# The “Forbidden” World of Chemistry

## Abstract

Hückel’s rule^{1}, the Woodward-Hoffmann rules^{2}, and, in a broader sense, Hückel Molecular Orbital (HMO) theory^{3} have had profound impact on chemistry in a way which is now well recognized and admired. Perhaps, the most important and time-lasting contribution of HMO theory has been the revelation of what in Valence Bond (VB) terms we call parity control of stereoselection^{4}, or, in more familiar language, the revelation of the fact that a ground state molecule, a transition state, or, any molecular system, in general, can be thought of as the product of a “forbidden” or “allowed” union of two component fragments with the latter mode of union being energetically more favorable than the former one.

## Keywords

Lone Pair Nonbonded Interaction Orbital Symmetry Symmetry Species Ground State Molecule## Preview

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## References

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