Theoretical Study of the Double Proton Transfer in the CHX−XH···CHX−XH (X = O, S) Complexes

A theoretical study of double-proton-transfer processes in bimolecular complexes formed by combinations of molecules of the type CHX−XH (X = O, S) is reported. The reactions are rationalized in terms of the energy, chemical potential, and hardness of hydrogen-bonded and isolated species. Sanderson's rule to determine molecular chemical potential and hardness from the values of the constituent fragments is used to characterize the relaxation effects due to hydrogen bonding. In ten formation and seven double-proton transfer processes studied here, the principles of maximum hardness and minimum polarizability are verified. The mechanism for double proton transfer has been analyzed through the force acting on the system to bring reactants into products and the corresponding energy barriers have been qualitatively classified according to its through bond or through space nature.