Molecular oxygen can quench the triplet state of 6MP forming reactive oxygen species other than 1 O2. For instance, electron transfer between the triplet state of 6MP and O2 can give rise to the superoxide radical, as previously proposed for 6-thioguanosine and 6-thioguanine. Hence, the oxidation potential and adiabatic triplet energy for the neutral and the deprotonated forms of 6MP were calculated to estimate the Gibbs free energy associated with the electron transfer process (ΔGET) using the Rehm-Weller equation (see Methodology for details). The results from these calculations are reported in Table using both the planar and out-of-plane T1 minima calculated in this study. ), corrected for the zero-point-energy, and using the VIE calculated in this work; § § energy of the out-of-plane T1 minimum obtained using the UPBE0/IEFPCM/6-311+G(d,p) level of theory; † † energy of the planar T1 minimum obtained using the TD-PBE0/IEFPCM/6-311+G(d,p) level of theory.