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CHM 504
Infra-red Spectroscopy
Electromagnetic Spectrum
Objectives of IR spectroscopy
• To identify the functional groups present in
molecules
• To identify compounds by comparison with
spectra of known compounds in a database.
Molecular vibrations
Natural frequency of vibration depends on
•Mass of each atom; light atoms (e.g., H) vibrate faster.
•Strength of bond; strong bonds vibrate faster.
Modes of vibration
• Molecules with more than two atoms can
vibrate in several different ways. Each way
is called a MODE.
• Every mode of vibration has a natural
frequency.
• Each mode of vibration involves distortions
of
– Bond length : STRETCHING
– Bond angle : BENDING
Vibrations of H2O
Symmetric stretching
Asymmetric stretching
Bending
Interaction of matter with infra-red energy
• Molecules can absorb energy in the form of infra-red
radiation.
• The radiation being absorbed must have the same
frequency as one of the modes of vibration of the
molecule.
• When a molecule absorbs one photon of IR radiation
of appropriate frequency, the corresponding mode of
vibration increases its amplitude.
• The frequency of vibration does not change.
Vibrations and infra-red radiation
Infra-red radiation
• Amplitude of vibration increases
• Frequency of vibration unchanged
Energy of IR radiation
• IR radiation is a form of electromagnetic radiation.
• Every photon of electromagnetic radiation has a
quantum of energy, given by the equation
E = hn
where h is Planck’s constant, 6.626  10-34 J s.
• The frequency n is related to the wavelength l by
the equation
ln = c
where c is the speed of light.
Measurement of IR radiation
• Can be measured as either frequency (cycles per
second, Hz, s-1) or wavelength (micrometers,
mm).
• For historic reasons, usually measured as
wavenumber (cycles per centimeter, cm-1).
• Wavenumber is a kind of frequency; the greater
the wavenumber, the higher the energy.
• Symbol for wavenumber is n.
Wavenumber and wavelength
4
10
1
n (cm ) 
l (mm)
Absorption of IR radiation due to vibrations occur in
the range
10 000 - 100 cm-1 (1 - 100 mm)
Range of real interest in IR spectroscopy:
4000 - 400 cm-1 (2.5 - 25 mm)
Infra-red Spectrum
• Graph
• Horizontal axis:
– Wavelength (2.5 - 25 mm) OR
– Wavenumber (4000 - 400 cm-1) – usually.
• Vertical axis:
– Absorbance (A)
OR
– Transmittance (T) – usually (0 - 100%).
IR spectrum of 3-hydroxyacetophenone
Some definitions
I
Transmittance: T 
 100 %
I0
Absorbance:
I0
A  log
I
100
A  log
T
Spectral bands or peaks
• A complex molecule has many modes of
vibration.
• Each mode has a characteristic frequency.
• The molecule can absorb IR radiation of each
characteristic frequency.
• Each such absorption appears as a band or
peak in the IR spectrum.
Identification of molecules
• Each molecule has a unique set of modes of
vibration.
• Therefore each molecule has a unique spectrum.
• Unknown molecule can be identified by comparing
spectrum with spectra of known molecules. Exact
one-to-one matching of peaks sufficient to identify
molecule.
• Computer required to search database of known
spectra.
Qualitative information from IR spectra
• Many modes of vibration principally involve
specific bonds or functional groups.
• Peaks corresponding to those vibrations reveal
the presence of those functional groups.
• Each peak has 3 characteristics that can provide
information:
– Wavenumber
– Intensity (strong, medium, or weak)
– Shape (sharp, normal, or broad)
• Not every peak provides useful information.
Shapes of peaks
sharp
normal
broad
Analysis of IR spectrum
• Spectrum can be divided into 4 regions:
–
–
–
–
Region 1: 4000 - 2550 cm-1
Region 2: 2500 - 2000 cm-1
Region 3: 1900 - 1400 cm-1
Region 4: 1400 - 400 cm-1
• Different types of vibrations, corresponding to
different functional groups, are found in different
regions.
4000 - 2550 cm-1
• C–H, N–H, O–H, and (rarely) S–H stretching
vibrations.
• Can be distinguished based on wavenumber,
intensity, and shape.
• Very important in the identification of
–
–
–
–
Alcohols / phenols
Carboxylic acids
Primary / secondary amines
Amides
– Terminal alkynes (R–CC–H)
2500 - 2000 cm-1
• Triple bond stretching (R–CC & R–CN)
• Stretching in cumulative pairs of double bonds
(X=C=Y, where X and Y could be C, N, or O).
• In most spectra, this area is blank.
1900 - 1400 cm-1
• C=C, C=O, and C=N stretching peaks.
• Stretching of bonds that are intermediate
between single and double (1300 - 1600 cm-1).
For example,
• Often the most important part of the spectrum
Functional groups visible in 1900 1400 cm-1 region
• Aldehydes and ketones
• Other C=O containing functional groups,
including
–
–
–
–
Carboxylic acids
Acid chlorides and anhydrides
Esters
Amides
• C=C double bonds
• Aromatic rings
1400 - 400 cm-1
• Stretching of single bonds to atoms other than H,
e.g., C–C, C–O, C–N, C–Cl, etc.
• C–H bending peaks.
• Usually lots of peaks. Most cannot be identified or
interpreted.
• Region of spectrum that tends to be unique for a
given compound.
• Sometimes referred to as the fingerprint region.
Identification of functional groups
• Based on presence of key peaks in spectrum.
• Wavenumber, shape, and intensity of peak
should be considered.
• Some functional groups cannot be easily
identified using IR, e.g., those containing only
single bonds other than O-H, N-H.
– Alkyl halides
– Ethers
– Tertiary amines
Alkanes (alkyl groups)
• C–H stretching: 2850 - 2960 cm-1
• C–H bending: 1150 - 1390, 1450 - 1465 cm-1
• Many overlapping bands
• Presence of these peaks is not informative,
since most compounds contain alkyl groups.
Alkenes
Three types of diagnostic peaks.
1. C=C stretching (1640 – 1670 cm-1, w to m)
2. C–H stretching (3000 – 3100 cm-1)
3. C–H bending
IR spectrum of 1-hexene
Alkynes
1. CC stretching.
R–CC–H
2140 - 2100 cm-1 (medium)
R–CC–R’
2260 - 2190 cm-1 (weak or absent)
2. C–H stretching. 3330 - 3270 cm-1 (strong, sharp)
3. C–H bending.
A terminal alkyne (R–CCH) is easily recognised; an
internal alkyne is very difficult to spot using IR.
IR spectrum of 1-hexyne
Aromatic hydrocarbons
1. C=C stretching: two sets of peaks.
(i) 1600 - 1585 cm-1 (weak - medium)
(ii) 1500 - 1400 cm-1 (weak - medium)
• Each set typically contains two peaks.
• The second peak in each set may be weak, absent,
or appear as a “shoulder.”
2. C–H stretching.
3100 - 3000 cm-1.
(indistinguishable from alkene C-H stretch)
3. C–H bending
IR spectrum of toluene
Alcohols and phenols
1. O–H stretching.
broad)
3550 - 3200 cm-1 (strong,
 Broad because of H – bonding.
 In dilute solutions, sharp peak at ~3600 cm-1.
2. C–O stretching.
1260 - 1000 cm-1 (strong)
(Cannot distinguish between alcohols and phenols based on IR)
IR spectrum of (CH3)2CHCH2OH
The carbonyl group
Identifying a carbonyl-containing
functional group
• C=O stretching peak
– Very strong peak - often the strongest in the spectrum
• Consider other characteristic peaks
– O–H stretching of carboxylic acid
– C–H stretching of aldehyde
– N–H stretching and bending of amide
– C–O stretching of ester
• Absent such peaks, probably a ketone
Aldehydes
• C=O stretching. 1680 – 1740 cm-1
• C–H stretching.
2830 - 2690 cm-1
– 1-2 peaks, relatively weak
– No other peaks appear in this range
Ketones
• C=O stretching : 1670 – 1750 cm-1
• Can be distinguished from aldehydes by
absence of C–H peaks at 2830 - 2690 cm-1.
IR spectrum of CH2CH2CH2CHO
IR spectrum of CH3C(O)CH2CH2CH3
Carboxylic acids
• C=O stretching. 1680 – 1720 cm-1
• O–H stretching: very broad, distinctive peak, not
very strong, stretching from ~3300 to ~2500 cm-1
(centered at ~3050 cm-1); C–H stretching peaks
are usually superimposed.
IR spectrum of CH3(CH2)4CO2H
Esters
• C=O stretching. 1715 – 1770 cm-1.
• C–O stretching: 1250 - 1140 cm-1
– A very strong peak.
– Absent in ketones; can be used to distinguish
between esters and ketones.
– However: its presence does not guarantee an ester!
Could be a ketone with a C–O single bond elsewhere.
IR spectrum of CH3C(O)OCH2CH3
Amides
• C=O stretching: 1680 – 1630 cm-1.
• N–H stretching: 3400 – 3180 cm-1.
– Primary amides:
2 peaks
– Secondary amides: 1 peak
– Tertiary amides:
missing
IR spectrum of CH3CONH2
Primary and secondary amines
N–H stretching:
3400 - 3250 cm-1
– Primary (RNH2) : 2 peaks
– Secondary (R2NH) : 1 peak
– Relatively weak and sharp. Much weaker than amide
N–H stretching.
– Easy to distinguish from strong, broad OH stretching.
– May be obscured if OH, NH2 in same molecule.
IR spectrum of CH3(CH2)3NH2