Infrared spectroscopy Basic theory

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1 Infrared spectroscopy Basic theory Dr. Davide Ferri Paul Scherrer Institut

2 Importance of IR spectroscopy in catalysis IR Raman NMR XAFS UV-Vis EPR Number of publications Number of publications containing in situ, catalysis, and respective method Source: ISI Web of Knowledge (Sept. 2008)

3 Infrared spectroscopy Use of infrared radiation Excitation of vibrational and rotational modes (vibrational transitions) Identifies functional groups (-(C=C) n -, -C=O, -C=N, etc.) Access to molecular structure, interactions and lattice vibrations of solids pros economic non-invasive versatile (e.g. solid, liquid, gas and interfaces) very sensitive (concentration) fast acquisition (down to ns!) cons no atomic resolution

4 The electromagnetic spectrum INFRARED 1 mm-1 µm Hz source: Andor.com

5 The IR region Visible λ [µm]õ ṽ [cm -1 ] E [kj/mol] High Near overtone, combination vibrations Frequency Mid fundamental vibrations Low Far fundamental vibrations of heavy elements, lattice vibrations Microwave

6 The frequency unit Typically, the wavenumber (cm -1 ) is used The number of waves in 1 cm 1 cm Calculate the wavelength of the following light frequency a) 400 cm -1 b) 4000 cm -1 1 % = [cm -1 ] ν λ

7 IR spectroscopy is vibrational spectroscopy Pt IR radiation Ferri et al., JACS 123 (2001) 12074; Bonalumi et al., JACS 127 (2005) 8467

8 How many vibrations in a molecule? molecule linear non-linear number of vibrations 3N-5 3N-6 vibrational normal modes O=O Oxygen molecule (N=2) 1 fundamental mode Glucose (N=24) 66 fundamental modes Proteins (hemoglobin) N typically 10'000...

9 Vibrational transition potential energy v = 0 v = 1 v = 2 For each vibrational mode Fundamental band v = 0 to v = 1 Overtone bands v = 0 to v > 1 r eq interatomic distance v= vibrational number

10 The mid-ir spectrum acetone (liquid) ν(al-o) α-al 2 O θ-al 2 O 3 absorbance ν(o-h) frequency intensity γ-al 2 O γ-al 2 O C 180 C 200 C wavenumber / cm -1 Morterra, Catal. Today 27 (1996) 497

11 Which vibrations do appear in a spectrum? Selection rule µ Q 0 Molecular dipole moment µ must change due to vibration or rotation along its coordinate (so called, normal mode or normal coordinate, Q) Are these molecules infrared active or inactive? O=O H-Cl Ar

12 The H 2 O molecule N=3, non-linear, 3 fundamental modes µ µ µ 3657 cm cm cm -1 symmetric stretching asymmetric stretching scissoring (bending) All modes IR active

13 Gas and liquid phase H 2 O stretching bending Absorbance Wavenumber / cm Absorbance Wavenumber / cm -1

14 Vibrational transition (II) Rotational-vibrational transition potential energy r eq v = 0 v = 1 v = 2 interatomic distance gas phase molecules free rotation pure rotational transition microwave region v= vibrational number

15 The CO 2 molecule N=3, linear, 4 fundamental modes µ cm cm -1 asymmetric stretching scissoring (or bending) degenerate when isolated µ 1288 cm -1 symmetric stretching IR inactive 667 cm -1 scissoring (or bending)

16 The -CH 2 group 2925 cm cm cm -1 asymmetric stretching symmetric stretching scissoring / bending in-plane cm -1 twisting / bending out-of-plane 720 cm -1 rocking / bending in-plane cm -1 wagging / bending out-of-plane

17 Basic functional groups bending C-H O-H C=O C-O stretching O-H alkenes aromatics C-H C C C=C C-C wavenumber / cm -1

18 Assignments Functional groups (not complete!) aliphatic hydrocarbons aromatic compounds amines amides

19 oxygen-containing compounds nitrogen-containing compounds NIR (in nm!)

20 Frequency Real potential Fundamental band v = 0 to v = 1 potential energy v = 0 v = 1 v = 2 How can we approximate this excitation? interatomic distance v= vibrational number

21 Frequency Approximation: harmonic oscillator The stretching frequency of a bond can be approximated by Hooke s law. Two atoms and the connecting bond are treated as a harmonic oscillator composed of two masses (atoms) joined by a spring. ν = 2 1 k π µ k: force constant µ = m m m m 1 2 µ: reduced mass E 0 = ½ hν hν hν hν v = 1 v = 0 v = 2 v= vibrational number For molecules more than two atoms Normal mode analysis

22 Frequency Approximation: harmonic oscillator potential energy v = 1 v = 2 v = 0 Quite accurate for fundamental modes r eq interatomic distance v= vibrational number

23 Stretching modes ν = 1 k 2 π µ C-C C=C C C 1200 cm cm cm -1 larger k C-H C-D C-C C-O C-Cl 3000 cm cm cm cm cm -1 larger µ

24 Isotopic labeling The stretching frequency of HCl is 2890 cm -1. What is the frequency of DCl assuming the force constants (k HCl and k DCl ) are the same? H-Cl D-Cl