Calculation of proton affinities. All quantum chemical calculations were performed using Gaussian16 revision B.01 and the M06-2x density functional. The 6-311++G(d,p) basis set was employed for all compounds except for 3,5-diiodo-4pyridone-1-acetic acid where the D-Gauss Double Zeta Va-lence Polarized basis-set (DGDZVP) was used instead to account for the iodine atoms. The three-dimensional structures for all analytes were constructed using the 'Clean Structure in 3D' feature as implemented in MarvinSketch. Then, GaussView was used to add protons to generate protonated molecules (see Table for the location of the additional proton). The resulting structures were optimized and the difference between the electronic energies for the neutral and charged molecules was compared to the known proton affinity of a simple reference compound in an isodesmic reaction. Here, methanol was used when the proton affinity was calculated for an oxygen atom, ammonia was used when calculating the proton affinity of a nitrogen atom, and 2-methyl propene is used for MOP. See Table for individual proton affinity values and the associated content on for the Gaussian16 input and output files respective to the aforementioned calculations.