Theory of infra red absorption IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. 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 frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration. 1
Transcript
Theory of infra red absorptionIR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.
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 frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.
1
Theory of infra red absorptionMolecular rotations
Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions.
Molecular vibrations
The positions of atoms in a molecules are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main catagories of stretching and bending.
2
Some General Trends: • Stretching frequencies are higher than
corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.)
• Bonds to hydrogen have higher stretching frequencies than those to heavier atoms
• Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds.(Except for bonds to hydrogen).
3
Vibrations as a Key to Structure• Each type of vibration has a frequency that depends
upon the:
THE MASS OF THE VIBRATING ATOMS AND THE NATURE OF THE BOND BETWEEN
THEM• For a constant bond type (single, double or triple) the
frequency of the vibration is low for a bond between heavy atoms. Conversely, for a given bond type the frequency of vibrations is high for light atoms.
• Multiple bonds vibrate at a higher frequency than do single bonds
4
IR Spectroscopy• deal with the interaction of infrared radiation with matter
IR spectrum (%T against Frequency)• chemical nature and molecular structure of compound
IR region of the electromagnetic spectrum• wavelength 770 nm to 1000 m (wave number 12,900 to 10 cm-1)
IR region is often further subdivided into threesubregions
1.Near-infrared region (nearest to the visible)2.Mid-infrared region3.Far-infrared region
7
Table Infrared Spectral Regions
Region
Near
Middle
Far
Most used
Wavelength Range, m
wavenumberRange, cm-1
0.78 to 2.5
2.5 to 50
50 to 1000
2.5 to 15
12800 to 4000
4000 to 200
200 to 10
4000 to 670
Frequency (v)Range, Hz
3.8x1014 to 1.2x1014
1.2x1014 to 6.0x1012
6.0x1012 to 3.0x1011
1.2x1014 to 2.0x1013
8
IR Spectrum
9
Mid-infrared region
1. Group-frequency region
2. Finger print region
• wavenumber 4000 to 1300 cm-1 (2.5 to 8 m)• functional group
• wavenumber 1300 to 650 cm-1
10
Requirements for the absorption of IR radiation1. The natural frequency of vibration of the molecules must equal the frequency of the incident radiation
2. The frequency of the radiation must satisfy, E = hv,where E is the energy difference between the vibrational states involved
hEEE vibvib 2,1,
3. The change in vibration must stimulate changes in the dipole moment of the molecule
11
Requirements for the absorption of IR radiation
4. The degree of absorption is proportional to the square of the rate of change of the dipole during excitation.
5. The energy difference between the vibrational energy levels is modified by the addition or subtraction of rotational energy levels.
• Molecules with permanent dipole moments (μ) are IR active.
H + – Cl- O – O
Dipole moment No dipole moment
HCl, H2O Atoms, O2, H2, Cl2
IR active IR inactive 12
13
Molecules are made up of atoms Molecules are made up of atoms linked by chemical bonds. The linked by chemical bonds. The movement of atoms and movement of atoms and chemical bonds like spring and chemical bonds like spring and balls (vibration)balls (vibration)
What is a vibration in a molecule?
Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds
Vibrations
There are two main vibrational modes :
1.Stretching - change in bond length (higher frequency)
Stretching vibration14
Stretching Types
Symmetric Asymmetric
Bending Types
In-plane (Scissoring)
2. Bending - change in bond angle (lower frequency)
Out-plane (Twisting)
15
Scissoring or Deformation
• The two atoms connected to a central atom move toward and away from each other with deformation of the valence angle
16
Rocking or In-plane bending
• The structural unit swings back and forth in the symmetry plane of the molecule
17
Wagging or out-of-plane bending
• The structural unit swings back and forth in the plane perpendicular to the molecules symmetry plane
18
Twisting
• The structural unit rotates back and forth around the bond that joins it to the rest of the molecule
19
Modes of vibrationsModes of vibrations
Stretching: change in bond Stretching: change in bond distance. distance. Occurs at higher Occurs at higher energy: 4000-1250 cmenergy: 4000-1250 cm11..
-CH2-
H2
O
Bending: change in bond angle.Bending: change in bond angle. Occurs at lower Occurs at lower energy: 1400-666 cmenergy: 1400-666 cm11..
20
More complex types of stretching and bending are possible
Can a vibration change the dipole moment of a molecule?
Infrared active vibrations (those that absorb IR radiation) must result in a change of dipole moment
Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active.
Symmetrical stretching/bending does not. Not IR active
21
Fundamental Vibrations (Absorption Frequencies)A molecule has as many as degrees of freedom as the total degree of freedom of its individual atoms.
• Each atom has 3 degree of freedom (x,y,z)• A molecule of n atoms therefore has 3n degrees of freedom. • Non linear molecules (e.g. H2O) Vibrational degrees of freedom or Fundamental Vibrations = 3n – 6 = 3(3) - 6 = 3
HO
H HO
H HO
H
SymmetricalStretching (υs OH)
3652 cm-1
AsymmetricalStretching (υas OH)
3756 cm-1
Scissoring (δs HOH)1596 cm-1
22
For linear molecule (e.g. CO2) :Vibrational degrees of freedom or Fundamental Vibrations = 3n – 5 =
3(3)-5 = 4O C O O C O
SymmetricalStretching (υs CO2)
1340 cm-1
AsymmetricalStretching (υas CO2)
2350 cm-1
O C O O C O
Scissoring (bending outof the plane of the paper)
(δs CO2) 666 cm-1
Scissoring (bending in the plane of the paper)
(δs CO2) 666 cm-123
24
The theoretical no. of fundamental vibrations will seldom be observed because overtones (multiples of a given frequencies) and combination tones (sum of two other vibrations) increase the no. of bands. Other phenomena reduce the no. of bands including:
1. Fundamental frequencies that fall outside the 4000-400 cm-1 region.
2. Fundamental bands that are too weak to be observed.
3. Fundamental bands that are so close that they coalesce.
4. The occurrence of a degenerate band from several absorptions of the same frequency in highly symmetrical molecules.
5. The failure of certain fundamental vibrations to appear in the IR because of the lack of change in molecular dipole.
25
IR Correlation Diagram
26
Tra
nsm
itta
nce (
%)
100
80
60
40
20
04000 3500 3000 2500 2000 1500 1000
2.5 3.0 4.0 5.0 6.0 10.0 Frequency (cm-1)
Region I3600-2700 cm-1
Region II1800-1600 cm-1
/ Wavelength (microns, m)
O-H N-H C-Hbond
stretching
alcohols phenols carboxylic acids
aminesamides
alkynes alkenes alkanes
C=O
acid chloridesanhydrides
estersketones
aldehydescarboxylic
acidsamides
FingerprintRegion
(below 1500 cm-1)
C-H=C-H -C-H
Hooke’s Law states that two masses joined by a spring will vibrate such that
k
(1)
where = the frequency (rad/sec), but since 2
we have
k
2
1 (2)
27
where = the frequency of vibration, k is the force constant of the bond (dynes/cm), and is the reduced mass, or
21
21
MM
MM
(3)
where M1 is the mass of one vibrating body, M2 the mass of the other. But is in cyles per second (cps).During this time light travels a distance measured in cm/sec (I.e., the speed of light).
28
Therefore, if one divides by c, the result is the number of cycle per cm. This is , the wavenumber of an absorption peak (cm-1) and
c
(4)
It can be deduced that
k
c2
1 (5)
k
x 12103.5 (6)
29
Hooke’s Law
ύ = The vibration frequency (cm-1)c = Velocity of light (cm/s)f = force constant of bond (dyne/cm)
33• Frequency decreases with increasing atomic mass.• Frequency increases with increasing bond energy.
Stretching FrequenciesStretching Frequencies
Frequencies of various group vibrations in the group frequency region and in fingerprint region
34
Block diagram of IR spectrophotometer
source samplemonochromatordetector readout
Nernst GlowerGlobarIncandescent wire sourceHg Arc
GratingFilter
Thermal DThermocoupleThermopileThermisterBolometerPneumatic DPyroelectric D
RecorderXY plotterPrinter
35
Instrumentation
Three distinct types of instruments employed for IR absorption spectrometry1. Dispersive instruments with a
monochromator are used in the mid-IR region for spectral scanning and quantitative analysis2. Fourier transform IR systems are
widely applied in the far-IR region and becoming quite popular for mid-IR spectrometry3. Nondispersive instruments that use filters for wavelength selection or an infrared-absorbing-gas in the detection system are often used for gas analysis at specific wavelength
36
IR sources: general
• an inert solid that is heated electrically to a temperature between 1500 and 2200 K (provide continuous radiant)• the maximum radiant intensity at these temperatures occurs at between 5000 and 5900 cm-1 (2 to 1.7 m)
37
IR sourcesThe Nernst Glower (Continuous source)
• It is a useful and inexpensive source• It is composed of rare earth oxides formed into a cylinder having a diameter of 1 to 2 mm and a length of 20 mm• Platinum leads are sealed to the ends of the cylinder to permit passage of electricity; temperatures between 1200 and 2200 K results• Because of a negative temperature coefficient of electrical resistance, it must be used with ballast resistor in the heating circuit to prevent burnout
38
IR sourcesThe Nernst Glower (Continuous source)
• it is rather fragile, and its lifetime depends on the operating temperature and the care taken in handling it
(cont.)
39
IR sourcesThe globar (continuous source)• A globar is a silicon carbide rod, usually about 50 mm in length and 5 mm in diameter• current through the globar causes the rod to heat and emit radiation at temperature exceeding 1000 oC• the power consumption is normally higher than that of the Nernst Glower• water cooling is needed to cool the metallic electrodes attached to the rod• less convenient to use and more expensive because of the necessity for water cooling
40
c ) G l o b a r
5 c m
6 - 8 m m d i a .
S i C r o d h e a t e d t o 1 3 0 0 o C
w a t e r -c o o l e d b r a s s t u b e w i t h s l o t
+ t e m pc o e f f . o f r e s i s t a n c e
G 1 5 m6 5 0 c m - 1
N GG
1 0 m1 0 0 0 c m - 1
N G 1 m1 0 0 0 0 c m - 1
41
IR sources
Incandescent wire source• A source somewhat lower intensity but longer life than the Globar or Nernst glower
• It is a tightly wound spiral of nichrome wire heated to about 1100 K by an electrical current
• A rhodium-wire heater sealed in a ceramic cylinder has a similar properties as a source
42
IR sourcesThe Mercury arc
• For the far-infrared region of the spectrum 50 m)
• It provides sufficient energy for convenient detection
• It consist of a quartz-jacketed tube containing mercury vapor at a pressure greater than one atmosphere
• Passage of electricity through the vapor forms an internal plasma source that provides continuous radiation in the far-infrared region
43
IR sourcesThe Mercury arc
44
IR sources
The Tungsten filament lamp
• the near-infrared region of4000 to 12,800 cm-1
(2.5 to 0.78 m)
45
Monochromators
• To select radiation of any desired frequency from source and eliminate other frequencies.
• This is done by monochromator, which consists of a dispersion element and a slit system.
• Two types of dispersion elements are
1. Prisms
2. Gratings
46
Prisms
Requirements:• It must be transparent to IR radiation. This eliminates
common materials like glass & quartz, which are not
transparent to IR at wavelengths longer than 3.5 m. • The material used must be strong enough to be shaped &
polished.
The most common materials used to make prism are metal salts, such as– Potassium bromide
– Calcium fluoride
– Sodium chloride (Rock salt)
– Thallous bromide 47
Prisms• Sodium chloride is transparent to radiation between 2.5
and 15m. This wavelength range is sometimes termed the rock salt region.
• Potassium bromide or cesium bromide can be used over
the range 2.1 - 26m.• Calcium fluoride in the range of 2.4 – 7.7m• The prism is prepared from a single large crystal of the
metal salt which is cut and polished to the required shape.
• The larger the crystal, the better the resolution.• Surfaces must be smooth to prevent random scattering of
the radiation by the prism surfaces. 48
Prisms• The crystal must be high quality otherwise scattering of
radiation takes place and hence a loss of signal and resolution.
• The metal salts used to make prisms are water soluble, so if the surface becomes wet it dissolves. On drying it is left etched. So, it is necessary to keep the prisms dry.
49
GratingsThe advantages of gratings are:• They can be made with materials such as aluminum that
is stable in the atmosphere and not attacked by moisture.• They can be used over a wide wavelength range.• Resolution is constant over the operable range.
50
Slit systemsThe radiation from the source is dispersed by the
dispersion element, which a small part is used. This small section is separated from the rest of the beam by the exit slit.
Problems involved in IR Spectroscopy:• At longer wavelengths, the energy of each proton is
decreased.• The power from the light source drops off at long
wavelengths and the response of the detector becomes poor.
• If prisms are used, they lose transparency at longer wavlengths.
51
Slit systemsThe problem is reduced by the use of a
“PROGRAMMED SLIT SYSTEM”
In this system, the slit opens up automatically to permit a wider wavelength range to reach the detector and therefore to increase the intensity of the signal.
The advantage is that more radiation falls on the detector and the instrument responds more reliably.
The disadvantage is that use of wider slits results in an automatic loss in resolution. This may lead to interference from unresolved bands in quantitative analysis, and loss of fine structure in qualitative analysis.
52
Infrared Detectors
General types of infrared detectors:
1. Thermal Detectors
2. Pyroelectric Detectors
3. Photoconducting Detectors
Dispersive spectrophotometer
Fourier Transform IR
53
Infrared Detectors
Thermal Detectors• widely used in the IR region of the spectrum
• responses depends upon the heating effect of radiationProblem:
The problem of measuring infrared radiation by thermal means is compounded by thermal noise from surrounding
Solution:Thermal detectors are usually encapsulated and carefully shielded from thermal radiation emitted by other nearby objects 54
• a thermocouple is made by welding together at each end two wires made from different metals.• If one welded joint (called the hot junction) becomes hotter than the other joint (the cold junction), a small electrical potential develops between the joints
In IR spectroscopy, the cold junction is carefully screened in a protective box and kept at a constant temperature. The hot junction is exposed to the IR radiation, which increases the temperature of the junction. The potential difference generated in the wires is a function of the temperature difference between the junctions and, therefore, of the intensity of IR radiation falling on the hot junction.
A well-designed thermocouple detector is capable of responding to temperature difference of 10-6 K. This figure corresponds to a potential difference ofabout 6 to 8 V/W
To enhanced sensitivity, several thermocouples may be connected in series to give what is called a
‘thermopile’
57
Infrared DetectorsThermal detectors: Bolometer
A bolometer is a type of resistance thermometer constructed of thin metal conductor such as platinum wire or nickel, or from a semi conductor.IR radiation heats up this conductor which causes its electrical resistance to change.The degree of change the conductor’s resistance is a measure of the amount of radiation that has fallen on the detector.A germanium bolometer operated at 1.5 K, is an ideal detector for radiation in the 5 to 400 cm-1 (2000 to 25 mm) range.
58
Infrared DetectorsThermal detectors: Bolometer
• A bolometer is made one arm of the wheatstone bridge.• A similar strip of metal is used as the balancing arm of the bridge.• The strip is not exposed to IR radiation.• When no radiation falls on the bolometer, the bridge
remains balanced. • If radiation falls on the bolometer, the bridge becomes unbalanced due to change in the electrical resistance which causes a current to flow through the galvanometer G.• The amount of current flowing through the galvanometer is a measure of the intensity of the radiation falling on the detector.
59
Infrared DetectorsThermal detectors: Thermistor
A thermistor is made of a fused mixture of metal oxides which show a negative thermal coefficient of electrical resistance.
The thermistor is normally placed in a bridge circuit with a reference thermistor that is not irradiated. The resistance can be measured by a null-comparison method
60
Golay detector • It consists of small hollow cylinder which is closed by a blackened film at one end and by a flexible metalized diaphragm at the other.
• It is filled with a nonabsorbing gas such as xenon and sealed.
• IR radiation falls on the blackened film and heats the enclosed gas in the confined space.
•The resulting pressure increase in the gas deforms the metalized diaphragm which separates two chambers.
• Light from a lamp is made to fall on the diaphragm which reflects the light on to a photocell. 61
Golay detector • Motion of the diaphragm changes the output of cell.
• The signal seen by the phototube is modulated in accordance with the power of the radiant beam incident on the gas cell.
• The response time is about 10-2 sec, much faster than that of the bolometer, thermistor or thermocouple.
• The advantage of this detector is that its useful wavelength range is very wide.
• The response is linear.
62
Golay detector
63
Pyroelectric Transducers (PTs) or Detector• Pyroelectric substances act as temperature-dependent
capacitors and are constructed from single crystalline wafers of pyroelectric materials.
• Triglycine sulfate is sandwiched between two electrodes. One electrode is IR transparent, a temperature dependent capacitor is produced.
• The current across the electrodes is Temperature dependent.
• Changing its temperature by irradiating it with infrared radiation alters the charge distribution across the crystal, which can be detected as a current in an external electric circuit connecting the two sides of the capacitor.
64
Pyroelectric Transducers (PTs) or Detector
continued..• The magnitude of this current is proportional to the
surface area of the crystal and the rate of change of polarization with temperature.
• Pyroelectric crystals lose their residual polarization when they are heated to a temperature called the curie point.
• The curie point for triglycine sulfate is 470c.• Pyroelectric detectors exhibit fast response times,
which is why most of the FT-IR instruments uses them.
65
Pyroelectric detector
Change in detector temp. by IR absorption changes lattice spacing and polarization – charge moves.
Fast response time: 1s-1ms
Ignore steady background
TGS, DTGS Tc~50oC; LiTaO3 Tc~610oC
66
SAMPLE CELLS
• IR Spectrometry deals with the measurement of the IR spectrum.
• It is used for the characterization of solid, liquid and gas samples.
• Samples of different phases must be treated differently.
• The material used to contain the sample must always be transparent to IR radiation.
• This limits the selection to certain salts, such as Sodium chloride & Potassium bromide.
67
Solid Samples
Four techniques are available for preparing solid samples for IR spectrometry:
1. Solids run in solution:
• Solution of solid is prepared in a suitable solvent then the solution is run in one of the cells for liquids.
• When solids are in solutions, the absorption due to solvent has to be compensated by keeping the solvent in a cell of same thickness as that containing the sample.
68
Solid Samples
2. Solids films:
• Sample is deposited on the surface of a KBr or NaCl cell by evaporating a solution of the solid to dryness.
• IR radiation is then passed through the thin layer deposited.
• Useful for rapid qualitative analysis and difficult to carryout quantitative analysis.
69
Solid Samples
3. Mull technique: • The sample may be ground to a powder.
• The powder can be made into a thick slurry or mull by grinding it with a greasy, viscous liquid, such as Nujol (paraffin oil) or chlorofluorocarbon greases and spread between IR transmitting windows.
• Disadvantage is the absorption band of sample will coincide with absorption band of nujol mull.
• So, the solid sample in the nujol has to be used in combination with hexachlorobutadiene because the absorption band of nujol and hexachlorobutadiene appear in different regions, their use in combination permits the recording of IR spectrum.
• Good for qualitative studies, but not for quantitative analysis.70
Solid Samples4. Pressed pellet technique or KBr pellet method: • A finely ground solid sample is mixed with about 100 times its
weight of dried powdered potassium bromide (other alkali metal halides can also been used).
• Mixing is carried out using pestle and mortar or a small ball mill.
• The mixture is compressed under very high pressure (10,000 to 15,000 pounds per square inch) to yield a transparent disk about 1 cm in diameter and 1-2 mm thick.
• The disk is prepared in vacuum to eliminate air or moisture.
• The disk is then held in the instrument beam for spectroscopic examination.
• Being ionic, KBr transmits most of the IR region with a lower cutoff of about 400 cm-1.
• Cesium iodide absorbs below 200 cm-1 and is used because of its greater transparency at low frequencies. 71
Solid SamplesAdvantages of pellet technique over Nujol method:
• KBr pellets can be stored for long periods of time.
• As concentration of sample can be suitably adjusted in the pellets, it can be used for quantitative analysis.
• Resolution of the spectrum in KBr is superior to that obtained with mulls.
Disadvantages:
• Not suitable for some polymers which are difficult to grind with KBr.
• High pressure involved during the formation of pellets may bring about polymorphic changes in crystallinity in the samples, which may cause complications in IR spectrum.
• It always has a absorption band at 3450 cm-1 from the OH group of moisture present in the sample, so care must be taken while investigating the region of the OH band in the sample. 72
Liquid Samples• Liquid samples are poured without any preparation into
circular or rectangular cells made of NaCl, KBr or ThBr and their IR spectrum determined directly.
• The sample thickness should be so selected that the transmittance lies between 15 and 70 percent.
• For most liquids this will represent a thin layer of 0.01 – 0.05 mm in thickness.
73
Liquid SamplesFor double beam work, matched cells are used. • One cell is used to contain the sample and the other cell
to contain a reference material.• Matched cells are similar in total thickness and in cell
wall thickness.• All cells must be protected from water, because they are
water soluble.• Organic liquid samples should be dried before being
poured into the cells, otherwise the cell surfaces become opaque and cause erroneous results to be obtained.
74
Gas Samples• The gas sample cell are similar to liquid sample cell and
are made of KBr, NaCl or ThBr. • Larger cells are used for the small number of molecules
of a sample that is contained in a gas.• The cells are larger, usually they are about 10 cm long,
but they may be up to 1 mt long.• Multiple reflections can be used to make the effective
path length as long as 40 mt, so that constituents of the gas can be determined.
• The gas must not react with the cell windows or the reflecting surfaces.
• Gas analysis are performed with IR but the method is not commonly used because of its lack of sensitivity.75
An Infrared Spectrometer
76
IR Spectrometer
77
FT-IR Spectrometer• Uses an interferometer.
• Has better sensitivity.
• Less energy is needed from source.
• Completes a scan in 1-2 seconds.
• Takes several scans and averages them.
• Has a laser beam that keeps the instrument accurately calibrated.
Components
• Source
• Michelson Interferometer
• Sample
• Detector
78
Michaelson Interferometer
• Beam splitter
• Stationary mirror
• Moving mirror at constant velocity
• Motor driven Micrometer screw
• He/Ne laser; sampling interval, control mirror velocity
79
Source
Stationary mirror
Moving mirror
Sample
Detector
Beam Splitter
PMT
HeNe laser
80
Photoconducting detector
• Consists of a thin film of a semi-conducting material, such as lead sulfide, mercury/cadmium telluride or indium antimonide deposited on a nonconducting glass surface and sealed into an evacuated envelope to protect the semiconductor from the atmosphere.
• Absorption of IR radiation promotes non-conducting electrons to a higher energy conducting state, thus decreases the electrical resistance of the semiconductor.
• The voltage drop across the thin film is a measure the power of the IR radiation.
81
Photoconducting detector• A lead sulfide photoconductor for near IR can be
operated at room temperature.• Mercury/cadmium telluride can be used in the
mid-IR and far IR, but must be cooled with liquid nitrogen (77 K) to minimize thermal noise.
• It provides superior response characteristics.• Great for GC-Irs.
82
Principle of operation of FTIR spectrometerDetermines datapoint sampling interval
Photodiode
83
Advantages of FTIR
• High S/N ratios - high throughput• Rapid (<10 s)• Reproducible• High resolution (<0.1 cm-1)• Inexpensive
IR (especially FTIR) very widely used for Qualitative & quantitative analysis of
• Gases, liquids, solids
84
Comparison of Fourier Transform & Dispersive IR
SL. NO. DISPERSIVE IRFOURIER TRANSFORM IR
(FTIR)
1There are a many moving parts, resulting in mechanical slippage and wear
Only the mirror moves during an experiment
2Calibration against reference spectra is required to measure frequency
Use of a laser provides high frequency accuracy (to 0.01 cm-1)
3Stray light within the instrument causes spurious readings
Stray light does not affect the detector, since all signals are modulated
4
In order to improve resolution only a small amount of the IR beam may be allowed to pass through the slits
A much larger beam may be used at all times. Data collection is easier
85
Comparison of Fourier Transform & Dispersive IR
SL. NO. DISPERSIVE IRFOURIER TRANSFORM IR
(FTIR)
5Only radiation of a narrow frequency range falls on the detector at any one time
All frequencies of radiation fall on the detector simultaneously
6
Slow scan speeds make dispersive instruments too slow for monitoring systems undergoing rapid change
• An IR spectrum is a plot of per cent transmittance (or absorbance) against wavenumber (frequency or wavelength). A typical infrared spectrum is shown below.
88
• A 100 per cent transmittance in the spectrum implies no absorption of IR radiation. When a compound absorbs IR radiation, the intensity of transmitted radiation decreases. This results in a decrease of per cent transmittance and hence a dip in the spectrum. The dip is often called an absorption peak or absorption band.
FEATURES OF AN IR SPECTRUM
• Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb infrared radiation at different characteristic wavenumbers.
• No two molecules will give exactly the same IR spectrum (except enantiomers)
• Simple stretching: 1600-3500 cm-1
• Complex vibrations: 400-1400 cm-1, called the “fingerprint region” 89
Baseline
Absorbance/Peak
IR Spectrum
Describing IR AbsorptionsIR absorptions are described by their frequency
and appearance.• Frequency () is given in wavenumbers (cm-1)• Appearance is qualitative: intensity and
shape• conventional abbreviations:
vs very strong
s strongm medium w weakbr broadsh sharp OR shoulder
90
• In general, the IR spectrum can be split into four regions for interpretation:
• 4000 2500 cm-1: Absorption of single bonds formed by hydrogen and other elements e.g. OH, NH, CH
• 2500 2000 cm-1: Absorption of triple bonds e.g. C≡C, C≡N
• 2000 1500 cm-1: Absorption of double bonds e.g. C=C, C=O
• 1500 400 cm-1: This region often consists of many different, complicated bands. This part of the spectrum is unique to each compound and is often called the fingerprint region. It is rarely used for identification of particular functional groups.
91
Summary of IR AbsorptionsSummary of IR Absorptions
• Secondary amines give one peak• Tertiary amines give no peak
96
NH
HN
H
Hsymmetric asymmetric
N-H 3300 - 3400 cm-1
•C-H aldehyde, two peaks (both weak) ~ 2850 and 2750 cm-1
3000 divides
UNSATURATED
SATURATED
•C-H sp stretch ~ 3300 cm-1
•C-H sp2 stretch > 3000 cm-1
•C-H sp3 stretch < 3000 cm-1
The C-H stretching regionBASE VALUE = 3000 cm-1
97
• CH2 bending ~ 1465 cm-1
• CH3 bending (asym) appears near
the CH2 value ~ 1460 cm-1
• CH3 bending (sym) ~ 1375 cm-1
98
THE C-H BENDING REGIONTHE C-H BENDING REGION
The triple bond stretching region
• C N 2250 cm-1 • C C 2150 cm-1
99
===
=
The cyano group often gives a strong, sharp peak due to its large dipole moment.
The carbon-carbon triple bond gives a sharp peak, but it is often weak due to a lack of a dipole. This is especially true if it is at the center of a symmetric molecule.
R C C R
This region stretches from about 1800 to 1650 cm-1 - RIGHT IN THE MIDDLE OF THE SPECTRUM
The base value is 1715 cm-1 (ketone)
The bands are very strong !!! due to the large C=O dipole moment.
100
C=O is often one of the strongest peaks in the spectrum
THE CARBONYL STRETCHING THE CARBONYL STRETCHING REGIONREGION
CR
O
H
CR
O
O C R
O
CR
O
ClCR
O
OR'CR
O
RCR
O
NH2CR
O
OH
169017101715172517351800
1810 and 1760
BASEVALUE
acid chloride ester aldehyde
carboxylicacid amideketone
anhydride
( two peaks )
EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCYC=O IS SENSITIVE TO ITS ENVIRONMENTC=O IS SENSITIVE TO ITS ENVIRONMENT
THESE VALUES ARE WORTH LEARNING all are +/- 10 cm-1
101
Ketones are at lower frequency than Aldehydes because ofthe second electron-donating alkyl group.
Acid chlorides are at higher frequency than ketones because of the electron-withdrawing halide.
Esters are at higher frequencies than ketones due to the electron-withdrawing oxygen atom. This is more important than resonance with the electron pair on the oxygen.
Amides are at lower frequencies than ketones due to resonance involving the unshared pair on nitrogen. The electron-withdrawing effect of nitrogen is less important than the resonance.
SUMMARYSUMMARY
Note the electronegativity difference, O versus N, weights thetwo factors (resonance/ e-withdrawal) differently in esters than in amides.
Acids are at lower frequency than ketones due to H-bonding.102
The C=C stretching region
• C=C double bond at 1650 cm-1 is often weak or not even seen.
• C=C benzene ring shows peak(s) near 1600 and 1400 cm-
1 , one or two at each value - CONJUGATION LOWERS THE VALUE.
• When C=C is conjugated with C=O it is stronger and comes at a lower frequency.
103
The C-O stretching region
• The C-O band appears in the range of 1300 to 1000 cm-1
• Look for one or more strong bands appearing in this range!
• Ethers, alcohols, esters and carboxylic acids have C-O bands
104
The N=O stretching region
• N=O stretching -- 1550 and 1350 cm-1
asymmetric and symmetric stretchings
• Often the 1550 cm-1 peak is stronger than the other one
105
The C-X stretching regionThe C-X stretching region
• C-Cl 785 to 540 cm-1, often hard to find amongst the fingerprint bands!!
• C-Br and C-I appear outside the useful range of infrared
spectroscopy.
• C-F bonds can be found easily, but are not that common.
