The predicted barrier heights for TS2 manifest trends that are qualitatively similar to TS2 TS1 the TS1 results, with the TS2 process always significantly less favorable. The TS2 barrier predictions are slightly more sensitive than the TS1 predictions and span a range from 21.2 kcal mol -1 with the lowest barrier predicted for R=F (30.1 kcal mol -1 ) and the highest R=SiH 3 (51.2 kcal mol -1 ). The most competitive case, R=F, still predicts TS1 to be the more favorable pathway by 6.5 kcal mol -1 . Our results confirm that it is highly unlikely that the TS2 barrier could be made competitive with the TS1 barrier by substituent selection alone in the uncatalyzed RNCO + H 2 O case, also noting that this conclusion is agnostic to other factors such as catalysts, solvent effects, etc. Figure presents a scatterplot between the substituted TS1 and TS2 barrier heights, which correlate excellently, indicating that both barriers exhibit similar relationships to substituent identity. The supplementary information (Tables ) also contains the 298 K Gibbs free energy correction for each reaction which is generally insensitive to substituent choice and is close to 11 kcal mol -1 for all species.