AM1 AND MNDO CI QUANTUM CHEMICAL STUDY OF THE HYDROLYSIS OF FUNCTIONAL-GROUPS ON THE SURFACE OF SILICA

Using the AM1 and MNDO/CI methods in the cluster approximation, we have calculated the cross sections of the potential energy surface for the reaction = SiX + H2O reversible = SiOH + HX (X = OCH3, OPh, NH2, Cl, H, CH3). We have found that the activation energy of the limiting step, H+ transfer in a four-center intermediate, decreases in the order CH3 > H > OPh > OCH3 > Cl > NH2. We have studied the effect of the number (n) of H2O molecules reacting with the = SiCl group on the stabilization of the intermediate and the activation energy of H+ transfer. We have established that E not-equal H+ monotonically decreases with an increase in n, but the increase in the heat of formation of the associate occurs significantly faster. For all X, in the transition state the polarity of the attacking H2O molecule increases, while the changes in the ionicity of the Si-X bond depend on the nature of X: for X = OCR, NH2 the charge q(O,N) is reduced, q(Si) increases; in the remaining cases, q(Y) and q(Si) increase. The value of E not-equal H+ is due more to the proton-acceptor properties of the first atom in X than to the ionicity of the Si-X bond.