Infrared Absorption Spectroscopy

IR Spectroscopy• deal with the interaction of infrared radiation with matter

IR spectrum (%T against Frequency)• chemical nature and molecular structure of cpd

Applications• organic materials• polyatomic inorganic molecules• organometallic compounds

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

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

IR Spectrum

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

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Infrared Spectrometry• useful for quantitative analysis, although it is considerably more difficult to achieve accurate and precise results with IR spectrometry than with UV-visible methods

• Beer’s Law provides the basis of quantitative IR method as it does in UV-visible spectrophotometry

Electromagnetic radiationUV-visible electronic transitioninfrared vibration, rotation

Basis of Infrared Absorption

The IR spectrum can be obtained with gas-phase or with condensed-phase molecules.

For gas-phase, molecules vibration-rotation spectra are observed.

For condensed-phase, the rotaional structure is lost.

‘Vibrational spectroscopy’

Requirements for the absorption of IR radation1. 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

IR active / IR inactive

Types of Molecular Vibrations

IR Vibration of bonds

1.Stretching2.Bending

Stretching vibration

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1.Symmetric stretching2.Asymmetric stretching

C

H

H

C

H

H

Symmetric stretching

Asymmetric stretching

Methylene

(~2853 cm-1)

(~2926 cm-1)

Bending vibration

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In plane

Out of plane

Bending

Vibrational mode of methylene group

Number of Vibrational Modes

Fundamental vibrational modes = 3N-6 Nonlinear molecule

Linear molecule

Fundamental vibrational modes = 3N-5

Nonlinear molecule: ็H2O

Vibrational modes = 3(3) - 6 = 3

Linear molecule: CO2

Vibrational modes = 3N-5 = 3(3)-5 = 4

Molecular Vibration

A molecule is made up ofa number of atoms joined by chemical bonds. Such atoms vibrate about each other in the same way as weights held together by springs

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)

where = the frequency of vibration, k is the force constant of the bond (N/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).

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)

Example

Calculate the approximate wavenumber and wavelength of the fundamental absorption peak due to the stretching vibration of a carbonyl group C=O

k

x 12103.5

The mass of the carbon atom in kg is given by

kgx

atomxmolatomsx

molkgxM

2610

23

3

1

0.2

1/1002.6

/1012

Similar, for oxygen

kgxx

xM 26

23

3

2 107.2)1002.6(

)1016(

and the reduced mass is given by

21

21

MM

MM

kgx

kgx

kgxxkgx

26101.1

10)7.20.2(

107.2100.226

2626

The force constant for the typical double bond is about 1x103 N/cm. Substituting this value and into eq. (5) gives

13

26

312

106.1

101.1

/101/103.5

cmx

kgx

cmNxcmsx

The carbonyl stretching band is found experimentally to be in the region of 1600 to 1800 cm-1 (6.3 to 5.6 m)

Frequencies of various group vibrations in the group frequency region and in fingerprint region

Instrumentation

Three distinct types of instruments employed for IR absorption spectrometry

1. Dispersive instruments with a monochromator are used in the mid-IR region for spectral scanning and quantitative analysis

2. Fourier transform IR systems are widely applied in the far-IR region and becoming quite popular for mid-IR spectrometry

3. 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

Instrumentation

Block diagram of IR spectrophotometer

source samplemonochromatordetector readout

Nernst GlowerGlobarIncandescent wire sourceHg Arc

GratingFilter

Thermal DThermocoupleThermopileThermisterBolometerPneumatic DPyroelectric D

RecorderXY plotterPrinter

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)

IR sourcesThe Nernst Glower (Continuous source)

• useful and inexpensive source• rare earth oxides formed into a cylinder having a diameter of 1 to 2 mm and a length of perhaps 20 mm• platinum leads are sealed to the end of the cylinder to permit passage of electricity; temperatures between 1200 and 2200 K result• because of a negative temperature coefficient of resistance, it must be used with ballast resistor in the heating circuit to prevent burnout

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.)

IR sourcesThe Nernst Glower (Continuous source)

IR sourcesThe globar (continuous source)• 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

IR sources

Incandescent wire source• somewhat lower intensity but longer life than the Globar or Nernst glower

• 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

IR sourcesThe Mercury arc

• for the far-infrared region of the spectrum 50 m)

• provide sufficient energy for convenient detection

• consist of a quartz-jacketed tube containing mercury vapour at a pressure greater than one atmosphere

• passage of electricity through the vapour forms an internal plasma source that provides continuous radiation in the far-infrared region

IR sourcesThe Mercury arc

IR sources

The Tungsten filament lamp

• the near-infrared region of4000 to 12,800 cm-1

(2.5 to 0.78 m)

Infrared Detectors

General types of infrared detectors:

1. Thermal Detectors

2. Pyroelectric Detectors

3. Photoconducting Detectors

Dispersive spectrophotometer

Fourier Transform multiplex instrument

Infrared Detectors

Thermal Detectors• widely used in the IR region of the spectrum

• responses depends upon the heating effect of radiation

Problem:

The problem of measuring infrared radiation by thermal means is compounded by thermal noise from surrounding

Infrared Detectors

Solution:

Thermal detectors are usually encapsulated and carefully shielded from thermal radiation emitted by other nearby objects

Infrared DetectorsThermal detectors: Thermocouples

• 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

Metal A

Metal B welded junction(cold)

welded junction(hot)

Infrared DetectorsThermal detectors: Thermocouples

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.

Infrared DetectorsThermal detectors: Thermocouples

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 a called a

‘thermopile’

Infrared DetectorsThermal detectors: Thermistor/Bolometer

A bolometer is a type of resistance thermometer constructed of strips of metals such as platinum or nickel, or from a mixture of metal oxide; the latter devices are sometimes called thermistors. These materials exhibit a relatively large change in resistance as a function of Temperature.

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