basics of ir.doc

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.

Basic concept ofIR spectroscopy

Presented By:

Shaikh Farida Banu

M.Sc.-I


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Infrared spectroscopy (IR spectroscopy) is thespectroscopythat deals with the infrared region

of the electromagnetic spectrum, that is light with a longerwavelengthand

lowerfrequency than visible light. It covers a range of techniques, mostly based on absorption

spectroscopy. As with all spectroscopic techniques, it can be used to identify and

study chemicals. A common laboratory instrument that uses this technique is a Fourier transform

infrared(FTIR)spectrometer.

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the

near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy

near-IR, approximately 140004000 cm1 (0.82.5 m wavelength) can

excite overtone orharmonicvibrations. The mid-infrared, approximately 4000400 cm1 (2.5

25 m) may be used to study the fundamental vibrations and associated rotational-

vibrational structure. The far-infrared, approximately 40010cm1(251000 m), lying adjacent to

the microwaveregion, has low energy and may be used forrotational spectroscopy. The names

and classifications of these subregions are conventions, and are only loosely based on the

relative molecular or electromagnetic properties.

Theory

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are

characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of

the absorbed radiation matches the transition energy of the bond or group that vibrates. The

energies are determined by the shape of the molecularpotential energy surfaces, the masses of

the atoms, and the associated vibronic coupling.

In particular, in the BornOppenheimerand harmonic approximations, i.e. when the molecular

Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic

oscillatorin the neighborhood of the equilibriummolecular geometry, the resonant frequencies are

associated with the normal modes corresponding to the molecular electronic ground state

potential energy surface. The resonant frequencies are also related to the strength of the bond

and the mass of the atoms at either end of it. Thus, the frequency of the vibrations are associated

with a particular normal mode of motion and a particular bond type.

Number of vibrational modes

In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes

in the dipole. A permanent dipole is not necessary, as the rule requires only a change in dipole

moment.[1]

A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules

with N atoms in them, linear molecules have 3N 5 degrees of vibrational modes, whereas

nonlinear molecules have 3N 6 degrees of vibrational modes (also called vibrational degrees of
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unit time) and its wavenumber k(the number of waves per unit length) [3]. The

velocity for EM-waves proves to be a constant for the medium in which the waves

are propagating, that is 3 108 m/s in vacuum. It can be easily seen that

c=

(1)

the wavelength being inversely proportional to the frequency. Using the quantumcharacter of EM radiation, there is the relation

E=h

(2)

where Eis the photon energy and h is Planks constant [4]. Each quantum of radiant

energy or photon, has an energy proportional to its frequency [5]. By this means,

a photon with a higher frequency contains more energy.

The EM spectrum can be divided into several regions differing in frequency only. It

extends from gamma (1020Hz) and X-rays (31016


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vibrational and rotationa lstates.

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net

change in the dipole moment of the molecule. The alternating electrical field of the radiation

(remember that electromagnetic radation consists of an oscillating electrical field and an

oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole

moment of the molecule. If the frequency of the radiation matches the vibrational frequencyof the molecule then radiation will be absorbed, causing a change in the amplitude of

molecular vibration.

Figure 2: A schematic representation of the quantized electronic and vibrational energy levels

of a molecule. It is clear that to raise the electronic energy state of a molecule from the

ground state to the excited state will cost more energy than to raise the vibrational energy

state.

An example of an electronically excited atom comes by considering the hydrogen

atom. The ground state of the hydrogen atom corresponds to having the atom's

single electron in the lowest possible orbits. By giving the atom additional energy,

the electron is able to move into a higher orbit. Now, we deem the hydrogen atom to

be in its excited state [6].

The vibrational energy of a molecule is not determined by the orbit of an electron but

by the shape of the molecule, the masses of the atoms and, eventually by the

associated vibronic coupling1). In the study of molecular vibrations I can start with a

classical model of the molecule where the nuclei are represented by mathematical

points with mass. The forces that hold the molecule together are assumed to be

similar to those exerted by massless springs. Each mass requires three coordinates

to define its position, such asx, y, andzin a Cartesian coordinate system. As aresult it has three independent degrees of freedom of motion. If there are Natomic

nuclei in the molecule, there will be a total of3Ndegrees of freedom of motion for all

the nuclear masses in the molecule. After subtracting the translational and rotational

degrees of freedom from the total 3Ndegrees of freedom, we are left with 3N

6 internal degrees of freedom for a nonlinear molecule and 3N 5internal degrees

of freedom for a linear molecule. These are the so-called normal modes of vibrations

and they result in specific natural vibrational frequencies for different molecular


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configurations. The typical frequencies of these vibrations match the frequency of IR

radiation (!).

For example, simple diatomic molecules have only one bond allowing only stretching

vibrations. More complex molecules may have many bonds, and vibrations can be

conjugated. The atoms in a CH2 group, commonly found in organic compounds, can

vibrate in six different ways; symmetrical and antisymmetricalstretching, scissoring,rocking, wagging and twisting.

Interaction of light and moleculesNow that I have discussed the energy level structure of molecules, I will address the

interaction of these molecules with EM waves. These interactions are at the very

heart of IR spectroscopy.

Roughly, there are five possible effects of interaction between radiation and

molecules. These are scattering,absorption, and emission. [8]. Absorption is the

process by which the energy of a photon is taken up by the matter, and this process

plays a key role in IR spectroscopy. There are several types of physical processes

that could lie behind absorption, depending on the quantum energy of the particular

frequency of EM radiation. For example, high energetic ultraviolet (UV) radiation cancause ionization and visible light usually causes electron transitions. As told before,

the energy levels for all physical processes at the atomic and molecular levels are

quantized, and if there are no available quantized energy levels with spacings which

match the quantum energy of the incident radiation, then the material will be

transparent to that radiation. In view of IR spectroscopy, I will now focus on the

absorption of IR radiation by matter.

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

with UV and visible light. Absorption of IR is restricted to excite vibrational and

rotational states of a molecule.

Even though the total charge on a molecule is zero, the nature of chemical bonds is

such that the positive and negative charges do not necessarily overlap in this case.Such molecules are said to be polar because they possess a permanent dipole

moment. For a molecule to absorb IR, the vibrations or rotations within a molecule

must cause a net change in the dipole moment of the molecule. The alternating

electrical field of the radiation interacts with fluctuations in the dipole moment of the

molecule. If the frequency of the radiation matches the vibrational frequency of the

molecule then radiation will be absorbed, causing a change in the amplitude of

molecular vibration. The result of IR absorption is heating of the matter since it

increases molecular vibrational energy. Molecular vibrations give rise to absorption

bands throughout most of the IR region of the spectrum. The far IR, lying adjacent

to the microwave region, has low energy and may be used for rotational

spectroscopy.
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Figure 3: Energy levels of a molecule during the absorption of a photon. Photons with specific

energies will be absorbed by the molecule if this energy is equal to the difference between the

energy levels; this is when the frequency of the IR radiation matches the vibrational frequency

of the molecule. Before absorption of the incident photon the molecule is in the ground state.

After absorption the molecule is in an excited vibrational state but still in the ground state of

the electronic energy level. Of course the molecule does not remain in this excited state

forever. The energy absorbed by a molecule is rapidly dissipated: it will be transformed into

kinetic energy as result of collisions or released again as photon.

IR SpectrometerIn an IR spectrometer molecules are irradiated with a whole range of IR frequencies

but are only capable of absorbing radiation energy at certain specific frequencies

which match the vibration frequencies of the molecule. The energy absorbed by a

molecule is rapidly dissipated. The excited molecule loses its energy in less than 10 -6seconds. The energy is either transformed into kinetic energy as result of collisions

or released again as photon. As the direction of the emitted photon is random in

space, and because the absorption process can be repeated for such a photon on its

way trough the medium, it can be seen that for a once-absorbed photon the

probability of re-emerging from the medium in the direction of the transmitted beam

is negligibly small [1]. A spectrometer usually measures the radiation intensity as a

function of the wavelength of the light behind a sample. At the vibrational

frequencies of the molecules an intensity decrease is obtained and a transmittance or

absorbance spectrum is plotted. In this way a sample can be characterized and

allows to determine which chemical bonds are present.
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Figure 4: FTIR spectrum of the Benzoic acid dimer recorded at room temperature 1 M CDCl3

solution [9]. The wavelength is often expressed as some unit directly proportional to the

photon energy, such as wavenumber which has a reciprocal relationship to wavelength.

Practical IR spectroscopy

The infrared spectrum of a sample is recorded by passing a beam of infrared light through the

sample. When the frequency of the IR is the same as the vibrational frequency of a bond,

absorption occurs. Examination of the transmitted light reveals how much energy was absorbed

at each frequency (or wavelength). This can be achieved by scanning the wavelength range

using a monochromator. Alternatively, the whole wavelength range is measured at once using

aFourier transforminstrument and then atransmittance orabsorbancespectrum is generated

using a dedicated procedure. Analysis of the position, shape and intensity of peaks in this

spectrum reveals details about the molecular structure of the sample.

This technique works almost exclusively on samples withcovalent bonds. Simple spectra are

obtained from samples with few IR active bonds and high levels of purity. More complex

molecular structures lead to more absorption bands and more complex spectra. The technique

has been used for the characterization of very complex mixtures. Spectra issues with infrared

fluorescence are rare.

Sample preparation

Gaseous samples require a sample cell with a longpathlength to compensate for the diluteness.

The pathlength of the sample cell depends on the concentration of the compound of interest. A

simple glass tube with length of 5 to 10 cm equipped with infrared windows at the both ends of

the tube can be used for concentrations down to several hundred ppm. Sample gas

concentrations well below ppm can be measured with a White's cell in which the infrared light is

guided with mirrors to travel through the gas. White's cells are available with optical pathlength

starting from 0.5 m up to hundred meters.

Liquid samples can be sandwiched between two plates of a salt (commonly sodium chloride, or

common salt, although a number of other salts such aspotassium bromide orcalcium fluoride are

also used).[2]The plates are transparent to the infrared light and do not introduce any lines onto

the spectra.

Solid samples can be prepared in a variety of ways. One common method is to crush the sample

with an oily mulling agent (usually Nujol) in amarble oragatemortar, with a pestle. A thin film of

the mull is smeared onto salt plates and measured. The second method is to grind a quantity of
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the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering

effects from large crystals). This powder mixture is then pressed in a mechanicalpress to form a

translucent pellet through which the beam of the spectrometer can pass. [2] A third technique is the

"cast film" technique, which is used mainly for polymeric materials. The sample is first dissolved

in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or

NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analyseddirectly. Care is important to ensure that the film is not too thick otherwise light cannot pass

through. This technique is suitable for qualitative analysis. The final method is to

use microtomy to cut a thin (20100 m) film from a solid sample. This is one of the most

important ways of analyzing failed plastic products for example because the integrity of the solid

is preserved.

In photoacoustic spectroscopythe need for sample treatment is minimal. The sample, liquid or

solid, is placed into the sample cup which is inserted into the photoacoustic cell which is then

sealed for the measurement. The sample may be one solid piece, powder or basically in any form

for the measurement. For example, a piece of rock can be inserted into the sample cup and the

spectrum measured from it.

It is important to note that spectra obtained from different sample preparation methods will look

slightly different from each other due to differences in the samples' physical states.

Comparing to a reference

Schematics of a two-beam absorption spectrometer. A beam of infrared light is produced, passed through

an interferometer(not shown), and then split into two separate beams. One is passed through the sample, the

other passed through a reference. The beams are both reflected back towards a detector, however first they pass

through a splitter, which quickly alternates which of the two beams enters the detector. The two signals are then

compared and a printout is obtained. This "two-beam" setup gives accurate spectra even if the intensity of the light

source drifts over time.

To take the infrared spectrum of a sample, it is necessary to measure both the sample and a

"reference" (or "control"). This is because each measurement is affected by not only the light-

absorption properties of the sample, but also the properties of the instrument (for example, what

light source is used, what infrared detectoris used, etc.). The reference measurement makes it

possible to eliminate the instrument influence. Mathematically, the sample transmission spectrum

is divided by the reference transmission spectrum.
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The appropriate "reference" depends on the measurement and its goal. The simplest reference

measurement is to simply remove the sample (replacing it by air). However, sometimes a

different reference is more useful. For example, if the sample is a dilute solute dissolved in water

in a beaker, then a good reference measurement might be to measure pure water in the same

beaker. Then the reference measurement would cancel out not only all the instrumental

properties (like what light source is used), but also the light-absorbing and light-reflectingproperties of the water and beaker, and the final result would just show the properties of the

solute (at least approximately).

A common way to compare to a reference is sequentially: first measure the reference, then

replace the reference by the sample and measure the sample. This technique is not perfectly

reliable; if the infrared lamp is a bit brighter during the reference measurement, then a bit dimmer

during the sample measurement, the measurement will be distorted. More elaborate methods,

such as a "two-beam" setup (see figure), can correct for these types of effects to give very

accurate results. The Standard additionmethod can be used to statistically cancel these errors.

Absorption bands

IR spectroscopy is often used to identify structures because functional groups give rise to

characteristic bands both in terms of intensity and position (frequency). The positions of these

bands is summarized in correlation tables as shown below.

Main article: Infrared Spectroscopy Correlation Table

Wavenumbers listed in cm1 .

Badger's Rule

For many kinds of samples, the assignments are known, i.e. which bond deformation(s) are

associated with which frequency. In such cases further information can be gleaned about the

strength on a bond, relying on the empirical guideline called Badger's Rule. This rule states that

the strength of a bond correlates with the frequency of its vibrational mode.

Uses and applications

Infrared spectroscopy is a simple and reliable technique widely used in both organic and

inorganic chemistry, in research and industry. It is used in quality control, dynamic measurement,

and monitoring applications such as the long-term unattended measurement of

CO2 concentrations in greenhouses and growth chambers by infrared gas analyzers.
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It is also used inforensic analysisin both criminal and civil cases, for example in

identifying polymer degradation. It can be used in detecting how much alcohol is in the blood of a

suspected drunk driver measured as 1/10,000 g/mL = 100 g/mL. [citation needed]

A useful way of analysing solid samples without the need for cutting samples uses ATR

orattenuated total reflectancespectroscopy. Using this approach, samples are pressed against

the face of a single crystal. The infrared radiation passes through the crystal and only interacts

with the sample at the interface between the two materials.

With increasing technology in computer filtering and manipulation of the results, samples in

solution can now be measured accurately (water produces a broad absorbance across the range

of interest, and thus renders the spectra unreadable without this computer treatment).

Some instruments will also automatically tell you what substance is being measured from a store

of thousands of reference spectra held in storage.

Infrared spectroscopy is also useful in measuring the degree of polymerization

inpolymermanufacture. Changes in the character or quantity of a particular bond are assessed

by measuring at a specific frequency over time. Modern research instruments can take infraredmeasurements across the range of interest as frequently as 32 times a second. This can be done

whilst simultaneous measurements are made using other techniques. This makes the

observations of chemical reactions and processes quicker and more accurate.

Infrared spectroscopy has also been successfully utilized in the field of semiconductor

microelectronics:[4]for example, infrared spectroscopy can be applied to semiconductors

like silicon, gallium arsenide,gallium nitride,zinc selenide, amorphous silicon,silicon nitride, etc.

The instruments are now small, and can be transported, even for use in field trials.

References

Applications of absorption spectroscopy of organic compounds by JOHN R.DYER

Principles of instrumental analysis by SKOOG &WEST 3Rd EDITION

Google. COM
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