Understanding the movement, encapsulation, and energy barrier of water molecule diffusion into and in silicalites using ab initio calculations

Quantum chemical calculations at the Hartree-Fock and MP2 levels have been performed to investigate water-silicalite interactions as well as the energy barrier and water orientations during diffusion into and in silicalite. Experimental geometries of water and silicalite have been used and kept constant throughout. The silicalite crystal structure has been represented by three fragments consisting of 20, 52, and 64 heavy atoms (oxygen and silicon atoms). Calculations have been performed using extended 6-31G and 6-31G* basis sets with BSSE (basis set superposition error) corrections. The results indicate obviously how the water molecule moves and turns via diffusion through the center of the silicalite pore in order to find the optimal route. The energy barriers for the water molecule to enter the pore and to diffuse from one channel to another have been clearly examined. The most stable binding site inside the pore is to be encapsulated in the intersection channel. It was also found that a water molecule enters and leaves the pores by pointing its dipole vector toward the center of the cavity.