While internal hydrogen-bonding may affect some of the features we already include, our model does not directly consider it. Adding a more explicit descriptor to capture such affects may be something we should explore as we improve our model. A more likely explanation for the error is that it is due to the amide nitrogen our model misses. Our logic for not including amides as ionizable sites in our model was because they often have a basic pK a value less than 2.0 and an acidic pK a value greater than 14.0. However, in this highly conjugated system, that amide nitrogen could be an important contributor to the pK a of 11.74 ± 0.01. An analogous system to consider is N-(2-pyrimidyl)benzamide, with its second ionization measured at 11.2 , demonstrating that acidic amide nitrogens can have pK a values in the appropriate range. Improving our model will likely involve conducting a more thorough investigation of which groups should be considered ionizable.