We started our QM/MM simulation from structure in the R state obtained after equilibration at 300 K. The specific proton transfer reaction we simulated involves the breaking of covalent bond between O 7 and H 1 and formation of a covalent bond between C 2 and H 1 . Subsequently C 2 and H 3 bond breaks and that proton moves back to the zeolite framework. Here H 3 was arbitrary chosen as the one dissociates during the reaction, among all the H-atoms bound to C 2 in the R state. Although the leaving proton has a possibility to bind to any one of the Brønsted acid oxygen atoms while forming P, only the O 8 and O 9 atoms are accessible to the reactive site (Figure ). This is true for both the zeolite frameworks. Thus we chose the It is noted that the chemical nature of all the Brønsted acid oxygens in the zeolite framework is the same, as clear from the charge analysis 85 (Table ). The atomic charges of AlO 4 oxygen atoms implicate the chemical nature of the Brønsted acid site. The O 7 atom has a different charge as it is bonded to an H-atom. The other three oxygen atoms have nearly same charges. Thus, the free energy barrier would be the same if the H-atom moves to O 8