Fourier Transformation Infrared FTIR Theory Principle Analysis by

Fourier Transformation Infra-red (FT-IR)

Theory Principle • Analysis by IR spectroscopy is based on the fact that molecules have specific frequencies associated with internal vibrations/rotational of groups of atoms • IR region 4000 cm-1 to ~ 200 cm-1

Theory • IR radiation does not have enough energy to induce electronic transition • Absorption of IR is restricted to compounds with small energy difference in the possible vibrational and rotational states.

• In a molecule, the atoms are not held rigidly apart • they can move, as if they are attached by a spring of equilibrium separation Re • For a molecule to absorb IR , the vibration or rotation must cause a net change in the dipole moment of the molecule.

• The alternating electrical field of the radiation interact with fluctuations in the dipole moment of the molecule. • If the frequency of the radiation matches the vibrational of the molecule the radiation will be absorbed and excite the molecule to a higher vibrational state

• An IR spectrometer measures the frequencies of the absorbed radiation at specific wavelength • The result is a plot of absorbed energy versus frequency and is called IR spectrum of the material Molecular Rotation – not important especially in solid and liquid. Molecular Vibration: vibrations fall into two main categories – stretching and bending

• In a simple diatomic molecule, there is only one direction of vibrating, stretching • This means there is only one band of infra red absorption. • Weaker bonds require less energy, as if the bonds are springs of different strengths. • More atoms will give more modes of vibrations • For a linear molecule with "n" atoms, there are 3 n-5 vibrational modes, if it is nonlinear, it will have 3 n-6 modes.

• For example, water (H 20), has 3 molecules, and is non linear: therefore it has (3*3)-6 = 3 modes of vibration • There is one important restriction, the molecule will only absorb radiation if the vibration is accompanied by a change in the dipole moment of the molecule. • A dipole occurs when there is a difference of charge across a bond

• If the two appositely charged molecules get closer or further apart as the bond bends or stretches, the moment will change • To calculate the frequency of light absorbed, requires Hookes law: Where: k = force constant indicating the strength of the bond m 1 and m 2 are the masses of the two atoms

Ø From this equation, we can see that if there is a high value of k, i. e. the bond is strong, it absorbs a higher frequency of light. Ø a C=C double bond would absorb a higher frequency of light than a C-C single bond Ø the larger the two masses, the lower the frequency of light absorbed.

Experimental procedure: • If the sample is a liquid, it can be tested straight away • If it is a solid, then it is ground up to a fine powder, and mixed with a few drops of liquid paraffin (Nujol) to form a paste • A thin layer of the liquid or paste is then spread between two sodium chloride plates, and placed in the machine

• Sodium chloride is used as it does not absorb strongly in the infrared region whereas glass does, however it does dissolve readily in water, so it must be cleaned with CH 2 CL 2.

• The reference cell is an identical rock salt plate, with a similar amount of Nujol (if used in sample). • The detector then compares the two beams it receives, and can remove any peaks due to the Nujol or the plates.

Interpreting the spectrum • The graph produced show percentage transmission against wave number • If no radiation is absorbed at a particular frequency, then the line on the graph will be be at 100% at the corresponding wave number • Identification possible due to differences in the chemical structure of materials characteristic vibration and yield unique IR spectra (fingerprint)

• In addition, the magnitude of the absorption related to concentration quantitative • Different types of bonds have characteristic regions of the spectrum where they absorb:

• Most functional groups absorb above 1500 cm-1. • The region below 1500 cm-1 is known as the "fingerprint region". • Every molecule produces a unique pattern here, so if an unknown sample produces a spectrum which matches that of a known compound, the sample can be confirmed to be that compound.

Example spectrum: A sample with chemical formula (C 3 H 6 O 2) gives the above spectrum

• This shows a strong absorption at just over 1710 cm -1 and a medium absorption at 2500 -3300 cm-1. • these bonds correspond to the C=O and O-H groups found in a carboxylic acid. • This information + the chemical formula tells us the structure must be propanoic acid:

For decades, dispersive spectrometers were the workhorse of IR analysis. From 1980 s onwards FTIR spectrometers almost entirely replaced dispersive The most significant feature of FTIR spectrometers is that radiation from all wavelength is measured simultaneously.

Applications IR spectroscopy – major tool determining structure, identification & quantitative analysis. There applications requiring special techiniques.

1. Surface Analysis 2. IR Microspectroscopy 3. Chromatography-FTIR 4. Time-resolved studies

1. Surface Analysis The are 3 available techniques and each provide different depth resolution and types of sample. a. ATR ( attenuated total reflectance) § probes the surface layer in the range ~0. 5 to 3 m

Attenuated light Prism ( 1) Sample ( 2) § light entering prism undergoes internal reflection § at the point of reflection light penetrates a small distance into the thin film Ø the depth depends on and § The light exiting the prism attenuated at specific freq. Corresponding to absorption by sample.

Applications of ATR • Investigating adhesion problem • surface treatment / modification • weathering of plastic parts, polymer films and fiber • identification very thin layer contiminants on surface

b. PAS ( photoacoustic spectroscopy) § sampling depth of ~1 to 10 m § IR energy absorbed converted to heat induces expansion of gas in contact - acoustic signal the incident beam intensity is modulated at a frequency in the acoustic range. § microphonic detection –yields IR spectrum. § useful in analysis samples w/o preparation – powders, pellets, films § surface coatings, alumina-based catalysis, etc

c. GIR ( grazing incidence reflectance) § detection ultrathin films ( ar least monolayer coverage) § IR beam intersects the surface at a high angle of incidence § very sensitive, applicable only to metallic surfaces Other applications 2. IR Microspectroscopy 3. Chromatography-FTIR 4. Time-resolved studies