A-level Spectroscopy
An image of a human brain from a live patient recorded using magnetic
resonance imaging - a 3D form of n.m.r. spectroscopy
Introduction
Spectroscopy is a collective name for the various techniques that use the
interaction between molecules and electromagnetic radiation to elucidate the
structure of molecules. Spectroscopic methods are fundamental to the study of
Chemistry, Molecular Biology, Medicine and Astrophysics.
This booklet covers the following techniques:-
A) Infrared Spectroscopy - from ‘What’s in a Medicine?’


Describe how infrared spectroscopy (i.r.) can be used for the elucidation
of molecular structure;
Interpret infrared spectra for salicylic acid and simple compounds
containing a limited range of functional groups (hydroxyl, carbonyl,
carboxylic acid and ester) given relevant information.
B) Mass Spectrometry


- from ‘What’s in a Medicine?’
Describe how mass spectrometry (m.s.) can be used for the elucidation
of molecular structure;
Interpret mass spectra (molecular ion and significance of the
fragmentation pattern) for salicylic acid and simple compounds containing
a limited range of functional groups (hydroxyl, carbonyl, carboxylic acid
and ester) given relevant information.
C) Nuclear Magnetic Spectroscopy


- from ‘Engineering Proteins’
Describe how nuclear magnetic resonance spectroscopy (n.m.r.) can be
used for the elucidation of molecular structure;
Interpret nuclear magnetic resonance spectra for simple compounds
given relevant information (reference to splitting on the resonances is
not required)
This work builds on AS topics of:

Interaction of radiation with matter (‘Elements of Life’ and ‘Atmosphere’);
Mass spectrometry (‘Elements of Life’).
A) Infrared (i.r.) spectroscopy


Used to identify bonds / functional groups
Can only identify the exact molecule by comparison with library
spectra
Experiment 1 RSC Video




Infrared radiation is passed simultaneously through the sample and a
reference cell.
The reference ensures that peaks due to water or carbon dioxide in the
air can be cancelled out.
The frequencies of i.r. radiation absorbed are determined by passing
through a rotating prism to focus one frequency at a time onto the
detector.
The spectrum shows the ________________ (cm-1) on the x axis
(which is 1/) and the ____________________ on the y-axis.
Calculations
1) c = 
2) Wavenumber = 1/(cm)
e.g. What wavenumber would appear on an i.r. spectrum if the frequency of
radiation absorbed by a molecule was 2.5 x 1013 Hz?
Theory



IR radiation corresponds to the energy required to make chemical bonds
vibrate more / move to a higher vibrational energy level.
Therefore, energy of certain wavelengths is absorbed by molecules.
The actual energy depends on the mass of the atoms and the strength of
the bond, so different bonds will absorb at different frequencies.
c.f.
spring
oscillations
m
Stronger bonds need more energy to make them vibrate, so absorb a
higher frequency of i.r. radiation (higher wavenumber)
e.g. hydrogen halides

 Molecules with more than 2 atoms can vibrate in different ways
e.g. sulphur dioxide

So these spectra will contain more absorptions
Most organic molecules contain a number of types of bond, so
characteristic absorptions will be seen for each bond.
e.g. ethanol

The following types of bond need to be recognised:Bond
O–H
Functional group
Alcohols
Absorbance (cm-1)
3200 – 3600 / strong and broad*
O–H
Carboxylic acids
C=O
Aldehydes / ketones /
carboxylic acids/ esters
2500 – 3200 / medium and very
broad*
1680 – 1750 / strong and sharp
C-O
Alcohols / esters / ethers
1050 – 1300 / medium
C-H
Alkanes / alkenes etc
2850 – 3100 / medium
*Broad due to Hydrogen Bonding between O-H groups
i.r. bands.ppt
Examples of infrared spectra
1) ethanol (CH3CH2OH)
displayed
formula
i.r. spectrum
Bond / (Functional group)
Absorption / cm-1
2) ethanoic acid (CH3COOH)
displayed
formula
i.r. spectrum
Bond / (Functional group)
Absorption / cm-1
3) Ethyl Ethanoate (CH3COOCH2CH3)
H
H
O
C
C
O
H
H
H
C
C
H
H
H
i.r. spectrum
Bond / (Functional group)
Absorption / cm-1
1750
1250
3000
4) a) i.r. spectrum of an alcohol with molecular formula C3H8O.
NB: This Alcohol is oxidised to compound 4)b) when heated under distillation
with acidified potassium dichromate and 4)c) when heated to reflux with
acidified potassium dichromate.
Clue?
Bond / (Functional group)
Displayed Formula of 4a
Absorption / cm-1
4) b) i.r. spectrum of the compound with molecular formula C3H6O
obtained by distilling compound 4)a) with acidified potassium dichromate
Bond / (Functional group)
Displayed Formula of compound 4b
Absorption / cm-1
4)c) i.r. spectrum of the compound with molecular formula C3H6O2 formed
when compound 4a is heated to reflux with acidified potassium dichromate
Bond / (Functional group)
Displayed Formula of Compound 4c
Absorption / cm-1
5)a) i.r. spectrum of an isomer of 4a which forms the same product 5)b)
whether it is heated to distil or reflux with acidified potassium dichromate
Bond / (Functional group)
Displayed Formula of Compound 5a
Absorption / cm-1
5)b) i.r. spectrum of the product of the reaction of 5a with acidified
potassium dichromate when heated to reflux or distillation.
Bond / (Functional group)
Displayed Formula of Compound 5b
Absorption / cm-1
6) Salicylic Acid (2-hydroxybenzoic acid - (HOC6H4COOH))
displayed
formula
i.r. spectrum
Bond / (Functional group)
Absorption / cm-1
7) Aspirin (CH3COOC6H4COOH)
i.r. spectrum
Bond / (Functional group)
Absorption / cm-1
2900 v. broad
1750
1700
1200
B) Mass Spectrometry



Use M+ (molecular ion) to measure Mr
Use M+2 isotope peaks to identify Cl or Br
Use fragmentation pattern to confirm structure of molecule
Experiment 2 – RSC video
AS-level





Vaporisation of atoms or molecules;
Ionisation of atoms or molecules;
Acceleration of ions;
Deflection of ions;
Detection of ions.
A2-level
The atoms or molecules are ionised by bombarding with high energy
electrons:-

e
e.g. CH3COCH3 +
[CH3COCH3]
M+
+
2e
Usually, the resulting molecular ion has such high energy that it splits up
into a smaller ion and an uncharged molecule (fragmentation)

+
e.g. [CH3COCH3]
M+m/e = 58
or
+
[CH3COCH3]
58
[CH3CO]
+
+
CH3
43
+
CH3CO
+
[CH3]
15
+
NB The first fragmentation route is more likely because fragments containing
+
the [R-C=O] group (acylium cations) are particularly stable.

The following peaks are often seen in the fragmentation patterns of mass
spectra – the highlighted peaks usually provide very useful clues in
determining the structure of a molecule
fragment
m/e
CH3
15
CH3CH2 or CHO
29
CH2NH2
30
CH2OH
31
CH3CO or C3H7
43
CONH2
44
COOH
45
C6H5
77
C6H5CH2
91
C6H5CO
105
Examples of fragmentation and the interpretation of mass spectra
1) Propanone (CH3COCH3)
displayed
formula
mass spectrum
m/z
58
43
15
Formula
m/z lost
Group lost
2) Propanal (CH3CH2CHO)
displayed
formula
mass spectrum
m/z
58
57
29
Formula
m/z lost
Group lost
3) Methyl Benzoate (C6H5COOCH3)
O
C
H
O
C
H
H
mass spectrum
m/z
136
105
77
Formula
m/z lost
Group lost
4) Ethyl Ethanoate (CH3COOCH2CH3)
H
H
O
C
C
O
H
H
H
C
C
H
H
H
mass spectrum
m/z
88
73
43
29
15
Formula
m/z lost
Group lost
5) Salicylic Acid (2-hydroxybenzoic acid - (HOC6H4COOH))
displayed
formula
mass spectrum
m/z
Formula
m/z lost
Group lost
138
*
120
92
* NB 3- or 4- hydroxybenzoic acid isomers cannot eliminate water –why not?
6) Aspirin (CH3COOC6H4COOH)
mass spectrum
m/z
180
138
120
43
Formula
m/z lost
Group lost
7) ethanamide (CH3CONH2)
displayed
formula
mass spectrum
m/z
59
44
43
Formula
m/z lost
Group lost
8) paracetamol (4-hydroxyphenylethanamide) (HOC6H4NHCOCH3)
displayed
formula
mass spectrum
m/z
151
109
108
43
Formula
m/z lost
Group lost
C) Nuclear Magnetic Resonance (n.m.r.) spectroscopy



The number of peaks – number of proton types
The chemical shift (δ) – what are the proton types
The integration – how many protons of each type
Experiment 3 – RSC video



Sample is placed in a very strong magnetic field
A pulse of radiofrequency radiation is applied
Radiofrequency signal emitted from sample is detected
Theory

Nuclei have a property called nuclear spin which generates a tiny
magnetic field. The nuclei therefore behave like tiny bar magnets.

When such nuclei are placed in a large magnetic field they will become
aligned with or against the direction of the external field.

The nuclei lined up with the field are slightly more stable (lower energy)
than those that oppose the external field.

The energy gap between these two states corresponds to radiofrequency
radiation.

If the sample is irradiated with a pulse of radio waves, the nuclei in the
lower energy state may be promoted to the higher energy state (the tiny
bar magnets ‘flip’ from being aligned with to against the external field).

The excited nuclei will then return to the ground state releasing fixed
quanta of energy which will be detected.

The energy gap depends on the chemical environment of the nuclei and can be
used to deduce the exact structure of the molecule.

Ethanal has two proton types, so produces 2 signals in the n.m.r. spectrum .
The important features of the spectrum are: The number of peaks – number of proton types
 The integration – how many protons of each type
 The chemical shift (δ) – what are the proton types
The following table can be used to link the chemical shift to the proton type (chemical
environment of H atom):type of proton
chemical shift δ / ppm
RCH3 / RCH2R (alkane)
0.8 - 1.4
RCOCH3 (carbonyls, esters)
1.8 - 2.2
RCH2Hal
3.2 - 4.6
ROCH3 (esters, ethers)
3.2 - 3.5
ROH (alcohol)
1.0 - 6.0
RC6H4H (arenes)
6.0 - 9.0
RC6H4CH3 (methylarene)
2.2 - 2.4
RCONHR (amides)
7.0 - 10.0
RCHO (aldehydes)
9.7 - 9.8
RCOOH (carboxylic acids)
9.0 - 12.0
RC6H4OH (phenols)
variable
RNHR (amines)
variable
[R represents an alkyl group]
Examples of the interpretation of n.m.r spectra
1) propanone (CH3COCH3)
displayed
formula
nmr spectrum
Proton
Ha
integration
inference
δ / ppm
inference
2) propanal (CH3CH2CHO)
displayed
formula
nmr spectrum
Proton
integration
inference
δ / ppm
Ha
9.7
Hb
2.4
Hc
1.1
inference
3) ethyl ethanoate (CH3COOCH2CH3)
Ha
Ha
O
C
C
Ha
O
Hb
Hc
C
C
Hb
Hc
Hc
nmr spectrum
Proton
integration
inference
δ / ppm
Ha
2.1
Hb
4.1
Hc
1.2
inference
4) Salicylic Acid (2-hydroxybenzoic acid - (HOC6H4COOH))
displayed
formula
nmr spectrum
Proton
integration
inference
δ / ppm
Ha
8.0
Hb
7.6
Hc
7.0
Where are the O-H groups?
inference
5) Aspirin (CH3COOC6H4COOH)
H
H
H
H
nmr spectrum
Proton
integration
inference
δ / ppm
Ha
1
11.3
Hb
4x1
7-8
Hc
3
2.1
inference
6) Mystery compound – “Why are there no aspirin in the jungle?”
n.m.r. spectrum
Proton
integration
Ha
1
9.7
Hb
1
9.1
Hc
2
7.4
Hd
2
6.7
He
3
2.0
Structure
inference
δ / ppm
inference
Combined Spectral Techniques
1) Predict the ir, nmr and mass spectra of propanal
a) IR spectroscopy
Bond / (Functional group)
Absorption / cm-1
b) Nmr spectroscopy
Proton
integration
inference
δ / ppm
inference
c) Mass Spectrometry
m/z
Formula
m/z lost
Group lost
Deduce the structure of the molecule from these spectra
a) ir spectrum
b) nmr spectrum
c) mass spectrum
45
60