A Kekulé-Crossing Model for the "Anomalous" Behavior of the b_2u Modes of Aromatic Hydrocarbons in the Lowest Excited ¹B_2u State

The Kekulé-crossing model presented in this paper shows that the frequency exaltation observed for certain b_2u modes in the 1¹B_2n states of aromatic hydrocarbons is a necessary result of the fact that these states are primarily covalent in nature, in the sense that they may be considered as linear combinations of the classical Kekulé structures. Thus, the ground and the 1¹B_2u states are very closely described by the states generated simultaneously (at the uniform geometry, D_6h or D_2h) by an avoided crossing between appropriate pairs of Kekulé-type structures along switching b_2u coordinates. The modes that undergo the frequency exaltation are the same ones that interchange the Kekulé pairs.¹⁰.