Do supra-antara paths really exist for 2 + 2 cycloaddition reactions? Analytical computation of the MC-SCF Hessians for transition states of ethylene with ethylene, singlet oxygen, and ketene

Abstract

The transition structures for the 2, + 2, cycloaddition of ethylene with ethylene, with singlet 02, and with ketene have been determined by MC-SCF gradient methods. The subsequent characterization of these critical points by analytical Hessian computation shows that in each case the critical point is a saddle point of index 2 (Le., there are two imaginary vibrational frequencies). Thus, the results show that the supra-antara reaction path may not exist at all for 2 + 2 cycloadditions.