For example, R2sub-I and R2sub-II each had f(-) values at the R2 substituent (-0.09 and -0.03, respectively) that were outside the observed range (-0.02 to 0.02), likely causing poor stability predictions in these structures. The other NCB which had a poor prediction was R2sub-IV. Our model likely failed in making an accurate prediction of stability for R2sub-IV because all other structures within our library have a neutral carbon atom at R2, so the electronic descriptors at this atom and C3 (directly bound to R2) are beyond the scope of our trained model.