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

The “transition structures” for the 2, 4+2acycloaddition of ethylene with ethylene, with singlet O2, 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 (i.e., 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.