In all cases, the isocyanate substituents are far enough away from the active site of each reaction mechanism to ensure that the reaction pathway remains qualitatively invariant to choice of -R group. All possible conformers of each RNCO species were searched for and considered in this analysis. They are denoted alphabetically with a subscript A corresponding to the lowest energy conformer if applicable. Conformers other than the lowest are only considered in the analysis if either barrier height is lowered by more than 1 kcal mol -1 relative to conformer A. We admit that many of the substituents chosen might be unrealistic for industrial applications but justify the selections for two reasons. 1) The selected subset of substituents (listed in Figure ) exhibits a wide array of electronic effects that allows for greater confidence that our predicted trends are robust and sufficiently span the effects normally exhibited by more standard substituents.