To compare the reaction pathways with each other, it is advantageous to represent them with a single number. For that, we propose a simplified chemical model. We can assume that pre-intermediate A and post-intermediate C are in equilibrium with reagents and products, respectively. A and B are connected through TS1, and the conversion from B to C is irreversible due to the largest barrier of 72 kcal/mol 5. By applying a steady state approximation for the catalyst (SO 2 ) and Intermediate #1 (B), details are given in ESI, we can describe this scheme with a single