1. Infrared Spectroscopy
Dr AKM Shafiqul Islam
School of Bioprocess Engineering
University Malaysia Perlis

2. 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

3. Infrared spectroscopy is very useful for obtaining qualitative information about the molecules. But molecule must possess certain properties in order to undergo absorption.

4. 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

5. THE ELECTROMAGNETIC SPECTRUM
high
Frequency (n)
low
high
Energy
low
MICRO-
WAVE
X-RAY
ULTRAVIOLET
INFRARED
RADIO
FREQUENCY
Nuclear
magnetic
resonance
Vibrational
infrared
Ultraviolet
Visible
2.5 mm
15 mm
1 m
5 m
200 nm
400 nm
800 nm
BLUE
RED
short
Wavelength (l)
long

6. Types of Energy Transitions in Each Region
of the Electromagnetic Spectrum
REGION
ENERGY TRANSITIONS
X-ray
Bond-breaking
UV/Visible
Electronic
Infrared
Vibrational
Microwave
Rotational
Radio Frequency
Nuclear and
(NMR)
Electronic Spin

7. 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

8. 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

9. 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.

10. 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 =m1m2/(m1+m2) , also called “reduced mass”

11. 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.

12. DIPOLE MOMENTS Bonds which do not absorb infrared include:
Symmetrically substituted alkenes and alkynes
Many types of C-C Bonds
Symmetric diatomic molecules
H-H Cl-Cl

13. 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

14. 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.

15. IR Instrumentation

16. 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:
IR can be used to distinguish one compound from another.
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.

17. Infrared Spectroscopy (IR)
The Infrared Spectrum
A plot of absorption intensity (% Transmittance) on the y-axis vs. frequency (wavenumbers) on the x-axis.

18. Infrared Spectroscopy (IR)
Principal Frequency Bands (from left to right in spectrum)
OH 3600 cm-1 (Acids - Very Broad, Alcohols - Broad)
NH cm-1 (2, 1, 0 peaks – 1o, 2o, 3o)
C≡N 2250 cm-1 (Nitrile)
C≡C 2150 cm-1 (Acetylene)
C=O cm-1 (1715) (Carbonyl)
C=C 1650 cm-1 (Alkene); 4 absorptions (aromatic)
CH cm-1 (Methylene Group)
CH cm-1 (Methyl Group)
CO cm-1 (Alcohol, Acid, Ester, Ether, Anhydride)
-CH (Saturated Alkane absorptions on Right side of 3000 cm-1)
=C-H (Unsaturated Alkene absorptions on Left side of 3000 cm-1)
=C-H (Aromatic absorptions) – Verify at 1667 – 2000 cm-1
≡C-H (Unsaturated Alkyne absorptions on Left side of 3000 cm-1)

19. Infrared Spectroscopy (IR)
Suggested approach for analyzing IR Spectra
Step 1. – Check for the presence of the Carbonyl group (C=O) at 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 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 cm-1)
Anhydrides - Check for 2 C=O absorptions near 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)

20. 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 cm-1 Confirm present of CO near 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 cm-1
N-H Bend at 800 cm-1
Ethers - Check for C-O group near cm-1 and absence of OH

21. 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 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 cm-1

22. 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 ( cm-1)
4 Medium to strong absorptions in region 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 hexa-substituted.
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.

23. 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.

24. 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