The role of the electrostatic potential, electric field and electric fieldgradient on the acidity of AFI and CHA zeotypes

We consider the role of short- and long-range interactions in determining t he acidity of microporous solids. Within the context of an interatomic forc efield approach and a correct treatment of long-range electrostatics we dem onstrate the role of the electrostatic potential, electric field and electr ic field gradient in determining the vibrational frequency of a Bronsted ac id centre. We demonstrate the applicability of these concepts for AlPO4-5, AlPO4-34, SSZ-24 and SSZ-13, in whose unit cells a single Bronsted acid sit e was introduced in the four different oxygens. Furthermore, we show, by us ing the energy required to elongate the OH bond-essentially the first stage of proton transfer-as a measure of acidity, how the vibrational frequency is related to the acid strength.