Vibrational spectroscopy is an energy sensitive method and is used to

(1)

Infrared Spectroscopy (IR)

Vibrational spectroscopy is an energy sensitive method and is used to

characterize compounds in terms of the strengths and number of the bonds present. One can get/detect:

the presence of known compounds (finger print)

the components of an unknown compound (functional groups)

and thus a likely structure of a compound

changes in the concentration of a species during reaction

the properties of bonds (bond strength, force constants)

Since a bond in a molecule behaves like a spring, the harmonic/anharmonic oscillator model is used to describe the 3N-6 or 3N-5 different ways a non- linear or linear molecule, respectively, consisting of N atoms can vibrate.

These vibrational modes (normal modes) give rise to absorption bands of

characteristic energies/frequencies/wave numbers, intensities, and widths

(change of dipole moment required), which are detected and analyzed.

(2)

Vibrational energy levels in harmonic approximation

Please note that vibrations normally are more or less anharmonic

Zero point vibration/energy!

(3)

Vibrational levels in harmonic/anharmonic approximation

Potential curve of an harmonic oscillator (E _n : Vibrational levels, E ₀ : Zero point energy)

Potential curve of an anharmonic oscillator (E ₀ : Zero point energy, E _D : Dissociation energy) E _VIB = hν _osc (n + ½) - h ² ν ² /(4E _D )· (n + ½) ² (Δn = ±1, ±2, ...) In case of an anharmonic vibration, the distances of

neighbouring levels become smaller with increasing n (the arrows symbolize transition probabilities and intensities.

Δn = ±1

(4)

Normal modes of vibration

The three normal modes of H ₂ O and their wavenumbers

The four normal modes of (linear) CO ₂

3N – 6 modes (3N – 5 if linear)

m f /

~ ~

ν

(5)

Typical wavenumbers of stretching/bending vibrations

“molecule“ stretching bending C - H 2800 - 3000

N - N 3300 - 3500

H ₂ O 3600 - 3000 1600

C = O 1700

C = C 1600

SO ₃ ^2- 970 (ν _s ) 930 (ν _as )

620 (γ)

470 (δ)

(6)

IR - Spectrometer

grating, double beam

Fourier Transform (FT)

Sample

(7)

3500 3000 2500 1300 1150 1000 850 700 ν/cm^-1 400

Sr(HPO OH)

₂ ₂

RT

¹²²⁶

1235

1225

12411164 1155 1170 1168

758

779 694

685 686

2881 2439 2433

2434 2440 2899 3032

3019

2400

2432 2300

2303 3176

317529632783 3191 32143080

RT TT

TT Sr(SeO OH)

₂ ₂

Tr an sm is si on

ν

_B

Vibrational spectroscopy is an energy sensitive method and is used to

Infrared Spectroscopy (IR)

Vibrational spectroscopy is an energy sensitive method and is used to

characterize compounds in terms of the strengths and number of the bonds present. One can get/detect:

 the presence of known compounds (finger print)

 the components of an unknown compound (functional groups)

 and thus a likely structure of a compound

 changes in the concentration of a species during reaction

 the properties of bonds (bond strength, force constants)

Since a bond in a molecule behaves like a spring, the harmonic/anharmonic oscillator model is used to describe the 3N-6 or 3N-5 different ways a non- linear or linear molecule, respectively, consisting of N atoms can vibrate.

These vibrational modes (normal modes) give rise to absorption bands of

characteristic energies/frequencies/wave numbers, intensities, and widths

(change of dipole moment required), which are detected and analyzed.

Vibrational energy levels in harmonic approximation

Please note that vibrations normally are more or less anharmonic

Zero point vibration/energy!

Vibrational levels in harmonic/anharmonic approximation

Potential curve of an harmonic oscillator (E n : Vibrational levels, E 0 : Zero point energy)

Potential curve of an anharmonic oscillator (E 0 : Zero point energy, E D : Dissociation energy) E VIB = hν osc (n + ½) - h 2 ν 2 /(4E D )· (n + ½) 2 (Δn = ±1, ±2, ...) In case of an anharmonic vibration, the distances of

neighbouring levels become smaller with increasing n (the arrows symbolize transition probabilities and intensities.

Δn = ±1

Normal modes of vibration

The three normal modes of H 2 O and their wavenumbers

The four normal modes of (linear) CO 2

3N – 6 modes (3N – 5 if linear)

m f /

~ ~

ν

Typical wavenumbers of stretching/bending vibrations

“molecule“ stretching bending C - H 2800 - 3000

N - N 3300 - 3500

H 2 O 3600 - 3000 1600

C = O 1700

C = C 1600

SO 3 2- 970 (ν s ) 930 (ν as )

620 (γ)

470 (δ)

IR - Spectrometer

grating, double beam

Fourier Transform (FT)

Sample

Sample

Sr(HPO OH)

RT

RT TT

TT Sr(SeO OH)

Tr an sm is si on

ν

(OH) ν(PH)

δ(OH)

γ(OH)

Cs 2 CrCl 5 ·4H 2 O BaSO 3

KMn(SeO 2 OH) 3

Examples

the presence of known compounds (finger print)

the components of an unknown compound (functional groups)

and thus a likely structure of a compound

changes in the concentration of a species during reaction

the properties of bonds (bond strength, force constants)

Potential curve of an harmonic oscillator (E _n : Vibrational levels, E ₀ : Zero point energy)

Potential curve of an anharmonic oscillator (E ₀ : Zero point energy, E _D : Dissociation energy) E _VIB = hν _osc (n + ½) - h ² ν ² /(4E _D )· (n + ½) ² (Δn = ±1, ±2, ...) In case of an anharmonic vibration, the distances of

The three normal modes of H ₂ O and their wavenumbers

The four normal modes of (linear) CO ₂

H ₂ O 3600 - 3000 1600

SO ₃ ^2- 970 (ν _s ) 930 (ν _as )

Cs ₂ CrCl ₅ ·4H ₂ O BaSO ₃

KMn(SeO ₂ OH) ₃