Cluster quantum-chemical MINDO/3 study of the nature of hydroxyl groups on a calcium oxide surface

A modified MINDO/3 method was applied to study water molecule interaction with a pure calcium oxide using a supermolecular approach. The calcium oxide was modelled by a Ca₃₂O₃₂ four-layer molecular cluster containing all sets of the most chemically active low-coordinated calcium and oxygen ions (Ca(LC)/^2-) and O(LC)/^2-). It is shown that the dissociative adsorption of water molecule on a pair of acid-base centers of CaO is energetically more favorable than the molecular forms of adsorption on an acidic sites. O-H stretching frequencies of six kinds of hydroxyl groups on a CaO calculated by a harmonic oscillator approach can be ascribed to the corresponding local structures.