Cagelike clusters as models for the isolated hydroxyls of silica:: Ab initio B3-LYP calculations of the interaction with ammonia

Structural, energetical, and vibrational features of cagelike clusters mimicking the isolated hydroxyl group at the surface of siliceous materials, both free and interacting with an ammonia molecule, are studied ab initio at the B3-LYP/DZP level of calculations. Two minimal model clusters, H3SiOH and F3SiOH, and four cagelike clusters differing in size and in the geometrical strain of the Si-O ring structures have been studied. The computed value of 3762 cm(-1) for the anharmonic omega(01) (OH) of cagelike clusters with an anharmonicity constant of 78 cm(-1) is in fair agreement with the experiment, as it is the computed value of 1454 +/- 10 kJ/mol for the deprotonation energy obtained by averaging values for the four different cagelike clusters. For the interaction with the NH3 molecule, the computed average value of 841 +/- 69 cm(-1) for the shift of the omega(01) (OH) stretching frequency is in excellent agreement with the experimental value measured on amorphous silica at low temperature; the computed average value of 29 +/- 3 kJ/mol for the heat of interaction of NH3 is definitely underestimated with respect to the microcalorimetric measurements, probably because of the neglect of dispersive interactions and long-range forces not accounted for by the cluster approach.