F

FF

FF

F

S

F

FF

FF

F

S

F

FF

FF

F

S

Lecture 9 – Worksheet: The Rule of Mutual Exclusion Model 1: IR and Raman activity Only vibrations that lead to a change in the dipole moment are IR active.

Only vibrations that lead to a change in the polarizability, α,  are Raman active.

For CO2, the symmetric stretch gives rise to a change in polarizability with the polarizability being larger for a stretched (+Q) molecule than a compressed molecule (-Q): it is Raman active. The polarizability also changes during the antisymmetric stretch. However, the polarizability is the same for +Q and for –Q.

The symmetric stretch is Raman active but IR inactive.

The antisymmetric stretch is IR active but Raman inactive.

vibration change in dipole

change in polarizability

C OO

+Q

C OO

-Q

No

C OO

+Q

C OO

-Q

Yes

Critical thinking questions 1. Work out which of the stretching vibrations of an octahedral molecule are IR and which are Raman

active. You may need to use the analogy between orbitals and vibrations covered in worksheet 5.

Stretching mode IR active Raman active

α α

+Q  -­‐Q  

α

+Q  -­‐Q  

Raman activity

FF

FF

Xe

FF

FF

Xe

FF

FF

Xe

FF

FF

Xe

2. Use the orbital matching analogy to work out the four stretching modes and the IR and Raman activity of a square planar molecule, like XeF4. (The xenon 5s, 5p and 5dx2-y2 orbitals are directed along the Xe-F bonds).

Stretching mode IR activity Raman activity

3. There is a relationship between the Raman activity of a mode and the type of atomic orbital to which it is

analogues. Can you spot this?

4. CO2, SF6 and XeF4 all possess centres of inversion (centres of symmetry). Are any vibrations active in

both the Raman and IR spectra of these molecules?

5. Check that the rule you deduced in question 4 does not apply if the molecule does not possess a centre of inversion. For example, what is the IR and Raman activity of the symmetric and antisymmetric stretches of H2O?