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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.

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slide2

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
slide3

IR region of the electromagnetic spectrum

  • wavelength 770 nm to 1000 mm
  • (wave number 12,900 to 10 cm-1)

IR region is often further subdivided into three

subregions

  • Near-infrared region (nearest to the visible)
  • Mid-infrared region
  • Far-infrared region
slide4

Table Infrared Spectral Regions

wavenumber

Range, cm-1

Region

Wavelength (l)

Range, mm

Frequency (v)

Range, Hz

Near

0.78 to 2.5

12800 to 4000

3.8x1014 to 1.2x1014

Middle

2.5 to 50

4000 to 200

1.2x1014 to 6.0x1012

Far

50 to 1000

200 to 10

6.0x1012 to 3.0x1011

Most used

2.5 to 15

4000 to 670

1.2x1014 to 2.0x1013

slide6

Mid-infrared region

1. Group-frequency region

  • wavenumber 4000 to 1300 cm-1 (2.5 to 8 mm)
  • functional group

2. Finger print region

  • wavenumber 1300 to 650 cm-1
  • เกิดจากโครงสร้างของโมเลกุลที่สมบูรณ์
slide7

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 radiation

UV-visible electronic transition

infrared vibration, rotation

slide8

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’

slide9

2. The frequency of the radiation must satisfy, E = hv,

where E is the energy difference between the

vibrational states involved

3. The change in vibration must stimulate changes in

the dipole moment of the molecule

IR active / IR inactive

Requirements for the absorption of IR radation

1. The natural frequency of vibration of the molecules

must equal the frequency of the incident radiation

slide10

Types of Molecular Vibrations

IR Vibration of bonds

  • Stretching
  • Bending

Stretching vibration

เกี่ยวข้องกับการเปลี่ยนแปลงความยาวระหว่างอะตอม

ที่เกิดพันธะกัน

  • Symmetric stretching
  • Asymmetric stretching
slide11

Methylene

Symmetric stretching

(~2853 cm-1)

Asymmetric stretching

(~2926 cm-1)

slide12

Bending vibration

การเปลี่ยนแปลงมุมระหว่างสองพันธะ

  • Scissoring
  • Rocking
  • Wagging
  • Twisting
slide13

In plane

Out of plane

Bending

slide14

Vibrational mode

of methylene

group

slide15

Number of Vibrational Modes

Nonlinear molecule

Fundamental vibrational modes = 3N-6

Linear molecule

Fundamental vibrational modes = 3N-5

slide16

Nonlinear molecule: ็H2O

Vibrational modes = 3(3) - 6 = 3

slide17

Linear molecule: CO2

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

slide18

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

slide19

Hooke’s Law states that two masses joined by a spring

will vibrate such that

(1)

where = the frequency (rad/sec), but since

we have

(2)

slide20

where = the frequency of vibration, k is the force

constant of the bond (N/cm), and is the reduced mass,

or

(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).

slide21

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

(4)

It can be deduced that

(5)

(6)

slide22

Example

Calculate the approximate wavenumber and wavelength

of the fundamental absorption peak due to the stretching

vibration of a carbonyl group C=O

The mass of the carbon atom in kg is given by

slide23

Similar, for oxygen

and the reduced mass m is given by

The force constant for the typical double bond is about

1x103 N/cm. Substituting this value and m into eq. (5) gives

slide24

The carbonyl stretching band is found experimentally

to be in the region of 1600 to 1800 cm-1 (6.3 to 5.6 mm)

slide25

Frequencies of various group vibrations in the group

frequency region and in fingerprint region

slide26

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

slide27

Instrumentation

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

slide28

Block diagram of IR spectrophotometer

readout

detector

source

sample

monochromator

Recorder

XY plotter

Printer

Grating

Filter

Thermal D

Thermocouple

Thermopile

Thermister

Bolometer

Pneumatic D

Pyroelectric D

Nernst Glower

Globar

Incandescent wire source

Hg Arc

slide29

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 mm)
slide30

IR sources

The 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
slide31

IR sources

The Nernst Glower (Continuous source)

(cont.)

  • it is rather fragile, and its lifetime depends on the
  • operating temperature and the care taken in handling it
slide32

IR sources

The Nernst Glower (Continuous source)

slide33

IR sources

The 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
slide34

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
slide35

IR sources

The Mercury arc

  • for the far-infrared region of the spectrum (l> 50 mm)
  • 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
slide36

IR sources

The Mercury arc

slide37

IR sources

The Tungsten filament lamp

  • the near-infrared region of
  • 4000 to 12,800 cm-1
  • (2.5 to 0.78 mm)
slide38

Infrared Detectors

General types of infrared detectors:

1. Thermal Detectors

Dispersive

spectrophotometer

2. Pyroelectric Detectors

3. Photoconducting Detectors

Fourier Transform multiplex

instrument

slide39

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

slide40

Infrared Detectors

Solution:

Thermal detectors are usually encapsulated and

carefully shielded from thermal radiation emitted

by other nearby objects

slide41

Metal A

Metal B

welded junction

(hot)

welded junction

(cold)

Infrared Detectors

Thermal 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
slide42

Infrared Detectors

Thermal 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.

slide43

Infrared Detectors

Thermal 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 of

about 6 to 8 mV/mW

To enhanced sensitivity, several thermocouples

may be connected in series to give what a called a

‘thermopile’

slide44

Infrared Detectors

Thermal 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