| Abstract
| - Structural, energetical, and vibrational features of cagelike clusters mimicking the isolated hydroxylgroup at the surface of siliceous materials, both free and interacting with an ammonia molecule, arestudied ab initio at the B3-LYP/DZP level of calculations. Two minimal model clusters, H3SiOH andF3SiOH, and four cagelike clusters differing in size and in the geometrical strain of the Si−O ring structureshave been studied. The computed value of 3762 cm-1 for the anharmonic ω01 (OH) of cagelike clusters withan anharmonicity constant of 78 cm-1 is in fair agreement with the experiment, as it is the computed valueof 1454 ± 10 kJ/mol for the deprotonation energy obtained by averaging values for the four differentcagelike clusters. For the interaction with the NH3 molecule, the computed average value of 841 ± 69 cm-1for the shift of the ω01 (OH) stretching frequency is in excellent agreement with the experimental valuemeasured on amorphous silica at low temperature; the computed average value of 29 ± 3 kJ/mol for theheat 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 thecluster approach.
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