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Practical FTIR analysis Introduction

David Cameron

Professor of Material Technology

Advanced Surface technology Research Laboratory (ASTRaL)

University of Lappeenranta

Finland

• Basics of FTIR spectroscopy

• Identification of structural features

• Some practical examples

• Exercises

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Outline

• Change in molecule dipole moment during vibration can absorb energy from an incident electromagnetic wave.

• Only certain vibrations able to absorb energy, depends on selection rules

• Selection rules depend on molecule symmetry - shape of molecule

• Vibrational frequency can be approximated by ball and spring model

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IR absorption

mass of atom 1 m1

mass of atom 2 m2

interatomic bond

• Simply, vibration frequency of molecule depends on Hooke’s Law

ν = fundamental vibrational frequency

κ = force constant

μ = reduced mass

• Weak bonds = lower frequency

• Heavier atoms = lower frequency

• Frequency is also affected by the adjacent atoms

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Vibrational frequency

2

1

21

21

mm

mm

• Number of vibrational modes for a molecule with N atoms

= 3N – 6 (non-linear molecule)

= 3N – 5 (linear molecule)

• For many molecules, some of these modes appear at same frequency or not IR active.

• For methane CH4, 9 modes but fewer visible

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Vibrational modes

methane CH4

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Vibrational modes

As the molecule vibrates, its dipole moment may change. If it is irradiated with an electromagnetic wave which is

oscillating at the same frequency as the dipole moment is changing, it can absorb energy from the radiation.

The dipole moment comes from the different charges on the atoms and the changing distance between them.

C

(+) O (-) O (-)

symmetrical stretching

no change in dipole moment

C

(+) O (-) O (-)

asymmetrical stretching

change in dipole moment

net dipole moment

Which modes are visible depends on selection rules which relate to the symmetry of the molecule.

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C-H stretch modes

C-H bend modes

rotational modes

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Measurement set-up

gas in gas out

IR beam

gas measurement cell

IR transparent window

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Aliphatic hydrocarbon

H - C C -H

single, double or triple bonded

Aromatic hydrocarbon

ring structures

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Comparison of molecules

octane 2 CH3 groups 6 CH2 groups

both C8H18

Vibrational frequency of C-H bond depends on local environment frequency

higher lower

C-H in CH3 (methyl) C-H in CH2 (methylene) C-H in CH (methyne)

ratio of CH3 / CH2 vibration absorption should be higher in (b) than (a)

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Previous Next

iso-octane 5 CH3 groups 1 CH2 groups 1 CH group

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Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Peak bond

2970 C-H in CH3 asym. stretch

2920-2935 C-H in CH2 asym. stretch

2880-2890 C-H in CH3 sym. stretch

1466 C-H in CH3 asym. bend C-H in CH2

bend

1370-1375 C-H in CH3 sym. bend

lower wavenumber

C-C skeletal modes

2970 2935 2880

1466

1385

1350

1184

2970 2920

2890

1472

1370

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Unsaturated C – C bonds increase the vibrational frequency of C-H groups

C = C C C aromatic compounds

vinyl

vinylidene

alkenyl

aryl

conjugated

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Unsaturated hydrocarbon 1-hexene

= C-H 3075

-C-H bands

C=C stretch 1620

vinyl C-H in-planebend 1440 vinyl C-H out of plane bend

896

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aryl

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Aromatic compounds

Multiple bands above 3000 cm-1 (weaker than aliphatic compounds) Can be complex, structure determined by position of groups round ring

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Acetylenic groups

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Functional groups

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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OH groups

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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C-O bonding

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Amine and amino groups

Interpretation of Infrared Spectra, A Practical Approach, J Coates in Encyclopedia of Analytical Chemistry, R.A. Meyers (Ed.) pp. 10815–10837 Ó John Wiley & Sons Ltd, Chichester, 2000

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Other groups

>C=O

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Example : amyl acetate

CH3 stretch

C=O stretch

C-O stretch

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Other groups