Kekule mode

As we are about to tell now, the theme of the Kekule modes involves stories within stories. The longer and fascinating one is that of the Kekule modes of benzene and aromatics.1 3 209 The complementary story deals with Kekule modes in antiaromatic molecules188 210 and is likely to continue in the future with further advances in spectroscopic techniques.218-222 Finally, another off-shoot of the main theme relates to the twin states in the case of aromatic transition states.225... 

The Kekule mode of benzene is the skeletal b2u mode a i4, which has been discussed all along and is shown in Scheme 34a. Another b2u mode is the hydrogenic movement shown in in Scheme 34 b and is labeled as co 15. These modes are both IR and Raman inactive for DSh benzene in the ground state. The electronic transition to the 11 B2u excited state is disallowed, and hence, the observations of the b2u... 

Despite our natural satisfaction with this explanation, we noted that it nevertheless does not provide a clear physical mechanism for the effect. For example, it was not evident why the frequency exaltation is state selective, i.e., it is not observed in any other state, and the frequency of all other modes are reduced upon excitation. It was deemed necessary, therefore, to articulate the avoided crossing model, derive a clear physical origin of the frequency exaltation of the Kekule mode, its state, and mode specificity, and establish the connection of the phenomenon to the jr-distortivity in the ground state. This was achieved in 1996.209... 

Origins of the Exalted Frequency of Kekule Modes in the VB2u State of Benzene... 

It follows that the excited state will have a Kekule mode with a larger force constant and therefore also an exalted frequency, as expressed in eq 17... 

Ti-CH STO-3G).209 Thus, adding to the common a-curve, a repulsive a-curve leads to a flat potential in the ground state, along the skeletal Kekule mode. In contrast, adding to the a-curve the attractive a-curve for the twin excited state leads to a steep potential, along the Kekule mode. 

Scheme 35. Frequency Up-Shifts of the Kekule Mode in Benzene and Its Derivatives (Refs 3, 234, and 235)a...

One might have thought that the state- and mode-specificity of the frequency exaltation is dictated by the symmetry of the states and vibrational mode. Scheme 35 shows, however, that benzene derivatives where the DSh symmetry is clearly absent exhibit this frequency upshift of Kekule mode.3 234 235 Moreover, in all these systems, no other state and no other mode... 

The frequency exaltation of the Kekule mode is mirrored by the structural manifestations in the twin states, discussed with reference to Figures 16 and 17. Thus, the repulsive jr-curve in the ground state softens the potential and thereby enables the ground-state molecule to distort along the Kekule mode when angular strain is exerted. In contrast, the attractive jr-curve in the twin excited state stiffens the potential and restores the local Deh symmetry of the benzene nucleus. The two physical effects are in perfect harmony and find a natural reflection in the VB model. 

Scheme 37. Kekule Mode that Interchanges the Kekule (Ki, K2) Structures of Naphthalene and Is Predicted To Possess an Exalted Frequency in the Twin Excited State (Ref 3)...

Scheme 39. Kekule Modes that Interchange the Benzenic (B) and Annulenic (A) Kekule Structures of Anthracene (Ref 3)a...

Scheme 40. Frequencies of the big Kekule Mode in the Ground and Twin Excited States of Antiaromatic Species (Ref 188)...

C. The Kekule Mode in the Twin States of Antiaromatic Molecules... 

Despite the quantitative difference between aromatics and antiaromatics, the qualitative picture is the same The combination of a, T-distortive ground state and a -attractive excited state along the Kekule modes is manifested in soft potentials in the ground state and steep ones in the twin excited state. This in turn is expressed as exalted frequencies and symmetrized geometries in the twin excited state. 

The Kekule Modes in Twin States of Chemical Transition States... 

In this simplified model, the tt stabilisation energy is at its lowest in the regular hexagonal structure is at a minimum, with curvature (92Zs/9[8/3]2) = 12/3 1. Hence the prediction is that distortion along the b2u Kekule mode, in which three disjoint bonds strengthen at the expense of the other three, will produce a quadratic stabilisation of the tt system. 

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