Influence of grafted methoxysylyl groups on the interaction of the silica surface with dimethyl carbonate
The density functional theory method with B3LYP and M06-2x functionals as well as the limits of the second order Mǿller-Pleset perturbation theory with the basis set 6-31G(d,p) was used to calculate energy parameters (activation energy and reaction energy effect) of the interaction of the methylating and carboxylating reagent, dimethyl carbonate (DMC), with models of silica surface, which reproduce different coordination of surface silicon atoms. It is shown that, regardless of the size of the surface model, thermodynamically more likely are the routes that involve the attack of the oxygen atom by the DMC molecule of the surface Si atom surrounded by a large number of silicon-oxygen tetrahedra. When assessing the influence of the methoxy group previously grafted onto the silica surface using models with different number of –OCH3 groups, it has been found that the attack of the ether atom of the DMC molecule on the surface Si atom, which is associated only with the silanol groups and is located next to the silicon atom to which the methoxy groups were grafted. The results show that with each grafted methoxy group, the energy effect of the corresponding stage increases. This correlates with the experimental data, according to which the methoxylation reaction of the surface of silica with DMC occurs at elevated temperatures.
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