IR spectrophotometry
Energy of molecule = Electronic energy+ Vibrational
energy + Rotational energy
• IR spectroscopy is concerned with the study of
absorption of infrared radiation, which causes
vibrational transition in the molecule.
Hence, IR spectroscopy also known as Vibrational
spectroscopy.
• IR spectra mainly used in structure elucidation to
determine the functional groups.
IR SPECTROSCOPY
- Used for qualitative identification of organic and inorganic compounds
- Used for checking the presence of functional groups in molecules
- Can also be used for quantitative measurements of compounds
- Each compound has its unique IR absorption pattern
- Wave number with units of cm-1 is commonly used
- Wave number = number of waves of radiation per centimeter
IR SPECTROSCOPY
- The IR region has lower energy than visible radiation and
higher energy than microwave
The IR region is divided into
Near-IR (NIR): 750 nm – 2500 nm
Mid-IR: 2500 nm – 20000 nm
Far-IR: 20000 nm – 400000 nm
Most of the analytical applications are confined to the middle IR
region because absorption of organic molecules are high in this
region.
IR ABSORPTION BY MOLECULES
- Molecules
with covalent bonds may absorb IR radiation
- Absorption is quantized
- Molecules move to a higher energy state
- IR radiation is sufficient enough to cause rotation and vibration
- Radiation between 1 and 100 µm will cause excitation to
higher vibrational states
- Radiation higher than 100 µm will cause excitation to higher rotational
state
IR ABSORPTION BY MOLECULES
- Absorption spectrum is composed of broad vibrational absorption
bands
- Molecules absorb radiation when a bond in the molecule vibrates at
the same frequency as the incident radiant energy
- Molecules vibrate at higher amplitude after absorption
- A molecule must have a change in dipole moment during vibration in
order to absorb IR radiation
DIPOLE MOMENT (µ)
µ=Qxr
Q = charge and r = distance between charges
- Asymmetrical distribution of electrons in a bond renders the
bond polar
- A result of electronegativity difference
- µ changes upon vibration due to changes in r
- Change in µ with time is necessary for a molecule to absorb
IR radiation
DIPOLE MOMENT (µ)
- The repetitive changes in µ makes it possible for polar molecules to
absorb IR radiation
- Symmetrical molecules do not absorb IR radiation since they do not
have dipole moment (O2, F2, H2, Cl2)
- Diatomic molecules with dipole moment are IR-active (HCl, HF, CO, HI)
- Molecules with more than two atoms may or may not be IR active
depending on whether they have permanent net dipole moment
Molecular vibrations
What is a vibration in a molecule?
• “Any change in shape of the molecule- stretching of bonds,
bending of bonds, or internal rotation around single bonds”.
There are two types of vibrations:
1. Stretching vibrations
2. Bending vibrations
1.




a)
b)
Stretching vibrations:
Vibration or oscillation along the line of bond
Change in bond length
Occurs at higher energy: 4000-1250 cm-1
Two types:
Symmetrical stretching
Asymmetrical stretching
a) Symmetrical stretching:
• Two bonds increase or decrease in length
simultaneously.
H
C
H
b) Asymmetrical stretching
• in this, one bond length is increased and other is
decreased.
H
C
H
2. Bending vibrations
• Vibration or oscillation not along the line of
bond
• These are also called as deformations
• In this, bond angle is altered
• Occurs at low energy: 1400-666 cm-1
• Two types:
a) In plane bending: scissoring, rocking
b) Out plane bending: wagging, twisting
a) In plane bending
i.
•
•
•
Scissoring:
This is an in plane blending
2 atoms approach each other
Bond angles are decrease
H
C
H
ii. Rocking:
• Movement of atoms take place in the same
H
direction.
C
H
b) Out plane bending
i. Wagging:
• Two atoms move to one side of the plane. They
move up and down the plane. H
C
ii. Twisting:
H
• One atom moves above the plane and another
atom moves below the plane.
H
C
H
Group Frequency Region
• consisting of the absorption
bands of the functional
groups.
• frequency = 4000-1300cm-¹
• wavelength = 2.5-8
Fingerprint Region
• IR spectra is called
“fingerprints” because no
other chemical species will
have similar IR spectrum.
• Single bonds give their
absorption bands in this
region.
• Frequency=1300-650cm-1
• Wavelength=8-15.4
Group frequency and fingerprint region
TRANSMISSION (ABSORPTION) TECHNIQUE
Solid Samples
Three sampling techniques
Mulling, pelleting, thin film
Pelleting
- 1 mg ground sample is mixed with 100 mg of dry KBr powder
- Mixture is compressed under very high pressure
- Small disk with very smooth surfaces forms (looks like glass)
Quantitative Analysis
• IR more difficult than UV-Vis because
– narrow bands (variation in e)
– complex spectra
– weak incident beam
– solvent absorption
• IR mostly used for rapid qualitative but not quantitative
analysis
– Beer’s law failure
– Long optical path-length required