Molecular orbitals zero-order reactions

The underlying theoretical approach is characterized by the type of the wavefunction and the choice of the basis set. Most current general-purpose semiempirical methods are based on molecular orbital theory and employ a minimal basis set for the valence electrons. Electron correlation is treated explicitly only if this is necessary for an appropriate zero-order description (e.g., in the case of electronically excited states or transition states in chemical reactions). Correlation effects are often included in an average sense by a suitable representation of the two-electron integrals and by the overall parametrization. 

If the picture is correct then we see that the observed order of activation energies which is largest for molecular reactions, small for radical-molecule reactions and nearly zero for radical-radical reactions, falls into a 1 1 relation with the acid-base model. The radical-radical reactions have the open orbital and the electron donor, hence little promotion energy to form an attractive pair. The radical-molecule reactions have one open orbital but require polarization of the molecule in order to form the complimentary acid or base. For the molecule-molecule addition type reaction, complimentary polarization of both species must take place for an attractive transition state to form and the activation energy is the highest. 

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