Spectroscopy in Inorganic Chemistry. Vibration and Rotation Spectroscopy

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1 Spectroscopy in Inorganic Chemistry

2 Symmetry requirement for coupling combination bands and Fermi resonance 2

3 3 V cm -1 (R, IR) E' stretches v cm -1 (R) A 1 ' stretch V cm -1 (IR) A 2 " bend 482 cm -1 (R, IR) E' bends

4 4 V cm -1 (R, IR) E' stretches v cm -1 (R) A 1 ' stretch V cm -1 (IR) A 2 " bend 482 cm -1 (R, IR) E' bends V 1 + V 3 = A 1 ' E' = E' (IR) overtone 2V 2 = A 2 '' A 2 '' = A 1 ' (R) 3V 2 = = A 2 '' (IR)

5 V cm -1 (R, IR) E' stretches v cm -1 (R) A 1 ' stretch V cm -1 (IR) A 2 " bend 482 cm -1 (R, IR) E' bends 2V 2 V 1 = A 1 ' (R) Fermi resonance = A 2 '' A 2 '' = A 1 ' (R) 5

6 V 3 V 1 V 2 V 4 6

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8 Vibrational coupling Acetylene C-C C-H C-H 3287 cm -1 C-D 2427 cm -1 8

9 Fundamental vibrational modes of C2HD Fundamental vibrational modes of acetylene 9

10 Microwave spectroscopy 10

11 Microwave spectroscopy Rotational spectroscopy Far IR & microwave Resolution: microwave 10-8 cm -1 (fingerprint) IR 1 cm -1 11

12 Microwave spectroscopy Fine and hyperfine structures (splittings due to large amplitude motions, quadrupole coupling, spin-spin or spin-rotation interaction) Internal motion Torisional barrier frequency and energy Exact Dipole moment determination Low frequency vibrational modes (C 3, PAH s) Intervibrational transitions: C 2 H 4 12

13 Low pressure gas phase experiments: 10 mtorr Polar molecules (ground state) 13

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15 tant, 5.) Using B=ħ22I, the rotational constant, B=ħ22I, the energy is further simplified: E=BJ(J+1) Diatomic Molecules moment of inertia rotational constant rigid rotor model nonrigid rotor model 15

16 Diatomic Molecules nonrigid rotor model the initial rotational energy state the initial rotational energy state Using the selection rule ΔJ= ±1 16

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18 Selection rules: dipole moment ΔJ= ±1 J=0 to j=1 I: calculated 18

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20 rigid rotor model nonrigid rotor model 20

21 Stark effect In the presence of a static external electric field the 2J+1 degeneracy of each rotational state is partly removed. The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to presence of an external electric field. 21

22 Zeeman effect splitting a spectral line into several components in the presence of a static magnetic field. 22

23 Rotational Raman spectra Molecules with anisotropic polarizability show pure rotational Raman spectra CH 4 CH 3 Cl The selection rule for rotational Raman Spectra is J = ± 2 the lines which are at frequencies less than that of exciting line with J = + 2 are called as Stokes lines the lines which are at frequencies higher than that of exciting line with J = - 2 are called as anti-stokes lines. 23

24 The separation between the Stokes lines becomes: 24

25 O 2 25

26 Application of IR and Raman spectroscopy IR FTIR: Fast scanning process measuring all of the infrared frequencies simultaneously employed a very simple optical device called an interferometer Low cost Without filter Mechanical Simplicity Sensitivity 26

27 r 1 r r 1 =r 2 ave r=0 r 2 ave Mirror r 2 =r 2 ave +λ/4 180 out of phase beam r 2 ave +λ/2 constant velocity T :as the time required for mirror to move distance of λ/2 f: Interfreogram frequency 27

28 Michelson Interferometer 28

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31 W 31

32 Absorbance IR Transmission IR Reflectance IR: specular reflectance diffuse reflectance IR-External reflection spectroscopy Photoacoustic Fourier transform infrared spectroscopy (PAS) 32

33 Absorbance IR 33

34 Transmission IR 34

35 Reflectance IR: specular reflectance diffuse reflectance diffuse reflectance Solids and powders, diluted in a IR transparent matrix if needed Information provided is from the bulk matrix. specular reflectance Sample must be reflective or on a reflective surface Information provided is from the thin layers 35

36 IR-External reflection spectroscopy 36

37 Photoacoustic Fourier transform infrared spectroscopy (PAS) 37

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44 In liquid and solid the collusion cause to elimination of fine structure. (collision occur before rotation) IR, near IR, UV, Vis Lattice vibrations in solids. 44

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