1. IR-Spectroscopy IR region Interaction of IR with molecules
The part of electromagnetic radiation between the visible and microwave regions 0.8 m to 50 m (12,500 cm cm-1) is called IR region
Most interested region in Infrared Spectroscopy is between 2.5m-25 m
(4,000cm-1-400cm-1), which corresponds to vibrational frequency of molecules
Interaction of IR with molecules
Only molecules containing covalent bonds with dipole moments are infrared active
Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed
Absorption of infrared radiation by a molecule rises the energy state of the molecule
increasing the amplitude of the molecular rotation & vibration of the covalent bonds
Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy)
Vibration - 10,000 cm-1 to 100 cm-1
The energy changes through infrared radiation absorption is in the range of 8-40 KJ/mol

2. IR-Spectroscopy Atoms in a molecule are constantly in motion
Spectroscopy Application
IR-Spectroscopy
Atoms in a molecule are constantly in motion
There are two main vibrational modes:
Stretching - (symmetrical/asymmetrical) change in bond length - high frequency
Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq.
The rotation and vibration of bonds occur in specific frequencies
Every type of bond has a natural frequency of vibration, depending on
the mass of bonded atoms (lighter atoms vibrate at higher frequencies)
the stiffness of bond (stiffer bonds vibrate at higher frequencies)
the force constant of bond (electronegativity)
the geometry of atoms in molecule
The same bond in different compounds has a slightly different vibration frequ.
Functional groups have characteristic stretching frequencies.

3. Infrared Spectroscopy
Characteristics of an IR Spectrum
In an IR spectrometer, light passes through a sample.
Frequencies that match the vibrational frequencies are absorbed, and the remaining light is transmitted to a detector.
An IR spectrum is a plot of the amount of transmitted light versus its wavenumber.
Frequencies in IR spectroscopy are reported using a unit called wavenumber ():
Wavenumber is inversely proportional to wavelength and is reported in reciprocal centimeters (cm–1). ~
 = 1/
Let us now consider the IR spectrum of 1-propanol, CH3CH2CH2OH.

4. Infrared Spectroscopy
Characteristics of an IR Spectrum—1-Propanol

5. Infrared Spectroscopy
Characteristics of an Infrared Spectrum
The IR spectrum is divided into two regions: the functional group region (at  1500 cm-1), and the fingerprint region (at < 1500 cm-1).
Figure 13.8
Comparing the functional group
region and fingerprint region of
two compounds

6. Infrared Spectroscopy
Characteristics of an Infrared Spectrum
The y-axis is % transmittance: 100% transmittance means that all the light shone on a sample is transmitted and none is absorbed. 0% transmittance means that none of the light shone on the sample is transmitted and all is absorbed.
Each peak corresponds to a particular kind of bond, and each bond type (such as O—H and C—H) occurs at a characteristic frequency.
Wavenumber, frequency and energy decrease from left to right.
Where a peak occurs is reported in reciprocal centimeters (cm-1).

7. Infrared Spectroscopy
IR Absorptions
Bonds absorb in four predictable regions of an IR spectrum.
Figure 13.10
Summary: The four regions
of the IR spectrum

8. IR Spectrum OH 3600 cm-1 Principal Correlation Chart
Spectroscopy Application
IR Spectrum
Principal Correlation Chart
OH 3600 cm-1
NH 3500 cm-1
CH 3000 cm-1
CN 2250 cm-1
CC 2150 cm-1
C=O 1715 cm-1
C=C 1650 cm-1
CO 1100 cm-1
Region freq. (cm-1) what is found there??
XH region OH, NH, CH (sp, sp2, sp3) stretches
triple bond CºC, CºN, C=C=C stretches
double bond C=O, C=N, C=C stretches
fingerprint many types of absorptions
C-O, C-N stretches
CH in-plane bends, NH bends
CH out-of-plane (oop) bends
Dispersive (Double Beam) IR Spectrophotometer
Prism or
Diffraction
Grating
Slit
Photometer
IR Source
Recorder
Split Beam
Air
Lenz
Sample

9. Infrared Spectroscopy
IR Absorptions

10. Infrared Spectroscopy
IR Absorptions
Even subtle differences that affect bond strength affect the frequency of an IR absorption.
The higher the percent s-character, the stronger the bond and the higher the wavenumber of absorption.

11. Infrared Spectroscopy
IR Absorptions
For a bond to absorb in the IR, there must be a change in dipole moment during the vibration.
Symmetrical nonpolar bonds do not absorb in the IR. This type of vibration is said to be IR inactive.

12. Infrared Spectroscopy
IR Absorptions in Hydrocarbons
Hexane has only C-C single bonds and sp3 hybridized C atoms. Therefore it has only one major absorption at cm-1.

13. Infrared Spectroscopy
IR Absorptions in Hydrocarbons
1-Hexene has a C=C and Csp2-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: Csp2-H at cm-1; Csp3-H at cm-1; C=C at 1650 cm-1.

14. Infrared Spectroscopy
IR Absorptions in Hydrocarbons
1-Hexyne has a CC and Csp-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: Csp-H at 3300 cm-1; Csp3-H at cm-1; CC at 2250 cm-1.

15. Infrared Spectroscopy
IR Absorptions in Oxygen Containing Compounds
The OH group of the alcohol shows a strong absorption at cm-1. The peak at ~3000 cm-1 is due to sp3 hybridized C—H bonds.

16. Infrared Spectroscopy
IR Absorptions in Oxygen Containing Compounds
The C=O group in the ketone shows a strong absorption at ~1700 cm-1. The peak at ~3000 cm-1 is due to sp3 hybridized C—H bonds.

17. Infrared Spectroscopy
IR Absorptions in Oxygen Containing Compounds
The ether has neither an OH or a C=O, so its only absorption above 1500 cm-1 occurs at ~3000 cm-1, due to sp3 hybridized C—H bonds.

18. Infrared Spectroscopy
IR Absorptions in Nitrogen Containing Compounds
The N—H bonds in the amine give rise to two weak absorptions at 3300 and 3400 cm-1.

19. Infrared Spectroscopy
IR Absorptions in Nitrogen Containing Compounds
The amide exhibits absorptions above 1500 cm-1 for both its N—H and C=O groups: N—H (two peaks) at 3200 and 3400 cm-1; C=O at 1660 cm-1.

20. Infrared Spectroscopy
IR Absorptions in Nitrogen Containing Compounds
The CN of the nitrile absorbs in the triple bond region at ~2250 cm-1.

21. Infrared Spectroscopy
Worked example
The infrared spectrum below is for one of the molecules shown. Identify the molecule and explain how you arrived at your choice.

22. Interpretation of the spectra
Looking in the region above cm−1, we can identify a band in the region 2840–3100 cm−1 that is due to a C–H bond and an absorption in the region 1700–1750 cm−1 that is due to the C=O bond. This eliminates molecules A and B, as neither of these contains a C=O bond. C is a carboxylic acid, which would be expected
to have a very broad O–H band in the region 2400–3400 cm−1. This band is not present in the spectrum, so the spectrum cannot be for molecule C. This means that we are left with molecule D. Molecule D contains C–H and C=O bonds and should also give rise to a band in the region 1000–1300 cm−1 as it contains a C–O bond; we can see from the spectrum that there is a band in this region.

23. Use of Infra-Red spectroscopy
Spectroscopy Application
IR-Spectroscopy
Use of Infra-Red spectroscopy
IR spectroscopy can be used to distinguish one compound from another.
No two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption spectrum.
A fingerprinting type of IR spectral library can be established to distinguish a compounds or to detect the presence of certain functional groups in a molecule.
Obtaining structural information about a molecule
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
Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information.
The known structure information is summarized in the Correlation Chart