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Infrared Spectroscopy
Dr AKM Shafiqul Islam
School of Bioprocess Engineering
University Malaysia Perlis
Introduction
• Spectroscopy is an analytical technique
which helps determine structure
• It destroys little or no sample
• The amount of light absorbed by the
sample is measured as wavelength is
varied
• Infrared spectroscopy is very useful for
obtaining qualitative information about the
molecules. But molecule must possess
certain properties in order to undergo
absorption.
IR Spectroscopy
The presence and also the environment of
functional groups in organic molecule can be
identified by infrared (IR) spectroscopy.
Infrared spectroscopy is nondestructive.
Moreover, the small quantity of sample
needed, the speed with which spectrum can
be obtained, the relatively low cost of the
spectrometer, and I wide applicability of the
method
combine
to
make
infrared
spectroscopy one the most useful tools
available to the organic chemist
THE ELECTROMAGNETIC SPECTRUM
high
Frequency (n)
low
high
Energy
low
X-RAY
INFRARED MICROWAVE
ULTRAVIOLET
Vibrational
infrared
Visible
Ultraviolet
2.5 mm
200 nm
400 nm
BLUE
short
15 mm
RADIO
Nuclear
magnetic
resonance
1m
800 nm
RED
Wavelength (l)
FREQUENCY
long
5m
Types of Energy Transitions in Each Region
of the Electromagnetic Spectrum
REGION
ENERGY TRANSITIONS
X-ray
UV/Visible
Infrared
Microwave
Bond-breaking
Electronic
Vibrational
Rotational
Radio Frequency
Nuclear and
Electronic Spin
(NMR)
Principles IR Spectroscopy
Energy: E=h
where:  is the frequency in hertz
In IR, frequency is commonly expressed as wave numbers
( , in Reciprocal cm, or cm-1)
Where
1
107


l (in cm) l (in nm)
Principles IR Spectroscopy
• Absorption of radiation in this region by a typical organic molecule
results in the excitation of vibrational, rotational, and bending modes,
while the molecule itself remains in its electronic ground state.
• Molecular asymmetry is a requirement for excitation by infrared
radiation and fully symmetric molecules do not display absorbance
in this region unless asymmetric stretching or bending transitions are
possible.
Symmetric stretch
Assymmetric stretch
Symmetric bending
Principles IR Spectroscopy
For the purpose of routine organic structure
determination, the most important absorptions in
the infrared region are the simple stretching
vibrations. For simple systems, these can be
approximated by considering the atoms as point
masses, linked by a “spring” having a spring
constant k and following Hooke’s Law.
Principles IR Spectroscopy
• Using this simple approximation, the equation
shown in below can be utilized to approximate
the characteristic stretching frequency (in cm-1)
of two atoms of mass m1 and m2, linked by a
bond with a spring constant k:
Where m=m1m2/(m1+m2) , also called “reduced
mass”
Absorption of Infrared Radiation
• Only bonds which have significant dipole
moments will absorb infrared radiation.
• Dipole is the polar covalent bond in which a pair
of electron is shared unequally.
• For absorption occur, there must be a charge in
the dipole moment (polarity) of the molecule. A
diatomic molecule must have a permanent
dipole in order to absorb, but larger molecule do
not.
DIPOLE MOMENTS
Bonds which do not absorb infrared include:
• Symmetrically substituted alkenes and alkynes
R
R
R
R
R C C R
• Many types of C-C Bonds
• Symmetric diatomic molecules
H-H
Cl-Cl
Molecular Vibrations
• Light is absorbed when radiation frequency =
frequency of vibration in molecule
• Covalent bonds vibrate at only certain
allowable frequencies
– Associated with types of bonds and movement of
atoms
• Vibrations include stretching and bending
IR Instrumentation
Light source: Nichrome wire that glows when an electrical current is passed through;
Interferometer: no monochrometer
Detector: thermocouple detector, whose output voltage varies with changes
caused by varying levels of radiation striking the detector.
IR Instrumentation
Infrared Spectroscopy (IR)
 No two molecules of different structure will have exactly the
same natural frequency of vibration, each will have a unique
infrared absorption pattern or spectrum.
 Two Uses:
1. IR can be used to distinguish one compound from another.
2. Absorption of IR energy by organic compounds will occur
in a manner characteristic of the types of bonds and atoms
in the functional groups present in the compound; thus,
infrared spectrum gives structural information about a
molecule.
 The absorptions of each type of bond (N–H, C–H, OH,
C–X, C=O, C–O, C–C, C=C, C≡C, C≡N, etc.) are regularly found
only in certain small portions of the vibrational infrared region,
greatly enhancing analysis possibilities.
Infrared Spectroscopy (IR)
The Infrared Spectrum
A plot of absorption intensity (% Transmittance) on the
y-axis vs. frequency (wavenumbers) on the x-axis.
Infrared Spectroscopy (IR)
Principal Frequency Bands (from left to right in spectrum)
OH
NH
C≡N
C≡C
C=O
C=C
CH2
CH3
CO
3600 cm-1 (Acids - Very Broad, Alcohols - Broad)
3300 - 3500 cm-1 (2, 1, 0 peaks – 1o, 2o, 3o)
2250 cm-1 (Nitrile)
2150 cm-1 (Acetylene)
1685 - 1725 cm-1 (1715) (Carbonyl)
1650 cm-1 (Alkene); 4 absorptions 1450-1600 (aromatic)
1450 cm-1 (Methylene Group)
1375 cm-1 (Methyl Group)
900 - 1100 cm-1 (Alcohol, Acid, Ester, Ether, Anhydride)
-CH
=C-H
=C-H
≡C-H
(Saturated Alkane absorptions on Right side of 3000 cm-1)
(Unsaturated Alkene absorptions on Left side of 3000 cm-1)
(Aromatic absorptions) – Verify at 1667 – 2000 cm-1
(Unsaturated Alkyne absorptions on Left side of 3000 cm-1)
Infrared Spectroscopy (IR)
Suggested approach for analyzing IR Spectra
Step 1. – Check for the presence of the Carbonyl group (C=O) at 1715
cm-1. If molecule is conjugated, the strong (C=O) absorption
will be shifted to the right by ~30 cm-1,i.e., ~1685 cm-1
If the Carbonyl absorption is present, check for:
 Carboxylic Acids - Check for OH group (broad absorption
near 3300-2500 cm-1)
 Amides
- Check for NH group
(1 or 2 absorptions near 3500 cm-1)
 Esters
- Check for 2 C-O group (medium
absorptions near 1300-1000 cm-1)
 Anhydrides
- Check for 2 C=O absorptions near
1810 and 1760 cm-1
 Aldehydes
- Check for Aldehyde CH group (2 weak
absorptions near 2850 and 2750 cm-1)
 Ketones
- Ketones (The above groups have been
eliminated)
Infrared Spectroscopy (IR)
Step 2. - If the Carbonyl Group is Absent Check for Alcohols,
Amines, or Ethers.
 Alcohols & Phenols
- Check for OH group (Broad
absorption near 3600-3300 cm-1
Confirm present of CO near
1300-1000 cm-1
 Amines
- Check for NH stretch (Medium
absorptions) near 3500 cm-1
Primary Amine
- 2 Peaks
Secondary Amine
- 1 Peak
Tertiary Amine
- No peaks
N-H Scissoring at 1560 - 1640 cm-1
N-H Bend at 800 cm-1
 Ethers
- Check for C-O group near 13001000 cm-1 and absence of OH
Infrared Spectroscopy (IR)
Step 3. – Refine the Structure Possibilities by Looking for
Double Bonds, Triple Bonds and Nitro Groups
 Double Bonds - Unsaturated C=C (and C≡C) stretch show
absorptions on the left side of 3000 cm-1
Alkene C=C weak absorption near 1650 cm-1
Aromatic C=C (4 absorptions 1450-1650 cm-1)
(Verify Aromatic at 1667 – 2000 cm-1)
 Triple Bonds
- R-C ≡ N Nitrile - medium, sharp absorption
(stretch) near 2250 cm-1
R – C ≡ C – R Alkyne - weak, sharp absorption
(stretch near 2150 cm-1)
R – C ≡ C – H Terminal Acetylene
(stretch near 3300 cm-1)
 Nitro Groups - Two strong absorptions 1600 – 1500 cm-1
and 1390 - 1300 cm-1
Infrared Spectroscopy (IR)
Step 3 (Con’t)
Aromatic Ring Absorptions
 Aromatic unsaturated C=C bonds show an absorption on
the left side of 3000 cm-1, but the aromaticity must be
verified in the overtone region (1667 – 2000 cm-1) and the
out-of-plane (OOP) region (900 - 690 cm-1)
 4 Medium to strong absorptions in region 1650 - 1450 cm-1
 Many weak combination and overtone absorptions appear
between 2000 and 1667 cm-1
The relative shapes and numbers (1 - 4) of the overtone
absorptions can be used to tell whether the aromatic ring is
monosubstituted or di-, tri-, tetra-, penta-, or hexasubstituted.
Positional (ortho (o), meta (m), para (p)) isomers can also be
distinguished.
Note: A strong carbonyl absorption can overlap these
overtone bands, making them unusable.
Infrared Spectroscopy (IR)
Step 3 (Con’t)
Aromatic Ring Absorptions (Con’t)
 The unsaturated =C-H “Out-of-Plane (OOP) bending
absorptions in the region 900 – 690 cm-1 can also be
used to determine the type of ring substitution.
 The number of absorptions and their relative positions
are unique to each type of substitution.
 Although these absorptions are in the “Fingerprint”
region they are particularly reliable for rings with Alkyl
group substitutions.
 They are less reliable for Polar substituents.
Infrared Spectroscopy (IR)
Step 4.
If none of the above apply then the compound is most
likely a:
Hydrocarbon
or
Alkyl Halide
Generally, a very simple spectrum
Hydrocarbons - Check for saturated Alkane absorptions
just on the right side of 3000 cm-1