Using valence bond theory to understand electronic excited states: Application to the hidden excited state (2(1)A(g)) 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 fr om C4H6 to C28H30. The ground rules needed to understand the results are qu alitatively outlined and used to discuss the asymptotic behavior of these m olecules as n goes to infinity. The theory enables to understand in a coher ent and lucid manner excited state properties, such as the makeup of the va rious 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 character istics, etc.