The optimized end points were then used to determine the minimum energy path using the quadratic string method (QSM) . The intermediate structures for QSM were obtained using a linear interpolation between the end points. Then, all the structures are optimized simultaneously using the restrained-MM path optimization procedure implemented in LICHEM. The initial restraint was set to 50 kcal/mol/Å 2 , and relaxed gradually. Including the end points (reactant and product), the reaction path had 16 structures in all, with 14 intermediates connecting the reactant and the product.