The “Forbidden” World of Chemistry

  • Nicolaos Demetrios Epiotis
Part of the Lecture Notes in Chemistry book series (LNC, volume 34)


Hückel’s rule1, the Woodward-Hoffmann rules2, and, in a broader sense, Hückel Molecular Orbital (HMO) theory3 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 stereoselection4, 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.


Lone Pair Nonbonded Interaction Orbital Symmetry Symmetry Species Ground State Molecule 
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    Molecules which shun deexcitation, i.e., “forbiddenness” relief, and exist as U-bound species are known and examples have been discussed in Ref. 4. The reason for U over H or D preference is maintenance of strong interfragmental overlap at the expense of excitation. A typical illustrator is acetylene which is U-bound in the linear Dαh geometry. This geometry is preferred over the transbent one although the latter involves H-bonding.Google Scholar
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    The C-D and C-A bond lengths were taken to be equal to the C-H bond length and tetrahedral geometries were used throughout. In the EHMO computation,the VOIE of D was set at -11.6 eV and that of A at -15.6 eV. In the ab initio computations, we used an STO-3G AO basis and set the nuclear charge of D at 0.9 and that of A at 1.1. The CI computations were carried out using a program kindly provided by Professor E. R. Davidson of this department.Google Scholar
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Copyright information

© Springer-Verlag Berlin Heidelberg 1983

Authors and Affiliations

  • Nicolaos Demetrios Epiotis
    • 1
  1. 1.Department of ChemistryUniversity of WashingtonSeattleUSA

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