Geometry of Benzene, cr or tt

FIGURE 6.20 The VBSCDs showing the crossing and avoided crossing of the Kekule structures of benzene along the bond alternating mode, b2u for (a) TT-only curves, (b) full ct+tt curves. 

Since the A St value for an isolated tt bond is of the order of 100 kcal/mol, Equation 6.25 places the tt electronic system in the region of large gaps. Consequently, the rr-component of benzene is predicted by the VBSCD model 

In order to show how delicate the balance is between the a and tt opposing tendencies, we recently (64) derived an empirical equation (Eq. 6.26) for An + 2 annulenes  

AE + a stands for the total (tt and a) distortion energies, the terms 5.0(2/ + 1) represent the resisting a effect, which is 5.0 kcal/mol for an adjacent pair of o-bond, whereas the negative term, —5.4(2/ ), accounts for the TT-distortivity. This expression predicts that for n = 7, namely, the C3oH3o annulene, the AE +a becomes negative and the annulene undergoes bond 

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