Infrared spectroscopy

Infrared spectroscopy
• Organic molecule absorb radiation in region of infrared spectroscopy.
• Where region lies
Introduction to IR
• The region lies at wavelength longer than visible light but shorter than
microwave.
• Wavelength is inversely proportional to frequency, this governed by
relation v = c/λ, c is the speed of light.
• Energy is directly proportional to frequency. E = hv, where h is the
Planck’s constant.
• h = 6.63 x 10-34 joule second.
• c = 3 x 108 ms-1
Energy transition in each EMR region
Infrared absorption process
• Molecules after absorption of infrared radiation are excited to higher
energy state.
• A molecule absorb only selected frequency of IR radiation.
• Absorbed energy serves to increase the amplitude of vibrational
motions of bonds in a molecule.
• Dipole moment
• A dipole moment happens when there is a difference in
electronegativity between two atoms in a bond, making one side
slightly negative and the other slightly positive.
Infrared absorption process
• Not all bond capable of absorbing infrared radiation.
• Those bonds that have dipole moment that change as function of
time are capable of absorbing infrared radiation.
• Electrical dipole must change at the same frequency as the IR
radiation for energy to be transferred.
• Example
• Symmetric bond such as H2 and Cl2 do not absorb infrared radiation.
• Symmetric bond with identical or nearly identical bond on each end.
Infrared absorption process
Uses of infrared spectrum
• Two of the same type of bond, in two different compounds are in two
slightly different environment.
• So, no two molecule have same infrared spectrum so infrared
spectrum can be used as fingerprint.
• Infrared spectrum is used to determine the structural information
about a molecule.
• Each type of bond (C-H, O-H, N-H, C-C, C-O, C=O) found in small
portion of vibrational region.
• For example C-H have absorption in the range 3000 ± 150 cm-1
Modes of stretching and bending
• Simplest type of modes of vibration are stretching and bending.

• There is symmetric and asymmetric stretch, asymmetric vibration

occur at higher frequency than the symmetric stretch.
• The other terms are scissoring, rocking, wagging and twisting.
Modes of stretching and bending
• These vibration arise by excitation from the ground state to the
lowest energy excited state.
• IR spectrum become complicated because of the presence of weak
overtone, combination and difference band.
• You might observe a weak overtone band at 2v and 3v, An absorption
in infrared at 500 cm-1, may have an accompanying peak of lower
intensity at 1000 cm-1 ----- an overtone.
Bond properties and absorption trend
• Bond strength and masses of bonded atoms affect the infrared
absorption frequency.
• Let see the heteronuclear diatomic molecule.
• Oscillation of spring is determined by force constant K of the spring,
its stiffness and masses of the two bonded atom (m1 and m2). The
natural frequency of bond is given by

• It is derived from Hooke’s law and reduced mass µ is given by

Bond properties and absorption trend
• So stronger bond has larger force constant and vibrate at higher
frequency than weaker bonds. Second bonds between atoms of
higher masses vibrate at lower frequency (higher wavenumber) than
lighter atoms.

• C-H stretch occur at 3000cm-1, As atoms bonded to carbon increase in

mass, reduced mass increased the frequency of vibration decrease.
• Bending occur at lower frequency than stretching because of the
lower value of bending force constant.

• Bonds are stronger in order sp > sp2 > sp3 and C-H vibration illustrate
this.
• Resonance effect the strength and length of bond its force constant K.
where normal ketone has C=O stretching at 1715 cm-1 the ketone
conjugated with double bond absorb at lower frequency 1675 and
1680 cm-1. This is because it give C=O a more single bond character.
Infrared spectrometer
• Two type of infrared spectrometer are in common use
• Dispersive infrared spectrometer
• Fourier transform infrared spectrometer
• FT-IR provide the infrared spectrum much more rapidly than the
dispersive instrument.
Dispersive infrared spectrometer
• Instrument produces a beam of infrared radiation from a hot wire.
• Mirror divide it in to two parallel beam of equal intensity radiation,
Sample is placed in one beam and other is used as reference.
• The bean then passes through the monochromator.
• Two alternating beams reaches the thermocouple detector.
• Detector sense the ratio between the intensities of reference and
sample beams.
• So detector determine which frequencies has been absorbed and
which frequencies are unaffected by light passing through sample.
Dispersive infrared spectrometer
Dispersive infrared spectrometer
• After the signal from detector is amplified, the recorder draw the
resulting spectrum of sample on chart.
• Dispersive instrument record spectrum in frequency domain.
• It is to plot as frequency (wavenumber) as light transmitted not light
absorbed.

• Is intensity of sample , Ir intensity of reference beam.

Fourier transform spectrometer
• FT-IR produce a pattern called interferogram.
• It contain all frequencies that makeup infrared spectrum.
• Interferogram is a plot of intensity versus time (time domain
spectrum), However chemist is interested in spectrum that is plot of
intensity versus frequency (frequency domain spectrum).
• Mathematical operator Fourier transform produce a spectrum
identical to dispersive instrument.
• FT-IR get dozen of interferogram of the same sample. Fourier
transform is performed on sum of accumulated interferogram a
spectrum with better signal to noise ratio can be plotted.
• FT-IR has greater speed and sensitivity than dispersive instrument.
• Source energy passes through beam splitter placed at 45˚, it separate
them in to two perpendicular beam. One goes to fixed mirror and
other to moving mirror.
• When two beam meet, the pathlength differences cause the
constructive and destructive interferences.
• The combined beam containing these interference pattern called
interferogram.
• Interferogram is oriented toward the sample, the sample absorb all
wavelength that normally found in infrared spectrum.
• The computer compare the modified interferogram to reference laser
beam to have standard comparison.
• A mathematical operator convert it to typical infrared spectrum.
Preparation of sample for IR
• Compound must be placed in sample holder.
• Glass and plastic absorb strongly throughout the infrared region of
the spectrum.
• Cell must be constructed of ionic substances typically of sodium
bromide or sodium chloride.
• Potassium bromide has range 4000-400cm-1, it is expensive
• Sodium chloride has the range 4000-650cm-1, it is low cost, sodium
chloride began to absorb at 650cm-1 and few band appear below
650cm-1.
Preparation of sample for IR
• A drop of liquid is placed between the plates of sodium chloride or
bromide called as salt plates.
• The plate inserted in holder that fit in spectrometer.
• There are three methods of preparing solid sample, first mix the finely
ground solid with potassium bromide and press it under pressure.
• This result in KBr pallets, inserted in holder, but it absorb water.
• Create suspension of finely ground substance in mineral oil, place the
thick suspension between salt plates.
• Third method is to dissolve solid in the organic solvent carbon
tetrachloride. But it has its disadvantages.
What to look for when examining the IR
spectra
• Spectrum exhibit two strong absorption peak at 3000 and 1715 cm-1,
these are due to C-H and C=O stretching.
• C=O absorb at 1715 cm-1, its shape and intensity are also unique to
it.
• Shape and intensity help to distinguish between C=O and C=C, C=O
absorb at 1850-1630 cm-1 and C=C 1680-1620 cm-1.
• C=O absorb strongly and C=C absorb weakly.
• Shape and fine structure also give clue about the identity.
• O-H absorb at 3650-3200 cm-1, N-H absorb at 3500-3300 cm-1.
• N-H has one or two absorption band of low intensity where as O-H
has broad absorption peak.
Correlation chart and tables
• You may consult infrared correlation tables to be familiar with
frequencies at which various functional group absorb.
• But at the beginning it might be prove useful to memorize the base
values, they are only eight of them.
• You can look at correlation chart in the chapter.
How to analyze IR spectra
• First look for the C=O peak, check related to it acid, amide, anhydride,
ester, aldehyde and ketone.
• If C=O is absent, look for alcohols, amines, ethers, double bond, triple
bond, nitro group and hydrocarbons.
How to analyze IR spectra
How to analyze IR spectra
Hydrocarbons: Alkanes, Alkenes and Alkynes
• Alkanes
• C-H stretch occur at frequency less than 3000 cm-1.
• CH2: Methylene group have characteristic bending absorption at
1465 cm-1.
• CH3: Methyl group have characteristic bending absorption at 1375
cm-1.
Example
Alkene
• =C-H stretch for sp2 C-H, occur at value greater than 3000cm-1.
• =C-H out of plane bending at 1000-650 cm-1.
• C=C stretch occur at 1660-1600 cm-1. conjugation moves it toward
lower frequency.
Alkyne
• Triple bond C-H, sp C-H stretch occur near 3300 cm-1.
• C triple C stretch occur at 2150 cm-1, conjugation move that toward
lower frequency.
Different hybridized carbon and their IR
stretch
Increasing s character move the stretch
toward left.
C-H stretch region
• Acetylenic (3300 cm-1), vinylic or aromatic (>3000), aliphatic (<3000
cm-1) or aldehydic (2850 and 2750 cm-1).

• C=C stretching
• Simple alkene show C=C stretching at 1640 cm-1, C=C frequency
increases as alkyl group added to double bond.
• Monosubstituted alkene at 1640 cm-1, 1,1 disubstituted alkene at
1650 cm-1, tri and tetrasubstituted alkene near 1670 cm-1, trans
disubstituted alkene at higher frequencies.
You can read
• Conjugation effect page 39
• Ring size effect with internal double bond page 39-40
• Ring size effect with external double bond page 41
• C-H bending vibrations for alkene page 41-42
Aromatic compounds
• =C-H stretch value greater than 3000 cm-1.
• =C-H out of plane bending at 900 – 690 cm-1. These bands can be
used to assign the substituted pattern on benzene.
• C=C stretch absorption occur at 1600 and 1475 cm-1.
• Overtone and combination band appear between 2000 and 1667 cm-
1, these weak absorption can be used to assign the substitution
pattern.
Aromatic compound
Alcohols and phenols
• O-H stretch: Free O-H has sharp peak at 3650-3600 cm-1, this band
appear in combination with hydrogen bonded O-H peak when alcohol
dissolved in a solvent.
• They hydrogen bonded O-H peak is a broad peak at 3400 – 3300 cm-
1, when alcohol is not dissolved in a solvent then this the only band
present. When dissolved in a solvent then hydrogen bonded O-H
bond are present with relatively weak free on the left.
• C-O stretching vibration occur in the range 1260 – 1000 cm-1. this
band can be used to assign primary, secondary and tertiary band to
alcohols.
Alcohol and phenols
Alcohol and phenols
Ethers
• C-O stretch
• The most prominent band is due to C-O stretch, 1300 – 1000 cm-1.
• Absence of C=O and O-H is required to ensure that C-O stretch is not
due to ester or alcohol.
• Phenyl and alkyl ether give two strong bands at 1250 and 1040 cm-1.
• Aliphatic ether give one strong band at 1120 cm-1.
Ethers
Carbonyl compounds
• They are aldehyde, ketone, acid, ester, amides, acid chloride and
anhydrides.
• They absorb strongly in the range 1850 to 1650 cm-1.
• Here is the base values of various C=O stretching vibrations.
• Electronegative effect tend to draw the electron between carbon and
oxygen through its electron withdrawing effect. So, the C=O bond
become somewhat stronger.
• Like the esters
• Second resonance observed with the unpaired electron on nitrogen
atom so nitrogen is less electronegative than oxygen atom, and easily
accommodate positive charge. It introduce somewhat single bond
character to C=O and lower the frequency of absorption.
• In acid chloride, electronegative atom strengthen the C=O bond through
enhanced inductive effect shift the value to higher frequency.
• Anhydride shifted to higher frequency then the esters.
• Carboxylic acid exist in monomeric form in very dilute solutions at about
1760 cm-1.
• However, in concentrated solutions it tend to dimerize via hydrogen
bonding resulting in lowering of carbonyl frequency to 1710 cm-1.

• Ketone absorb at lower frequency then aldehyde group the electron

releasing weaken the C=O bond in the ketone.
• Decreasing the ring increase the frequency of absorption due to
increased angle strain.

• In book chapter detailed discussion of aldehyde, ketone, carboxylic

acid, ester, amide, anhydride and acid chloride is given.
Amines
• Primary amine show R-NH2 stretching in the range 3500-3300 cm-1.
• Secondary amine R2N-H show only one band in this range. Weak for
the aliphatic compound and stronger for aromatic secondary amine.
• Tertiary amine will not show any N-H stretch.
• N-H bend in the primary amine result in the broad band in the range
1640-1560 cm-1, secondary amine absorb near 1500 cm-1.
• N-H out of plane bending sometime observe near 800 cm-1.
• C-N stretch occur in the range 1350-1000 cm-1.
Example
Example
Nitriles, isocyanate, isothiocyanate and
imines
• Nitrile R-C triple bond N:
• Stretch is a medium intensity, -C=N show sharp absorption near 2250
cm-1. conjugation with double bond and aromatic ring move the
absorption toward lower frequency.
• Isocyanate
• R-N=C=O it give a broad intense absorption near 2270 cm-1.
• Isothiocyanate R-N=C=S
• It give one or two broad intense absorption near 2125 cm-1.
• Imines R2C=N-R: stretch in imine or oxime give variably intense
absorption in range 1690 1640 cm-1.
Example
Nitro compound
• -NO2:
• Aliphatic nitro compound it show asymmetric stretch strong near
1600-1530 cm-1 and symmetric stretch of medium intensity near
1390-1300 cm-1.
• Aromatic nitro compound
• Asymmetric stretch strong at 1550-1490 cm-1 and symmetric stretch
strong near 1355-1315 cm-1.
Example
Carboxylate salt, amine salt and amino acid
• There are regions for the sulphur compounds, phosphorous
compounds and alkyl halide and aryl halide as well.