This document discusses the basics of infrared spectroscopy. It explains that infrared absorption spectroscopy works because infrared radiation can induce molecular vibrations and rotations if the radiation frequency matches the natural frequency of the vibration or rotation. It also describes the different types of molecular transitions that occur in infrared spectroscopy, including rotational, vibrational-rotational, and vibrational transitions. Finally, it provides an overview of common infrared radiation sources and detectors.

1. ORGANIC
SPECTROSCOPY
BASICS OFBASICS OF
INFRAREDINFRARED
BY: MAHMOUD GALAL
ZIDAN FARAG
Mahmoud Galal Zidan
1
Copyright 2016 © All right
reversed to Mahmoud Zidan

2. THEORY OF INFRARED
ABSORPTION SPECTROSCOPY
•• IR photons have low energy. The only transitions that haveIR photons have low energy. The only transitions that have
comparable energy differences are molecular vibrations andcomparable energy differences are molecular vibrations and
rotations.rotations.
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3. THEORY OF INFRARED ABSORPTION
SPECTROSCOPY
•• In order for IR absorbance to occur two conditions must be met:In order for IR absorbance to occur two conditions must be met:
1.1. There must be a change in the dipole moment of the molecule asThere must be a change in the dipole moment of the molecule as
a result of a molecular vibration (or rotation). The change (ora result of a molecular vibration (or rotation). The change (or
oscillation) in the dipole moment allows interaction with theoscillation) in the dipole moment allows interaction with the
alternating electrical component of the IR radiation wave.alternating electrical component of the IR radiation wave.
Symmetric molecules (or bonds) do not absorb IR radiation sinceSymmetric molecules (or bonds) do not absorb IR radiation since
there is no dipole moment.there is no dipole moment.
2.2. If the frequency of the radiation matches the natural frequency ofIf the frequency of the radiation matches the natural frequency of
the vibration (or rotation), the IR photon is absorbed and thethe vibration (or rotation), the IR photon is absorbed and the
amplitude of the vibration increases.amplitude of the vibration increases.
Mahmoud Galal Zidan
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4. THEORY OF INFRARED ABSORPTION
SPECTROSCOPY
•• In order for IR absorbance to occur two conditions must be met:In order for IR absorbance to occur two conditions must be met:
1.1. There must be a change in the dipole moment of the molecule asThere must be a change in the dipole moment of the molecule as
a result of a molecular vibration (or rotation). The change (ora result of a molecular vibration (or rotation). The change (or
oscillation) in the dipole moment allows interaction with theoscillation) in the dipole moment allows interaction with the
alternating electrical component of the IR radiation wave.alternating electrical component of the IR radiation wave.
Symmetric molecules (or bonds) do not absorb IR radiation sinceSymmetric molecules (or bonds) do not absorb IR radiation since
there is no dipole moment.there is no dipole moment.
2.2. If the frequency of the radiation matches the natural frequency ofIf the frequency of the radiation matches the natural frequency of
the vibration (or rotation), the IR photon is absorbed and thethe vibration (or rotation), the IR photon is absorbed and the
amplitude of the vibration increases.amplitude of the vibration increases.
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5. ∆∆E = hE = hνν
•• There are three types of molecular transitions that occur in IRThere are three types of molecular transitions that occur in IR
a)a) Rotational transitionsRotational transitions
•• When an asymmetric molecule rotates about its center of mass, theWhen an asymmetric molecule rotates about its center of mass, the
dipole moment seems to fluctuate.dipole moment seems to fluctuate.
•• ∆∆E for these transitions correspond toE for these transitions correspond to νν < 100 cm< 100 cm-1-1
•• Quite low energy, show up as sharp lines that subdivide vibrationalQuite low energy, show up as sharp lines that subdivide vibrational
peaks in gas phase spectra.peaks in gas phase spectra.
b)b) Vibrational-rotational transitionsVibrational-rotational transitions
•• complex transitions that arise from changes in the molecular dipolecomplex transitions that arise from changes in the molecular dipole
moment due to the combination of a bond vibration and molecularmoment due to the combination of a bond vibration and molecular
rotation.rotation.
c)c) Vibrational transitionsVibrational transitions
•• The most important transitions observed in qualitative mid-IRThe most important transitions observed in qualitative mid-IR
spectroscopy.spectroscopy.
•• νν = 13,000 – 675 cm= 13,000 – 675 cm-1-1
(0.78 – 15(0.78 – 15 µµM)M)
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6. Vibrational Modes
1.1. StretchingStretching -- the rhythmic movement along a bond axisthe rhythmic movement along a bond axis
wit a subsequent increase and decrease in bond length.wit a subsequent increase and decrease in bond length.
2.2. BendingBending -- a change in bond angle or movement of a group ofa change in bond angle or movement of a group of
atoms with respect to the rest of the molecule.atoms with respect to the rest of the molecule.
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7. THE VIBRATIONAL MODES OF
WATER
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8. Mahmoud Galal Zidan
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9. Mechanical Model of Stretching Vibrations
1.1. Simple harmonic oscillator.Simple harmonic oscillator.
•• Hooke’s Law (restoring force of a spring is proportional to theHooke’s Law (restoring force of a spring is proportional to the
displacement)displacement)
F = -F = -kyky
Where:Where: FF = Force= Force
kk == Force ConstantForce Constant
(stiffness of spring)(stiffness of spring)
yy = Displacement= Displacement
•• Natural oscillation frequency of a mechanical oscillator depends on:Natural oscillation frequency of a mechanical oscillator depends on:
a)a) mass of the objectmass of the object
b)b) force constant of the spring (bond)force constant of the spring (bond)
•• The oscillation frequency is independent of the amount of energyThe oscillation frequency is independent of the amount of energy
imparted to the spring.imparted to the spring.
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10. •• Frequency of absorption of radiation can be predicted with a modifiedFrequency of absorption of radiation can be predicted with a modified
Hooke’s Law.Hooke’s Law.
Where:Where: νν = wavenumber of the abs. peak (cm= wavenumber of the abs. peak (cm-1-1
))
cc = speed of light (3 x 10= speed of light (3 x 101010
cm/s)cm/s)
kk == force constantforce constant
µµ = reduced mass of the atoms= reduced mass of the atoms
2
1
2
1






=
µπ
ν
k
c
yx
yx
MM
MM
+
•
=µ Where:Where: MMxx = mass of atom x in kg= mass of atom x in kg
MMyy = mass of atom y in kg= mass of atom y in kg
•• Force constants are expressed in N/m (N = kg•m/sForce constants are expressed in N/m (N = kg•m/s22
))
-- Range from 3 x 10Range from 3 x 1022
to 8 x 10to 8 x 1022
N/m for single bondsN/m for single bonds
-- 500 N/m is a good average force constant for single bonds when500 N/m is a good average force constant for single bonds when
predictingpredicting k.k.
-- kk == nn(500 N/m) for multiple bonds where(500 N/m) for multiple bonds where nn is the bond orderis the bond order
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11. Example 1:Example 1: Calculate the force constant of the carbonyl bond in theCalculate the force constant of the carbonyl bond in the
following spectrum.following spectrum.
Example 2:Example 2: Predict the wavenumber of a peak arising from a nitrilePredict the wavenumber of a peak arising from a nitrile
stretch.stretch.
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12. Anharmonic oscillatorsAnharmonic oscillators
•• In reality, bonds act as anharmonic oscillators because as atoms getIn reality, bonds act as anharmonic oscillators because as atoms get
close, they repel one another, and at some point a stretched bondclose, they repel one another, and at some point a stretched bond
will break.will break.
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13. IR Sources and Detectors
SourcesSources -- inert solids that heat electrically to 1500 – 2200 K.inert solids that heat electrically to 1500 – 2200 K.
•• EmitEmit blackbody radiationblackbody radiation produced by atomic and molecular oscillationsproduced by atomic and molecular oscillations
excited in the solid by thermal energy.excited in the solid by thermal energy.
•• The inert solid “glows” when heated.The inert solid “glows” when heated.
•• Common sources:Common sources:
1.1. Nernst glowerNernst glower -- constructed of a rod of a rareconstructed of a rod of a rare
earth oxide (lanthanide) with platinum leads.earth oxide (lanthanide) with platinum leads.
2.2. GlobarGlobar -- Silicon carbide rod with water cooled contactsSilicon carbide rod with water cooled contacts
to prevent arcing.to prevent arcing.
3.3. Incandescent wireIncandescent wire -- tightly wound wire heatedtightly wound wire heated
electrically. Longer life but lower intensity.electrically. Longer life but lower intensity.
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14. DetectorsDetectors – measure minute changes in temperature.– measure minute changes in temperature.
1.1. Thermal transducerThermal transducer
•• Constructed of a bimetal junction, which has a temperature dependantConstructed of a bimetal junction, which has a temperature dependant
potential (V). (similar to a thermocouple)potential (V). (similar to a thermocouple)
•• Have a slow response time, so they are not well suited to FT-IR.Have a slow response time, so they are not well suited to FT-IR.
2.2. Pyroelectric transducerPyroelectric transducer
•• Constructed of crystalline wafers of triglycine sulfate (TGS) that have aConstructed of crystalline wafers of triglycine sulfate (TGS) that have a
strong temperature dependent polarization.strong temperature dependent polarization.
•• Have a fast response time and are well suited for FT-IR.Have a fast response time and are well suited for FT-IR.
3.3. Photoconducting transducerPhotoconducting transducer
•• Constructed of a semiconducting material (lead sulfide,Constructed of a semiconducting material (lead sulfide,
mercury/cadmium telluride, or indium antimonide) deposited on a glassmercury/cadmium telluride, or indium antimonide) deposited on a glass
surface and sealed in an evacuated envelope to protect thesurface and sealed in an evacuated envelope to protect the
semiconducting material from the environment.semiconducting material from the environment.
•• Absorption of radiation promotes nonconducting valence electrons to aAbsorption of radiation promotes nonconducting valence electrons to a
conducting state, thus decreasing the resistance (conducting state, thus decreasing the resistance (ΩΩ) of the semiconductor.) of the semiconductor.
•• Fast response time, but require cooling by liquid NFast response time, but require cooling by liquid N22..
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15. •• Collect data in the time domain and convert to the frequency domain byCollect data in the time domain and convert to the frequency domain by
Fourier Transform.Fourier Transform.
Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers
•• Detectors are not fast enough to respond to power variations at highDetectors are not fast enough to respond to power variations at high
frequency (10frequency (101212
to 10to 101515
Hz) so the signal is modulated by aHz) so the signal is modulated by a MichelsonMichelson
interferometerinterferometer to a lower frequency that is directly proportional to the highto a lower frequency that is directly proportional to the high
frequency.frequency.Mahmoud Galal Zidan
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16. 1.1. Michelson InterferometerMichelson Interferometer
B.B. Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers
•• The source beam is split into twoThe source beam is split into two
beams.beams.
•• One beam goes to a stationaryOne beam goes to a stationary
mirror and the other goes to amirror and the other goes to a
moveable mirror.moveable mirror.
•• Movement of the mirror at aMovement of the mirror at a
constant rate and recombination ofconstant rate and recombination of
the two beams results in a signalthe two beams results in a signal
that is modulated by constructivethat is modulated by constructive
and destructive interferenceand destructive interference
((InterferogramInterferogram).).
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17. Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers
•• The frequency of theThe frequency of the
radiation (radiation (νν) is directly) is directly
related to the frequencyrelated to the frequency
of the interferogram (of the interferogram (ff).).
ν
ν
c
f m2
=
νν = frequency of radiation= frequency of radiation
ff = frequency of inteferogram= frequency of inteferogram
ννmm = velocity of the mirror= velocity of the mirror
cc = speed of light (3.00 x 10= speed of light (3.00 x 101010
cm/s)cm/s)
•• FT-IR spectrometers use a polychromatic source and collect the entireFT-IR spectrometers use a polychromatic source and collect the entire
spectrum simultaneously and decode the spectrum by Fourier Transform.spectrum simultaneously and decode the spectrum by Fourier Transform.
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18. Mahmoud Galal Zidan
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19. Mahmoud Galal Zidan
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20. 2.2. FT-IR instrumentFT-IR instrument
Multiplexing (FT) SpectrometersMultiplexing (FT) Spectrometers
•• Mirror length of travel rangesMirror length of travel ranges
from 1 to 20 cm.from 1 to 20 cm.
•• Use multiple scans and signalUse multiple scans and signal
averaging to improve S/N.averaging to improve S/N.
•• Scan rates from 0.1 to 10 cm/sScan rates from 0.1 to 10 cm/s
•• Detectors are usually pyroelectricDetectors are usually pyroelectric
or photoconducting.or photoconducting.
•• Cost $10,000 - $20,000Cost $10,000 - $20,000
•• Have virtually replacedHave virtually replaced
dispersive instruments.dispersive instruments.
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21. Performance CharacteristicsPerformance Characteristics
•• Range:Range: 7800 to 350 cm7800 to 350 cm-1-1
(less expensive)(less expensive)
25,000 to 10 cm25,000 to 10 cm-1-1
(Near to far IR, expensive)(Near to far IR, expensive)
•• Resolution:Resolution: 8 cm8 cm-1-1
to 0.01 cmto 0.01 cm-1-1
•• Qualitative:Qualitative: Very good, functional groups areVery good, functional groups are
identifiableidentifiable
•• Quantitative:Quantitative: Dispersive – poorDispersive – poor
FTIR - fairFTIR - fair
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22. BE CONTINUED
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23. FOR CONTACT
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M7moud.zidan@gmail.com
Zidan4@live.com
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Tel. : +201273937378
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Copyright 2016 © All right reversed to Mahmoud Zidan
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