introduction to infrared spectroscopy
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7/25/2019 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$infrared is energetic enough to e&cite molecularvibrations to higher energy levels
The wavelength of infrared absorption bands is characteristic
of specific types of chemical bonds, and infrared
spectroscopy finds its greatest utility for identification oforganic and organometallic molecules The high selectivity
of the method ma#es the estimation of an analyte in a
comple& matri& possible
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+&le of I
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Theory of Infrared Absorption
Spectroscopy -or a molecule to absorb I, the vibrations or rotations
within a molecule must cause a net change in the dipole
moment of the molecule The alternating electrical field ofthe radiation %remember that electromagnetic radiation
consists of an oscillating electrical field and an oscillating
magnetic field, perpendicular to each other' interacts with
fluctuations in the dipole moment of the molecule If the fre(uency of the radiation matches the vibrational
fre(uency of the molecule then radiation will be absorbed,
causing a change in the amplitude of molecular vibration
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*olecular otations
otational transitions are of little use to the
spectroscopist otational levels are(uanti.ed, and absorption of I by gases
yields line spectra
/owever, in li(uids or solids, these lines
broaden into a continuum due to molecular
collisions and other interactions
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*olecular otations %cont'
,
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0ibrational$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