Journal of Physical Chemistry C vol:111 issue:33 pages:12376-12382
Density function theory calculations were carried out on 14T clusters wherein one of the 14 Si atoms was substituted by either B, Al, or Ga. The clusters were neutralized by one alkaline cation. Two accessible oxygen atoms, O-1 and O-4, bonded to the substituted atom are considered for their basic properties. The computed proton affinities and NO+ binding energies show the same trends as experimentally observed. These data point out that the O-4 oxygen is the most basic for the B substitution and O-1 is the most basic for the Al and Ga substitutions. In view of the hard character of the oxygen atom, hard descriptors are computed. Among them, the charge neither reproduces correctly the effect of the alkaline cation nor the effect of the isomorphous substitution. The local hardness reproduces well the isomorphous substitution but fails to predict the most basic site or the effect of the alkaline cation. The molecular electrostatic potential appears to be the best hard descriptor. It predicts well the effect of the monovalent cation as well as that of the isomorphous substitution. It is also able to predict which site is the most basic for a given substitution: O-4 for B substitution and O-1 for Al and Ga substitutions. In comparison with previous studies using soft indices to describe the oxygen basicity in zeolites, the molecular electrostatic potential is found to be a reliable and rigorous descriptor that can be used to predict both Lewis basicity and Bronsted-Lowry basicity.