Download - Rotational Raman spectroscopy
Nils Walter: Chem 260
Rotational Raman spectroscopy
Experimental setup:
laser
Gross selection rule:anisotropic polarization
(example: H-H)
Specific selection rules:∆∆∆∆J = +2 (Stokes lines)
∆∆∆∆J = -2 (anti-Stokes lines)
Nils Walter: Chem 260
The vibration of molecules
Morse potential2)(
21
eRRkV −=
ννννhvEv
++++====
21
Solution for the harmonic oscillator:
=
µπν k
21
BA
BA
mmmm+
=µeffective mass
A B B
zero point energy
Nils Walter: Chem 260
=
µπν k
21
⇒⇒⇒⇒ 300-3000 cm-1 = Infrared
Vibrational transitions: Infrared spectroscopy
Gross selection rule:the electric dipole
moment of the molecule must change during the vibration
Nils Walter: Chem 260
Specific selection rules
∆∆∆∆v = ±±±±1
In reality: anharmonic oscillator
xe = anharmonicity constant
∆∆∆∆v = ±±±±1, ±±±±2, ±±±±3,...
kTE
lower
upper eNN ∆−
=
D0
Deqev xhvhvE νν
2
21
21
+−
+=
Nils Walter: Chem 260
The vibrating rotor
Born-Oppenheimer approximation:The energies of rotations and vibrations are so
different that Etotal = Erot. + Evib.
∆ ∆ ∆ ∆J = -1 ∆ ∆ ∆ ∆J = +1
Nils Walter: Chem 260
Vibrations of polyatomic molecules:How many are there?Each atom can move along one of three axes:
⇒⇒⇒⇒ 3N possible displacements (= degrees of freedom)⇓⇓⇓⇓
Three degrees of freedom correspond to translational motion:⇒⇒⇒⇒ 3N - 3 degrees of freedom left
⇓⇓⇓⇓Three (/two) degrees of freedom correspond to rotations:
⇒⇒⇒⇒ 3N - 6 (3N - 5 for linear molecule) degrees of freedom left for vibrations
Nils Walter: Chem 260
Vibrations of polyatomic molecules:How can one describe them?
CO2: C-O stretch vibrations are NOTindependent (cannot be excited
independently)
Normal stretch modes (linear combinations of C-O stretches) ARE
independent
inactive activ
e
symmetric antisymmetric:greenhouse gas
Nils Walter: Chem 260
What is all this good for???
←←←← low energy→→→→
Identification of organic compounds!!!B
enze
ne C
-H st
retc
h
-NO
2gr
oup
Con
juga
ted
Car
boxy
lic a
cid
O-H
stre
tch C
C