Molecular Structure and Bonding

p Electrons in Conjugated Organic Molecules

Lecture 2 Synopsis:

The Non Crossing Rule and Hückel Theory

Learning Objectives:

· To learn and understand the Non-Crossing Rule.Proof in terms of qualitative application of the Variational principle.

The Non- Crossing Rule:

Potential Energy Curves for Electronic States of the Same Symmetry Cannot Cross

· To learn the Hückel Approximations for conjugated p-electron systems.

see: P.W. Atkins, Physical Chemistry, Ed. 5, Pages 496-501. 

· To learn how these approximations lead to easily solvable equations for the energies of conjugatedp-electron systems and for the corresponding molecular orbitals.

To learn how to write down these equations for any conjugated p-electron system.

The non-crossing rule