F. Taulelle et al., Trapping of an activated HF molecule inside a double four-ring unit: A quantum chemical model of the microporous fluorinated gallium phosphate ULM-18, J AM CHEM S, 123(1), 2001, pp. 111-120
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.