INFRARED INFRARED SPECTROPHOTOMETRYSPECTROPHOTOMETRY

Infrared Region in Electromagnetic Infrared Region in Electromagnetic Radiation Radiation

NIR 0,78 – 2,5 µm 12800 – 4000 cm-1

MIR 2,5 – 50 µm 4000 – 200 cm-1

FIR 50 – 1000 µm 200 – 10 cm-1

General 2,5 – 15 µm 4000 – 670 cm-1

How IR Absorption Occur?How IR Absorption Occur?

Atoms in a molecule are always moving. Since there are bonds Atoms in a molecule are always moving. Since there are bonds between atoms, forward and backward moves appear, result in between atoms, forward and backward moves appear, result in vibration.vibration.

When vibration change the dipole an interaction with magnetic field of When vibration change the dipole an interaction with magnetic field of electromagnetic wave occurelectromagnetic wave occur

When electromagnetic wave frequency is the same with vibration When electromagnetic wave frequency is the same with vibration frequency, an absorption occurfrequency, an absorption occur

Vibration frequency is in the range of infrared frequencyVibration frequency is in the range of infrared frequency

Requirements of infrared absorption:Requirements of infrared absorption: * Vibration changes the dipole moment* Vibration changes the dipole moment * Electromagnetic frequency is the same as* Electromagnetic frequency is the same as vibration frequencyvibration frequency

Absorption of the energy increase vibration amplitudeAbsorption of the energy increase vibration amplitude

Type of VibrationType of Vibration

Vibration frequency and wave number

Vibration frequency, ν whwre µ = reduced mass k = Force constant (bond strength)k value = 5,0 x 105, 1,0 x 106 and 1,5 x

106 for single, double and triple bonds respectively

Wavenumber, σ (cm-1)

Substitution: π = 3,141591 c = 3,00 x 1010 cm/s m = Ar/(6,022 x 1023) g/atom

µπk

v2

1=

21

21

mm

mm

+=µ

21

21 )(

2

1

mm

mmk

c

+=π

σ

21

21 )(12,4

ArAr

ArArk +=σ

Vibration ModeVibration ModeWhen N atoms in a molecule move in space, the movements When N atoms in a molecule move in space, the movements

are projected to 3 coordinate axis, there become 3N are projected to 3 coordinate axis, there become 3N movements. movements.

When all atoms move simultaneously in a coordinate axis, the When all atoms move simultaneously in a coordinate axis, the molecule undergoes translation (not a vibration). Three molecule undergoes translation (not a vibration). Three coordinate axis will produce 3 movementscoordinate axis will produce 3 movements

When movements of all atoms cause the molecole rotates in When movements of all atoms cause the molecole rotates in a coordinate axis is also not a vibration. Three coordinate a coordinate axis is also not a vibration. Three coordinate axis would produce the same movementaxis would produce the same movement

Number of vibration = 3N – 6Number of vibration = 3N – 6In linear molecule, rotation around the linear axis does not In linear molecule, rotation around the linear axis does not

change the structure, so that is not a rotation. Hence, for change the structure, so that is not a rotation. Hence, for alinear moleculealinear molecule

Number of vibration = 3N - 5Number of vibration = 3N - 5

Can all Vibration be Shown in a Can all Vibration be Shown in a SpectrumSpectrum

1.1. Vibration of a symmetrical molecule does not Vibration of a symmetrical molecule does not change the dipole momentchange the dipole moment

2.2. Closer frequencies show as a peakCloser frequencies show as a peak3.3. Weak intensity does not produce a peakWeak intensity does not produce a peak4.4. Frequencies outside of spectrum range can not Frequencies outside of spectrum range can not

be seenbe seenAdditional peaksAdditional peaks1.1. Overtone Overtone νν = 2= 2νν, , 33νν2.2. Combination Combination νν = = νν11 + + νν22 3.3. Difference Difference νν = = νν11 - - νν22

ExampleExample

COCO22 a linear molecule with 3 atoms a linear molecule with 3 atoms

Number of vibration = 3 x 3 - 5 = 4Number of vibration = 3 x 3 - 5 = 4

Shows 2 peaks at 2330 cmShows 2 peaks at 2330 cm -1-1 and 667 cm and 667 cm -1-1

HH22O a bent molecule with 3 atomsO a bent molecule with 3 atoms

Number of vibration = 3 x 3 – 6 = 3Number of vibration = 3 x 3 – 6 = 3

Three peaks exist at 3760 cmThree peaks exist at 3760 cm -1-1, ,

3650 cm3650 cm -1 -1 and 1595 cm and 1595 cm -1-1

Typical of Infrared SpectrumTypical of Infrared Spectrum