introduction to infrared spectroscopy

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Introduction to InfraredSpectroscopy

Chapter 16Instrumental Analysis

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Definition of Infrared

Spectroscopy The absorption of light, as it passes through a

medium, varies linearly with the distance the light

travels and with concentration of the absorbingmedium !here a is the absorbance, the "ree#lower$case letter epsilon is a characteristic constantfor each material at a given wavelength %#nown as

the e&tinction coefficient or absorption coefficient', cis concentration, and l is the length of the light path,the absorption of light may be e&pressed by thesimple e(uation a) epsilon times c times l

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Infrared Spectroscopy

Infrared spectroscopy is the measurement of the wavelength

and intensity of the absorption of mid$infrared light by a

sample *id$infrared is energetic enough to e&cite molecularvibrations to higher energy levels

The wavelength of infrared absorption bands is characteristic

of specific types of chemical bonds, and infrared

spectroscopy finds its greatest utility for identification oforganic and organometallic molecules The high selectivity

of the method ma#es the estimation of an analyte in a

comple& matri& possible 

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+&ample of I 

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Theory of Infrared Absorption

Spectroscopy -or a molecule to absorb I, the vibrations or rotations

within a molecule must cause a net change in the dipole

moment of the molecule The alternating electrical field ofthe radiation %remember that electromagnetic radiation

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 fre(uency of the radiation matches the vibrational

fre(uency of the molecule then radiation will be absorbed,

causing a change in the amplitude of molecular vibration 

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*olecular otations

otational transitions are of little use to the

spectroscopist otational levels are(uanti.ed, and absorption of I by gases

yields line spectra

/owever, in li(uids or solids, these lines

 broaden into a continuum due to molecular

collisions and other interactions 

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*olecular otations %cont'

,

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0ibrational$otational

Transitions In general, a molecule which is an e&cited vibrational state

will have rotational energy and can lose energy in a

transition which alters both the vibrational and rotationalenergy content of the molecule

The total energy content of the molecule is given by the sum of the vibrational and rotational energies -or a molecule ina specific vibrational and rotational state, denoted by the

 pair of (uantum numbers %v,  J ', we can write its energy as

E (v , J )=E vib(v ) + E rot(J )

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Transitions %cont'

The energies of these three transitions form a verydistinctive pattern If we consider the lower

vibrational state to be the initial state, then we canlabel the absorption lines as follows

Transitions for which the  J  (uantum numberdecreases by 1 are called P-branch transitions,

those which increase by 1 are called R-branchtransitions and those which are unchanged arecalled Q-branch transitions

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*olecular 0ibrations

In order to predict e(uilibrium stable$isotope fractionations, it isnecessary to #now the characteristic fre(uencies of molecularvibrations It is also necessary to #now how much each vibrational

fre(uency in a molecule changes when a heavy isotope is substituted fora light one 0ibrational fre(uencies for isotopically substitutedmolecules are not always #nown, so it is often necessary to use sometype of force$field model to predict them

*olecular vibrations are also important in understanding infrared

absorption and the mechanisms and #inetics of chemical reactions-re(uencies are most commonly measured with infrared or amanspectroscopy otational$vibrational spectroscopy, isotope substitution,and many forms of force$field modeling are used to determinecharacteristic atomic motions

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0ibrational *otion

Subdivided into so$called normal modes of vibrationwhich rapidly increase with the number of atoms in

the molecule +ach of these normal vibrational modescontributes T to the average molar energy of thesubstance and is a primary reason why heat capacitiesincrease with molecular comple&ity

If there are 2vib modes of vibration, then thevibrational energy contributes 2vib%T' to the average

molar energy of the substance

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Stretching and 3ending

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Stretching 0ibrations

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3ending 0ibrations

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4uantum Treatment of

0ibrations Transitions in vibrational energy levels can be

 brought about by absorption of radiation, provided

the energy of the radiation e&actly matches the

difference in energy levels between the vibrational

(uantum states and provided also that the vibration

causes a fluctuation in dipole 

Infrared measurements permit the evaluation of the

force constants for various types of chemical bonds

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Infrared Instruments

An infrared spectrophotometer is an instrument that passesinfrared light through an organic molecule and produces aspectrum that contains a plot of the amount of light transmittedon the vertical a&is against the wavelength of infrared radiationon the hori.ontal a&is In infrared spectra the absorption pea#s point downward because the vertical a&is is the percentagetransmittance of the radiation through the sample

Absorption of radiation lowers the percentage transmittancevalue Since all bonds in an organic molecule interact withinfrared radiation, I spectra provide a considerable amount ofstructural data

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Infrared Instruments

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eferences

http55wwwacsorg

http55wwwcasorg

  http55wwwchemcenter5org

http55wwwsciencemagorg

http55wwwshuacu#5schools5sci5chem5tutorials5molspec5irspec5htm

http55www#erouacpharmu#yedu5asrg5wave5wavehphtml

http55hi(linde$gascom5international5web5lg5spg5li#elgspgnsf5Doc3yAlias5analinfra