Frontier Orbital Interactions in the Transition States of One-Step -Cycloadditions

2 Frontier Orbital Interactions in the Transition States of One-Step [4+2] -Cycloadditions 

What Are the Factors Contributing to the Activation Energy of [4+2]-Cycloadditions  

These intermolecular orbital interactions, which are of the a type, occur between the ends of the 7T-type MOs that are associated with the respective two reagents. We will see in the next subsection what the ends of these TT-type MOs are like. In the subsequent subsection we will deal with the energy effects of the new orbital interactions. The energy effects associated with the special case of [4+2]-cycloadditions will be considered thereafter. Finally, these new orbital interactions will be discussed for [2+2]-cycloadditions. 

The LCAO Model of tt-MOs of Ethene, Acetylene, and Butadiene Frontier Orbitals 

There are a variety of methods for the computation of the MOs that interact in the transition states of [4+2]-cycloadditions. The LCAO method (linear combination of atomic orbitals) is often employed, and the basic idea is as follows. The MOs of the -systems of alkenes, conjugated polyenes, or conjugated polyenyl cations, radicals, or anions all are built by so-called linear combinations of 2p AOs. In a somewhat casual formulation, one might say that the MOs of these -systems are constructed with the help of the 2pz AOs. These AOs are centered at the positions of the n C atoms that are part of the -system. LCAO computations describe a conjugated -electron system that extends over n s/ 2-hybridized C atoms by way of n Ji-type MOs. 

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