Using valence bond theory to understand electronic excited states: Application to the hidden excited state (2¹Ag) of C_2nH_2n+2 (n = 2-14) polyenes

A valence bond (VB) method is presented and applied to calculate the hidden excited states, 2¹A_g, and other covalent excited states of polyenes from C₄H₆ to C₂₈H₃₀. 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.