Using Valence Bond Theory to Understand Electronic Excited States:  Application to the Hidden Excited State (2Ag) of C₂nH₂n₊₂(n= 2−14) Polyenes

A valence bond (VB) method is presented and applied to calculate the hidden excited states, 2 1 A g, and other covalent excited states of polyenes from C 4 H 6 to C 28 H 30 . The ground rules needed to understand the results are qualitatively outlined and used to discuss the asymptotic behavior of these molecules as n goes to infinity. The theory enables to understand in a coherent and lucid manner excited state properties, such as the makeup of the various states, their energies and geometries, the puzzling increase of the C C frequency in the excited state, the opposite bond alternation properties of the ground and excited state, isomerization patterns, soliton characteristics, etc.