Trends in Transition State Resonance Energies

This relationship predicts pretty well the trends in resonance energies of delocalized 3e/3c TSs in Table 6.1. Of course, this value is subject to small variations depending on the deviation of the TS from linearity (see Section 6.6.7). 

Another relationship for 4e/3c TSs was derived for SN2 reactions (5,36,52), using the same semiempirical approach and is given in Equation 6.20. 

Why does the charge on the central alkyl group reduce the resonance energy of Sn2 transition states The answer is straightforward, once it is realized that the X A 1 X structure commonly contributes to the two Lewis structures. Thus, the higher the contribution of the triple ionic structure, the more similar the two Lewis structures, and the lower the resonance energy becomes. In the theoretical limit, where the X A+ X structure becomes the dominant structure in the TS, say 100% of the TS wave function, then the resonance energy should go to zero. It is seen that the semiempirical expression in Equation 6.20 mimics this limit. Other semiempirical expressions make similar predictions (5,53), but Equation 6.20 is the simplest one. 

Finally, B is related to the stereochemistry of the reaction and the stereochemistry can be reasoned most simply using Rule 3, based on the FO-VB representations of R and R  

Application of the equation for cycloaddition reactions shows that the B factor is simply proportional to the product of the frontier molecular orbital (HOMO-LUMO) overlaps between the two reactants (5). As such, the following relationship holds for the allowed versus the forbidden pathways  

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