Raman Spectroscopy, Exams of Chemistry

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Raman Spectroscopy

Theory and Aplications

Dr. Florian Paulat

(Lehnert Laboratory)

Paulat, F.; Praneeth, V. K. K.; Lehnert, N.

Inorg. Chem.

^2006

,45, 2835-

Historical background

• 1920s: Prediction of inelasticscattering of light by molecules(Kramers, Heisenberg and Dirac)• 1928: First report of inelasticscattering in water and alcoholvapors by Raman & KrishnanTechnical limitation: light source• 1960s: Development of Laseras intense monochromatic lightsources

Raman spectrometer

1) Nonresonance Raman

• ~ 0.1 % is elastically (Rayleigh) and ~ 0.0001-0.00001 % inelasticallyscattered (Raman: Stokes; Anti-Stokes)



LASER!

• IR: one photon



direct absorption of light in IR region

• Raman: two photons



UV, Vis and NIR excitation

• Stokes more intense than anti-Stokes (Boltzmann distribution)

rR – Enhancement Mechanism(A-Term)

i

E

E

E

n

m

g

e

A

m

g

e^

• In resonance: E

e,v

• E

g,m

≈ E

0

• A proport. to electronic transition moment squared



intense

electronic transition

(dipole allowed)

• Vibrational overlap integrals (Franck-Condon factor):a) = 0 for identical potential curves andb)

≠

0 only if displacement of potential curves (

Q>0)



only

totally

symmetric modes

(A

1g

)^

Albrecht,

^1961

rR – Enhancement Mechanism(B-Term)





   

   

    

    









     

^





s^

j^

j j

se j

n Q m g s g e m Q n g s g e E

s h e

D

B

,

-^ Vibronic coupling

of another excited state |s> with the resonant excited state |e>

-^ Energetic separation of |e> and |s> must be small • Both transition dipole moments from |g> to |e> and |s> must be nonvanishing 

excited states must belong to allowed electronic transitions

• <

|Q

|m,n> connect |g> and |e> vibrational levels that differ by one quantum;j

when they are multiplied by Franck-Condon factors having same quantum numbers,the nominator does not! vanish even if there is no excited-state displacement



Q



(totally) and

nontotally symmetric modes are enhanced via B-Term

• Which modes are enhanced?



group theory (direct product)

Albrecht,

^1961

2) Resonance Raman Spectroscopyof [Fe(TPP)Cl] Optimized structure (B3LYP/LanL2DZ)

Vibrational Assignment:• 78 Atoms



3N-6 = 228

vibrations!!!• What tools to solve problem?- DFT calculations- Polarized rR spectroscopy (D

4h

apply to the [M(TPP)] vibrations of[M(TPP)(Cl)])

Nonresonance Raman Spectrumof [Fe(TPP)Cl] (



exc.

= 1064 nm)

0.03 0.02 0.01 0.

1363

(^15541597 )

1495 1467

1371

(^12331275)

(^10301072) 1006 994

886

Intensity

0.015 0.010 0.005 0.

407

247 390 379

(^199257)

2000

1750

1500

1250

1000

750

500

0 3000 2000 1000

1274

1124

898 10181008

1054

1398

1598

1533 1508

1379

1260

Wavenumbers (cm

-1 )

500

400

300

200

90 60 30 0

(^202251241)

419388 Wavenumbers (cm

-1)

measured calculated

Polarized nonresonance Raman Spectrum of[Fe(TPP)Cl] (

 exc.

= 1064 nm)

2000

1800

1600

1400

1200

1000

800

600

400

200

0.016 0.012 0.008 0.004 0.

0.016 0.012 0.008 0.004 0. 2 Cl 2 LM: CH p

2 Cl 2 LM: CH

p

p dp p

p dp dp

p

Intensity

Wavenumbers (cm

-1 )

Electronic structure of[Fe(TPP)Cl]: Gouterman model

400

600

800

0 120000 100000 80000 60000 40000 20000

L -1 mol -1 / cm 

Wavelength (nm)

Soret

Q^ v

Q

Electronic structure of[Fe(TPP)Cl]: Gouterman model

400

600

800

0 120000 100000 80000 60000 40000 20000

L -1 mol -1 / cm 

Wavelength (nm)

^

Both excited states have E

u

symmetry (a

1u

x e

g^

= a

2u

x e

g^

= E

)u

^

Strong CI leads to large splitting 

Soret and Q-band

^

Q

: Vibronic mixing between Soret and Q excited states: Whichv^ vibrations are active?



E

u^

x E

u^

= (A

1g

) + B

1g

• B

2g

• A

2g

^

Distance between Q and Q

?v

Soret

Q^ v

Q

Polarized rR spectroscopy ofMetalloporphyrins ^

A-Term: totally symmetric modes



A

1g

vibrations

A-Term proport. to <e|

|g>

2



A-Term is dominant for intense

electronic transitions ^

B-Term: vibronic coupling



nontotally symmetric modes which

are active in mixing |e> with |s>



B

1g

, B

2g

and A

2g

Metalloporphyrin:



In Soret resonance enhancement of A

1g

Metalloporphyrin:



In Q resonance (vibronic mixing with Soret

excited state) enhancement of B

1g

, B

2g

and A

2g

modes

But: Q band is relative intense



additional A-Term enhancement of A

1g

rR: excitation profile [Fe(TPP)Cl] 30000

25000

20000

15000

8 6 4 2 0

sym

(C

-C^ m

) +

(C

-C

)

= 1556 cm

-1^ (

A1g

)

Rel. intensity

wavenumbers (cm

-1 )

Symmetry?

Polarized rR spectrum (Q

) ofv

[Fe(TPP)Cl] at



exc.

= 514.5 nm

1300

1400

1500

1600

1700

0 60000 45000 30000 15000

60000 45000 30000 15000 0

p^1594 dp^1576 p^1553 ap^1517 dp^1492

pdp^13691362 ap^1334

Intensität

Wellenzahl / cm

Wavenumbers (cm

-1 )

Intensity

polarized, depolarizedand anomalous polarizedbands