Density functional theory (DFT) calculations were carried out to understand the protonation state and electronic structure of 5-phenyl tetrazole. The pKa was calculated to be 3.00, which implies that 5-phenyl tetrazole is nearly fully dissociated even in relatively dilute aqueous KOH solutions. As shown in Figure , the mole fraction of non-dissociated 5-phenyl tetrazole, which we denote as (0), is as low as 10 -9 at in 13.8 wt.% at 80 °C (pH 13). The specific HOMO/LUMO levels of polymer electrolytes and organic cations have often been linked to chemical stability in the literature. The HOMO level of the model compound in the neutral and deprotonated form was found to be -6.95 and -6.26 eV, respectively. The LUMO level of the model compound in the neutral and deprotonated form was found to be -1.56 and -0.98 eV, respectively. For comparison, the LUMO levels for arylene-linked bisbenzimidazoles were previously calculated to be in the range from -2.3 to -1.5 eV using the same computational setup. The high LUMO level suggests the nucleophilic attack by OH - will be difficult, although there is not a unique link between stability and the LUMO level because the distribution of the LUMO on the compound also plays a role. The LUMO on 5-phenyl tetrazole is found to be fully delocalized, as shown in Figure . The increase in LUMO level upon deprotonation is expected because 5-phenyl tetrazole becomes negatively charged upon deprotonation, which will also make the nucleophilic attack by OH -more difficult.