A VB Model of Transition Structure Regions of the Potential Energy Surfaces for Forbidden and Allowed Cycloaddition Reactions

The MC-SCF potential energy surfaces for the 2 + 2 cycloaddition of two ethylenes, the 4 + 2 cycloaddition of butadiene and ethylene and the 1,3 dipolar cycloaddition of fulminic acid and acetylene are modelled using the coulomb and exchange integrals of Heitler-London VB theory. The VB parameters reproduce the MC-SCF results exactly by virtue of their construction using effective Hamiltonian theory. The origin of the critical points and the magnitude of their activation barriers can thus be rationalized in terms of an analysis along the reaction coordinate, while the nature of the critical points is discussed with respect to normal modes that exclude the reaction coordinate. The computed results provide some understanding of the reason why certain transition structures exist for one cycloaddition and not for others. © 1989, Springer. All rights reserved.