1.
The Raman spectrum of cyclohexane is shown below. The very intense peak in the
middle of the spectrum is Rayleigh scattering from the laser.
(a)
Explain the difference between Rayleigh and Raman scattering
Rayleigh scattering is due to elastic scattering of the light. There is no energy transfer to or from the
molecule to the light and its energy, frequency and wavelength are unaltered.
Raman scattering is due to inelastic scattering of the light. There is energy transfer to the molecule
(Stokes) or from the molecule (anti-Stokes) to the light.
(b)
(i)
What are the collective names given to the peaks in the spectrum at positive and
at negative Raman shift?
The peaks in the spectrum with positive Raman shift are known as Stokes lines.
The peaks in the spectrum with negative Raman shift are known as anti-Stokes lines.
(ii) Explain how the peaks at positive and negative Raman shift arise.
In Stokes scattering, the molecule becomes vibrationally excited and the scattered light loses energy
and so has lower energy (and so lower frequency and longer wavelength) than the laser light.
In anti-Stokes scattering, the molecule loses vibrational energy and transfers this to the scattered
light which therefore has higher energy (and so higher frequency and shorter wavelength) than the
laser light.
(iii)
Why are the peaks at negative Raman shift weaker than that at positive Raman shift?
The anti-Stokes lines only occur when vibrationally excited molecules scatter the light, as only
vibrationally excited molecules can lose vibrational energy.
At normal temperatures, very few molecules are vibrationally excited so the chance of this
occurring is much smaller than the chance of Stokes scattering.
2.
Which of the molecules below will exhibit a Raman spectrum?
(a) HBr (b) Cl2 (c) CO2 (d) HCO (e) CH4
All of them, The symmetric stretch (“breathing mode”) is present in all molecules and is Raman
active.
3.
Which vibrations of acetylene (C2H2) will show up in the Raman spectrum?
(a) and (c) will show up as strong bands in the Raman spectrum. (e) will appear but will be weak.
vibration
change in polarizability
Raman active
(a) symmetric CH stretch
+Q
H
C
a
C
H
C
H
-Q
+Q
-Q
H
C
Yes
(b) asymmetric CH stretch
+Q
H
C
a
C
-Q
H
C
No
H
+Q
-Q
C
H
(c) C≡C stretch
+Q
H
C
a
C
H
C
H
-Q
+Q
Yes
-Q
H
C
(d) symmetric bend
+Q
H
a
H
C
C
No
-Q
C
H
-Q
C
H
+Q
(e) asymmetric bend
+Q
a
H
C
C
Yes
H
-Q
-Q
+Q
H
C
Bends tend to give weak peaks
in the Raman as the change in
polarizability is small.
C
H
4.
Which vibrations of carbonyl sulfide (OCS) will show up in the Raman spectrum?
(a) C=S str. (b) C=O str. (c) bend
(a) and (b) will show up as strong bands in the Raman spectrum. (c) will appear but will be
weak.
vibration
change in polarizability
Raman active
(a) C=S stretch
+Q
O
C
a
S
-Q
-Q
O
C
+Q
Yes
S
(b) C=O stretch
+Q
O
C
a
S
-Q
-Q
O
C
S
+Q
Yes
(c) bend
+Q
O
C
-Q
-Q
O
C
a
S
S
Yes
+Q
Bends tend to give weak peaks
in the Raman as the change in
polarizability is small.