Lecture 8

Question 1

What info do polyatomic MOs give us?

Answer 1

• The geometric structure of a polyatomic molecule
• How the stability of a polyatomic molecule changes if it is oxidised, reduced or interacts with
  a photon
• The way in which the molecule binds and/or reacts with another molecule. (This is mainly
  determined by HOMO-LUMO interactions; Frontier MO theory.)

Question 2

How do you know if an orbital diagram shows a sigma or pi orbital?

Answer 2

• Sigma orbitals will lie along the interncuelar axis whereas pi will be above and below the atoms
• Sigma* and pi* will contain nodes

Question 3

Describe the molecular orbitals of CH3I

Answer 3

sigma*
LUMO- occupied with a free electron
MO has a central node which leads to dissociation of CH3I once that MO is occupied by an electron

Question 4

Describe the molecular orbitals of phenol

Answer 4

• pi
  HOMO of phenol, electron density above and below the ring
• pi*
  LUMO of phenol, electron moving from HOMO to the LUMO becomes more acidic after absorbing the photon