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 mimi cking the isolated hydroxyl group at the surface of siliceous materials, bo th free and interacting with an ammonia molecule, are studied ab initio at the B3-LYP/DZP level of calculations. Two minimal model clusters, H3SiOH an d F3SiOH, and four cagelike clusters differing in size and in the geometric al 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 exper iment, as it is the computed value of 1454 +/- 10 kJ/mol for the deprotonat ion energy obtained by averaging values for the four different cagelike clu sters. For the interaction with the NH3 molecule, the computed average valu e of 841 +/- 69 cm(-1) for the shift of the omega(01) (OH) stretching frequ ency is in excellent agreement with the experimental value measured on amor phous silica at low temperature; the computed average value of 29 +/- 3 kJ/ mol for the heat of interaction of NH3 is definitely underestimated with re spect to the microcalorimetric measurements, probably because of the neglec t of dispersive interactions and long-range forces not accounted for by the cluster approach.