From a Frontier Molecular Orbital (FMO) reactivity perspective, x -HOMO correlates much more strongly to ∆G int1 form. (R 2 = 0.97, Figure ) compared to q O . Relative to the free peroxide π * HOMO energy of -4.4 eV, the Group IV, V, and VI metal species have lower HOMO energies of around -5 eV, -6 eV, and -7 eV, respectively. The higher-lying HOMOs from Group IV are more peroxide-like energetically and able to interact more strongly with the +2.0 eV acceptor orbital on CO 2 (which is actually a degenerate LUMO+2/3 with CAM-B3LYP/6-31+G*).