intensity of the signals.

a) methylpropene

b) propene

c) 2-chloropropane

d) propanone

e) methylamine

f) ethyl propanoate

g) 1,2-dibromopropane

h) dimethylethyl propanoate

i) but-2-ene


2 signals: ratio 6:2 (3:1)

C CH3CH3

O

CH CH3CH3

Cl

CH2 C CH3

CH3

CH2 CH CH3

3 signals: ratio 2:1:3

2 signals: ratio 6:1

1 signal

CH3 CH2 C

O

O CH2 CH3

CH3 NH2

2 signals: ratio 3:2

4 signals: ratio 3:2:2:3


3 signals: ratio 2:1:3

2 signals: ratio 6:2 (3:1)

CH CH3CH2

BrBr

CH3 CH2 C

O

O C CH3

CH3

CH33 signals: ratio 3:2:9

CH CHCH3 CH3


• Samples are dissolved in solvents free of 1H atoms, e.g. CCl4, CDCl3.

CH3

Si CH3

CH3

CH3

• 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


There are four signals here – each has the same area and so represents the same number of H atoms


8 7 6 5 4 3 2 1 0

chemical shift


• The spectra are recorded on a scale known as the chemical shift (), which is how much the field is shifted from the field for TMS compared to the field for TMS.

= Field for TMS - Field measured x 106

Field for TMS

• The is a measure in parts per million (ppm) of how far the magnetic field required for absorption is shifted away from that for TMS.

0 1 2 3 4 5 6 7 8 9 10

chemical shift (ppm)

• The depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift.


Data from AQA datasheet

For each of the following compounds, predict the number of signals and the relative intensity of the signals.

a) but-1-ene

b) 1,2-dichloropropane

c) propanal

d) 2-methylpropan-2-ol

e) pentane

f) ethyl methylpropanoate


a) but-1-ene

b) 1,2-dichloropropane

c) propanal

d) 2-methylpropan-2-ol

e) Pentane

f) ethyl methylpropanoate

CH3 CH2 CH CH2

CH2 CH CH3

Cl Cl

CH3 CH2 C

O

H

CH3 C CH3

OH

CH3

CH3 CH2 CH2 CH3CH2

CH3 CH C

O

O

CH3

CH2 CH3


•Number of signals•Relative “size”•Chemical shift

CH3 CH2 C

O

H

CH3 CH C

O

O

CH3

CH2 CH3

a) propanalb) ethyl methylpropanoate


Coupling / Splitting / Multiplicity

Number of H’s next door +1© www.chemsheets.co.uk A2 032 17-Jul-12

CH3 CH2 C CH3

O

0 H next doorsinglet (s)

1 H next doordoublet (d)

2 H next doortriplet (t)

3 H next doorquartet (q)

more H next doormultiplet (m)

n+1


signal singlet doublet triplet quartet

appearance

number of lines

1 2 3 4

number of H’s next door

0 1 2 3

relative size 1:1 1:2:1 1:3:3:1


CH3 CH C

O

O

CH3

CH2 CH3

C CH CH3

Br

CH3

Br

Br

CH3 CH2 CH3


CH3 CH2 CH2 CH3


CH3 CH2 CH2 CH3CH2

CH3 C

O

CH2CH2 C

O

CH3

CH3 C

O

CH2CH2 C

O

CH2 CH3


CH3 CH2 OH


Number of H’s next door +1

But you don’t couple to

• H’s that are equivalent

• H’s on O’s


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


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

b) propene

c) 2-chloropropane

d) propanone

e) methylamine

f) ethyl propanoate

g) 1,2-dibromopropane

h) dimethylethyl propanoate

i) but-2-ene


2 signals: ratio 6 : 2 (3 :1) s s

C CH3CH3

O

CH CH3CH3

Cl

CH2 C CH3

CH3

CH2 CH CH3

3 signals: ratio 2 : 1 : 3 d m d

2 signals: ratio 6 : 1 d m

1 signal

CH3 CH2 C

O

O CH2 CH3

CH3 NH2

2 signals: ratio 3 : 2 t q

4 signals: ratio 3 : 2 : 2 : 3 t q q t


3 signals: ratio 2 : 1 : 3 d m d

2 signals: ratio 6 : 2 (3 :1) d q

CH CH3CH2

BrBr

CH3 CH2 C

O

O C CH3

CH3

CH33 signals: ratio 3 : 2 : 9 t q s

CH CHCH3 CH3


•Number of signals•Relative “size”•Chemical shift•Multiplicity

a) 1,2-dichloropropaneb) but-1-ene

CH3 CH2 CH CH2

CH2 CH CH3

Cl Cl


• 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, CN, 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.


© a2 032 17-jul-12. protons and neutrons can be regarded as spinning about their axis. in many...

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