The data in Figure clearly indicate that the substituent selection plays a significant role in the energetics of the water catalyzed transition states. The previous relationship between substituent and barrier height in the the H 2 O + RNCO case no longer holds with these different mechanisms, at least not for the same reasons. In the case of TSN2 and TSN3, there is a weaker correlation between the RNCO C-O* occupation and the barrier height, as the dominant E interactions have changed with the new mechanism. We observe that in TSN2 and TSN3, the NBO method predicts a transition state where the nitrogen exhibits two lone pairs. The first lone pair is of almost entirely p-orbital character, while the other, from the broken N --C bond, is significantly hybridized. The p-orbital character of this second lone pair correlates quite strongly with barrier heights (R 2 =0.84 and 0.85 for TSN2 and TSN3, respectively) and can be explained in terms of Bent's rules. Strongly electronegetive substituents direct more p-orbital character from the nitrogen in the R-N bond, which increases the p-orbital character of the second nitrogen transition state lone pair and increases the proclivity of a N --C double bond breakage. In the TSO2 and TSO3 cases, we cannot make the same argument, but we do note that the RNCO C-O* occupation correlates extremely well with the barrier heights (R 2 = 0.93 and 0.92 for TSO2 and TSO3).