Towards modelling bioactive glasses: Quantum chemistry studies of the hydrolysis of some silicate structures

Density functional theory (DFT) and semi-empirical methods are used to investigate the hydrolysis of the Si–O bond in a series of model silicates, yielding energy barriers to the reaction and the geometrical changes which occur at a molecular level. The reaction is found to take place through a transition state involving a five-co-ordinate silicon atom and the transfer of a proton from the approaching water molecule to the bridging oxygen atom. The height of the energy barrier is found to vary both with the size of the cluster and with the type of atoms used to terminate the model. The presence of a second water molecule in the reaction lowers the energy barrier to hydrolysis. Comparison between DFT and semi-empirical results suggests that the latter should be used with caution.