Infrared Spectrometry
Infrared
Electromagnetic spectrum
UV- NMR
IR
VIS
(Gamma X Ultraviole Visible Radio
rays) rays t waves
Violet Blue Green Yellow Orange Red
380 760
nm nm
E = h = hc/ = hc = hc
where h is Planck’s constant, = = wavenumber (cm-1),
= frequency (Hz), c = speed of light (cm/sec) and =
wavelength (cm)
NMR: Nuclear Magnetic Resonance
Infrared
Infrared
IR spectrometer
Infrared
transmittance, % IR Spectrum
bends
C- H
1- butanol
CH3 CH2 CH2 CH2 OH
stretch
C-O
stretc h
O- H
stretch
C- H
1
wavenumbers, cm
Infrared
transmittance %
Fingerprint Region
bends
C-H
1-butanol
CH3CH2CH2CH2OH
stretch
C-O
stretch
O-H
stretch
C-H
fingerprint region
wavenumbers, cm
1
Infrared
Infrared Spectrometry
• The IR area : 780 nm – 1000 µm.
• The area is split in 3 regions: 10 / m
4
– Near-IR (NIR): 780 nm – 2.5 µm
– Mid-IR (MIR): 2.5 – 25 µm (mostly used, region
for vibrational energy changes)
– Far-IR (FIR): 25 – 1000 µm
• IR can be used for quantitative (Beer-
Lambert Law) and qualitative analysis.
However, IR is mostly used for qualitative
work.
Infrared
Infrared Spectrometry – Sample
Preparation
• Samples: gas, solid or liquid
• IR cells: sodium chloride or potassium
bromide plates (salt plates).
Infrared
Requirements for IR
• A match between a molecular vibration
frequency and the frequency of radiation.
• A change in dipole moment of the bond
during the vibration
Both criteria above have to apply, before a
bond vibration is observable in IR
Infrared
Polar Bonds Show the most Intense
Absorptions
– IR absorption only occurs when the vibration
causes a change in the dipole moment of the
molecule
– The larger the change in dipole moment, the
stronger the absorption
– This selection rule has important implications:
• The most prominent peaks are usually* those due to
polar bonds (C=O, O-H, C-O, etc.)
* C-H stretches and bends are often intense because there are
many in organic molecules
• Bonds with zero dipole do not absorb (e.g.
symmetrical C=C, CC), even if the frequency of the
radiation exactly matches that of the bond vibration.
Infrared
Symmetrical Bonds
– Thus, symmetrical bonds that have identical, or
nearly identical groups on each end will not
absorb IR. Alkenes and alkynes are mostly likely
to be affected by this restraint.
C
H3
C
H
C
H
3
C
H
C
H
3
3
2
C
C
C
C
H
C
H
C
H
C
H
3
3
C
C
C
H
C
H
C
C
H
C
H
C
H
3
3
Symmetric Pseudosymmetric
Infrared
What does an IR Spectrum tell us?
• The IR Spectrum of an organic compound
provides powerful information:
– For an unknown substance: peak positions provide
information about the structure (e.g. what
functional groups are present).
– For a known substance: the spectrum can be used
as a fingerprint, and compared with a reference
spectrum.
R DB c h / 2 n / 2 1
c = number of Group 14 atoms (C, Si
etc)
h = number of H + halogen atoms
Infrared
n = number of Group 15 atoms (N, P
Peak positions depend on bond strength
and atomic mass
– The wave number (, cm 1) of a bond depends on
the bond strength and the masses of the
bonded atoms.
– The theoretical relationship follows the
behaviour of two masses joined by a spring
(Hooke’s Law):
1 k
2c ( x y /( x y )
where = frequency, mx and my = the masses of atoms x
Infrared and y respectively and k = force constant of the bond
Peak position - Example
1 k
2c ( x y /( x y )
c = 3 x 10 10 cm/s; k = 5x10 5 dyn/cm
C = 12 / 6x10 23 = 19.8 x 10 -24 g 20 x 10 -24 g
H = 1 / 6x10 23 = 1.64 x 10 -24 g
(C- H) = 3050 cm-1
Infrared
Peak positions depend on bond strength
and atomic mass
Infrared
Peak positions depend on bond strength
and atomic mass
Infrared
Alkene
Infrared
Alkyne
Infrared
Toluene
Infrared
Alcohols and Ethers
Infrared
Infrared
Alcohol
Infrared
Amines
Infrared
Infrared
Carbonyl Compounds
• Carbonyl groups are present in aldehydes,
ketones, carboxylic acids, amides, acid
chlorides and anhydrides.
• The C=O group absorbs strongly in the
range 1850 – 1650 cm-1, because of its
large change in dipole moment.
• C=O stretching frequencies are sensitive
to attached atoms, resulting in different
absorption wavenumbers for the C=O group
for each of the compounds mentioned in
the first point above.
Infrared
Carbonyl Compounds
• The range of values for the C=O bands for
the different carbonyl compounds is
related to:
– Inductive effects: electron withdrawing and
donating effects
– Resonance effects
– Hydrogen bonding
• Inductive and resonance effects operate in
opposite ways to influence the C=O
stretching frequencies.
Infrared
C=C and C=O stretchings
Infrared
C=O Stretching Frequencies
Infrared
Aldehydes and Ketones
80
60
aromatic aldehyde 1705 cm-1
aromatic ketone 1690 cm-1
-1
-1
% transmission
aldehyde 1730 cm
ketone 1715 cm
40
20
0
aldehyde and ketone C=O
bond stretching frequencies
4000 3500 3000 2500 2000 1500 1000 500
-1
Infrared wavenumber (cm )
Carboxylic Acid and their
Derivatives
O H O
CH3 C C CH3
O H O
– This spectrum is a result of the molecule in its
usual H-bonded state.
– In the absence of H-bonding (e.g. very dilute
solution), the O-H is sharp around 3500 cm-1,
and the C=O at about 1760 cm-1.
Infrared
Ketone
Infrared
The IR of propionic anhydride
Infrared
Carboxylic acid
Infrared
Halogen Containing Compounds
• C-X absorption takes place at low
frequencies, where often a number of
other band appear (fingerprint region)
• Sodium chloride plates, which are often
used in IR, are only transparent above 650
cm-1
Infrared
The IR of CCl4
Infrared
The IR of CHCl3
Infrared
