Trapping of an activated HF molecule inside a double four-ring unit: A quantum chemical model of the microporous fluorinated gallium phosphate ULM-18

The penetration of a proton into the prenucleation building unit of a micro porous gallophosphate and its interaction with an encapsulated fluorine ani on have been investigated by means of DFT calculations. The inorganic part of the fluorinated gallophosphate ULM-18 has been modeled by a neutral, dou ble four-ring (D4R) unit of formula [(GaOH)(4)(HPO4)(4).H2O] encapsulating the fluorine ion. Assuming the cage to be rigid and to retain throughout th e calculations the geometry determined from X-ray diffraction (XRD), the po sition of F- has been optimized, either as an isolated guest species or in the presence of an incoming proton. In agreement with the XRD structure, th e fluorine atom has been shown to occupy in both cases a nonsymmetric posit ion in the cage, being attached to three gallium atoms out of four. The dis tribution of the molecular electrostatic potential inside and outside the ( F-)@[(GaOH)(4)(HPO4)(4).H2O] system has provided indications concerning the pathways that could be used by an incoming proton to penetrate the D4R uni t and to approach the fluorine anion. The migration of a proton from an ext ernal site of fixation to the interior of the D4R unit has been found possi ble through two faces out of six. In both cases, the process has been found exothermic by similar to0.17 eV and the energy barrier was estimated to si milar to0.8 eV. Inside the gallophosphate cage; the proton first adopts a p osition typical of a strong F...H...O bond made possible through an importa nt shift of the fluorine anion away from the tripod of bonded gallium atoms . Then, the F-...H+ system can easily evolve back and forth on a flat poten tial curve between one of the F...H...O bonded conformations and a situatio n characterized by the cleavage of the H...O link and the formation of a mo derately activated F-H molecule, with the fluorine still attached to three gallium atoms.