To train a machine learning model to predict NCB stability in buffer, we had to identify descriptors to effectively describe the NCBs in ways relevant to decomposition. Because decomposition in phosphate buffer occurs within the conserved, redox active 1,4-dihydropyridine ring, this motif was our focus while describing NCBs. DFT-level descriptors were calculated at each atomic position within the 1,4-dihydropyridine ring (N, C2, C3, C4, C5, and C6), as well at the first atom of each substituent (R1, R2, & R3) and the descriptor values of both hydrogens at the C4 position (C4H) were averaged for a total of ten descriptors from eleven atomic loci. Calculated descriptors include partial atomic charges derived from natural population analysis (NPA), as well as the electrophilic and nucleophilic condensed Fukui indices (f(+) and f(-), respectively). Further explanation of descriptors can be found in the supplemental information. Upon examination, these descriptors are able to effectively capture different substituent identities at each position, so no other features were considered in this study (Figure ).