Fourier Transform InfraRed Spectroscopy

Basics

Why to study FTIR?

Principle behind IR absorption

IR spectra analysis

FTIR instrumentation

Advantages and disadvantages of FTIR

Lab experiment

Outline

Spectroscopy is the study of the interaction of electromagnetic radiation with matter (chemical compounds)

There are many forms of spectroscopy – Radio, Microwave, IR, UV – Visible, X – ray, - ray spectroscopy

Each one gives useful information to identify substances and to determine various characteristics of their structure

Spectroscopy

Radiation Molecular effect

- rays Ionization

X – rays Ionization

UV Electronic transitions

Visible Electronic transitions

IR Molecular vibrations

Microwave Molecular rotations

Radio Nuclear spin transitions

IR: = 2.5 to 25 µm or = 4000 to 400 cm-1

Gives information about molecular vibrational energy levels

What is the need to study IR spectroscopy?

To identify the unknown compound

To know the purity of the sample

To determines the amount of components in a mixture

IR spectroscopy

All atoms within molecules are in constant vibration

All bonds with a change in dipole moment will absorb IR radiation at the appropriate vibrational frequency

H

Cl

H

Cl

H

ClDipole moment

+

IR radiation

Origin of IR absorption

The vibrational frequency will depend on the atoms associated with the bond as well as the bond strength

Factors affecting the frequency of molecular vibrations

Mass of the atoms - heavier molecules will result in slower movement and result in lower frequencies

Bond Strength - increasing number of bonds increases the frequency of vibration

Energies associated with the vibrational motion are in between 4000 cm -1 and 400 cm -1

Rotational energies are generally much smaller (below 300 cm -1)

No. of normal modes in a molecule:

for linear molecules – (3N-5)

for nonlinear molecules – (3N-6) , where N is the number of atoms

Carbon dioxide, a linear molecule, has 3 x 3 - 5 = 4 normal modes

Symmetric stretching

Antisymmetric stretching

In-plane bending

Out-of-plane bending

IR spectra and analysis

Sample

I0 I Transmittance (T) = I/I0

Absorbance (A) = log (1/T)

Abs

orba

nce

Wavenumber (cm-1)

FTIR spectra of isopropyl alcohol, CH(CH3)2OH

The absorption energy primarily depends on whether the bonds are single or double or triple or bonds to hydrogen

Single-bond regime800 - 300 cm-1

C–C, C–O, C–N

Double bond regime1900-1500 cm-1

C=C, C=O, C=N, N=O

Triple bond regime2300-2000 cm-1

C ≡ C, C ≡ N

C–H, N–H, O–H 3800–2700 cm-1

IR waveno.

IR data base: http://webbook.nist.gov/chemistry/

FTIR - A method for measuring all of the IR frequencies simultaneously, rather than individually as with dispersive instruments

FTIR Instrumentation

To separate IR light, grating is used

Grating

Light source

Detector

Sample

Slit

To select the specified IR light, slit is used

Dispersive IR spectrometer

Michelson Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector (DLATGS) (deuterated L-Alanine doped tri-glycine Sulfate)

Beam splitter

The heart of the FTIR is Michelson interferometer

FTIR spectrometer

Fixed mirror

Movable mirror

Fixed mirror

Movable mirrorSame-phase interference wave shape

Opposite-phase interference wave shape

Movable mirror

For monochromatic beam

Continuous phase shift

Sig

nal

str

eng

th

Ｉ

- - /2 0 /2

-2 - 0 2

Light source spectrum Signal o/p from interferometer(interferogram)

Path difference

I(t)

Path difference

I

b( )u

Wavenumber

FTIR seminar

Monochromatic light

Continuous spectrum

Az

Wavenumber

Path difference

IDichroic light

SAz

Wavenumber

Universal technique sensitivity 10-6 grams

fast and easy

relatively inexpensive

rich information

Advantages of FTIR

Cannot detect atoms or monoatomic ions

Cannot detect molecules comprised of two identical atoms N2 or O2

Aqueous solutions are very difficult to analyze - water is a strong IR absorber

Complex mixtures - samples give rise to complex spectra

Disadvantages of FTIR

Record and analyze the FTIR spectra of Polystyrene strip and Benzoic acid

1. Weigh the Benzoic acid and KBr in 1:100 ratio

2. Grind the samples using the mortar and pestle

3. Make pure KBr pellet and “Benzoic acid + KBr (1:100)” pellet using pelletiser

4. Record the spectra of the background (air for strip and KBr for powder or pellet)

5. Record the spectra of the sample

6. Analysis of the recorded spectra

Lab Experiment