INFRA RED ABSORPTION INFRA RED ABSORPTION SPECTROSCOPYSPECTROSCOPY

Kateřina HynštováKateřina Hynštová

Principle of IRPrinciple of IR

In the context of infra red spectroscopy the term In the context of infra red spectroscopy the term "infra red" covers the range of the "infra red" covers the range of the electromagnetic spectrum between 0.78 and electromagnetic spectrum between 0.78 and 1000 mm. 1000 mm.

WavenumbersWavenumbers

wavelength is measured in wavelength is measured in "wavenumbers„"wavenumbers„

wavenumber = 1 / wavelength wavenumber = 1 / wavelength in centimetersIt is useful to divide the infra in centimetersIt is useful to divide the infra

red region into three sections; red region into three sections; nearnear, , midmid andand far far infra red; infra red;

Regions of wavelength rangeRegions of wavelength range

Wavenumber rangeWavenumber range (cm-1) (cm-1) Near0.78 - 2.51 2800 – 4000Near0.78 - 2.51 2800 – 4000 Middle2.5 – 504000- 200Middle2.5 – 504000- 200 Far50 -1000200 - 10Far50 -1000200 - 10 The most useful I.R. region lies between The most useful I.R. region lies between

4000 - 670cm-1. 4000 - 670cm-1.

Theory of infra red absorptionTheory of infra red absorption

IR radiation does not have enough energy to IR radiation does not have enough energy to induce electronic transitions as seen with UV.induce electronic transitions as seen with UV.

Absorption of IR is restricted to compounds with Absorption of IR is restricted to compounds with small energy differences in the possible small energy differences in the possible vibrational and rotational states. vibrational and rotational states.

For a molecule to absorb IR, the vibrations or For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net rotations within a molecule must cause a net change in the dipole moment of the molecule. change in the dipole moment of the molecule.

Molecular rotationsMolecular rotations

Rotational transitions are of little use to the Rotational transitions are of little use to the spectroscopist. spectroscopist.

Rotational levels are quantized, and Rotational levels are quantized, and absorption of IR by gases yields line absorption of IR by gases yields line spectra. spectra.

However, in liquids or solids, these lines However, in liquids or solids, these lines broaden into a continuum due to molecular broaden into a continuum due to molecular collisions and other interactions. collisions and other interactions.

Molecular vibrationsMolecular vibrations The positions of atoms in a molecules are not The positions of atoms in a molecules are not

fixed; they are subject to a number of different fixed; they are subject to a number of different vibrations. vibrations.

SpectraSpectra

The twodimensional plotThe twodimensional plot The dependece of transmitance on The dependece of transmitance on

wavenumberwavenumber Each compound has its own specific Each compound has its own specific

spectraspectra

Examples of spectraExamples of spectra

Examples of spectraExamples of spectra

Percentage transmittancePercentage transmittance

A percentage transmittance of 100 would mean A percentage transmittance of 100 would mean that all of that frequency passed straight through that all of that frequency passed straight through the compound without any being absorbed.the compound without any being absorbed.

In practice, that never happens - there is always In practice, that never happens - there is always some small loss, giving a transmittance of some small loss, giving a transmittance of perhaps 95% as the best you can achieve.perhaps 95% as the best you can achieve.

A transmittance of only 5% would mean that A transmittance of only 5% would mean that nearly all of that particular frequency is absorbed nearly all of that particular frequency is absorbed by the compound. A very high absorption of this by the compound. A very high absorption of this sort tells you important things about the bonds in sort tells you important things about the bonds in the compound.the compound.

Scheme of spectrometerScheme of spectrometer

MethodsMethods

KBrKBr ATRATR TransmissionTransmission RAISRAIS

Reflection Adsorbtion Infrared Reflection Adsorbtion Infrared SpectroscopySpectroscopy

Reflection Adsorption Reflection Adsorption Infrared Spectroscopy Infrared Spectroscopy (RAIRS) offers a means (RAIRS) offers a means of probing vibrations at of probing vibrations at surfaces with high surfaces with high resolution and, with its resolution and, with its ease of implementation ease of implementation and strict selection rules, and strict selection rules, is a convenient technique is a convenient technique for verifying molecular for verifying molecular integrity and orientation at integrity and orientation at metal surfaces.metal surfaces.

Conditions of workConditions of work

RigorousRigorous Base lineBase line Moisture Moisture

COCO22

Conclusions Conclusions IR iIR identifies the components of a sample (liquid, solid or gas).dentifies the components of a sample (liquid, solid or gas). Infrared (IR) spectrometers measure the interaction of IR radiation Infrared (IR) spectrometers measure the interaction of IR radiation

with samples. The FTIR spectrometer measures the frequencies at with samples. The FTIR spectrometer measures the frequencies at which the samples absorb the radiation, and the intensities of the which the samples absorb the radiation, and the intensities of the absorptions.absorptions.

Intensity and frequency of samples absorption are depicted in a two-Intensity and frequency of samples absorption are depicted in a two-dimensional plot called a spectrum. Intensity is generally reported in dimensional plot called a spectrum. Intensity is generally reported in terms of absorbance - the amount of light absorbed by a sample, or terms of absorbance - the amount of light absorbed by a sample, or percent transmittance – i.e. the amount of light, which passes percent transmittance – i.e. the amount of light, which passes through it. through it.

What makes up an unknown sample, and how much of each What makes up an unknown sample, and how much of each component is present in that sample, can be valuable information component is present in that sample, can be valuable information supplied by this technique. Its many applications include research supplied by this technique. Its many applications include research and development of new products.and development of new products.

Sources of informationesSources of informationes

http://www.cem.msu.edu/http://www.cem.msu.edu/~parrill/AIRS/~parrill/AIRS/

http://www.wpi.edu/http://www.wpi.edu/Academics/Depts/Academics/Depts/Chemistry/Courses/Chemistry/Courses/CH2670/infrared.htmlCH2670/infrared.html

http://http://www.chem.ucla.edu/www.chem.ucla.edu/~webspectra/irintro.html~webspectra/irintro.html

http://www.spectro.com/http://www.spectro.com/pages/e/index.htpages/e/index.ht