1. 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.

2. IR-Spectroscopy IR region Interaction of IR with molecules
Spectroscopy Application
IR-Spectroscopy
IR region
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 sensitive
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 thr. infrared radiation absorption is in the range of 8-40 KJ/mol

3. IR-Spectroscopy 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

4. 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