Twist-Bent Bonds Revisited: Adiabatic Ionization Potentials Demystify Enhanced Reactivity

Explicit calculations of vertical and adiabatic ionization potentials of cyclopropane derivatives with modern DFT methods have underscored the possibility of unusually large reorganization energies (defined as the difference between vertical and adiabatic ionization potentials) of 0.5–1.0 eV for several compounds. Such is the case for ionization of the twist-bent σ-bond of trans-bicyclo[4.1.0]hept-3-ene (trans-3-norcarene), for which B3LYP*-D3 calculations predict an adiabatic IP of 7.92 eV. The corresponding value for the cis-norcarene is 8.34 eV. The significantly lower adiabatic IP provides an attractive explanation for the higher reactivity of the trans compound under oxidative conditions. Large reorganization energies are also found for the ionization of cyclopropane, bicyclo[1.1.0]butane, and bicyclo[2.1.0]pentane. In sharp contrast, an exceptionally small reorganization energy is associated with the ionization of tricyclo[1.1.1.0]pentane ([1.1.1]propellane)