DFT calculations were performed in Q-Chem 5.3. Thermodynamic quantities were calculated for the species involved in the EEC and EECC mechanisms (detailed in Section 3.A). For each quinone derivative involved, the gas-phase geometry was optimised at the B3LYP/6-311++G** level and thermodynamic quantities were calculated at T = 298.15 K and P = 1 atm. For unsymmetrical quinone derivatives, species involved were modeled as isomers with minimal steric hindrance. SCF calculations were considered to have converged when the wavefunction DIIS error was less than 10 -11 . At the end of each successful geometry optimisation, the structure was checked to be a real minimum on the potential energy surface and thermodynamic corrections were obtained by vibrational frequency analysis.