Carboborosilazane ceramics: initial reactions between TSDE and methylamine - a combined quantum chemical and first principles molecular dynamics study

Cl2BCH(CH3)SiCl3, in words 1-(trichlorosilyl)-1-(dichloroboryl)ethane (TSDE), is a precursor for the synthesis of Si/B/N/C high-demand ceramics. For the preparation of a ceramic of approximate stoichiometry Si2B2N5C4, methylamine is used as a network-forming agent. One way to get information about preferred coordinations in the amorphous network is a quantum chemical investigation of the reactions during the cross-linking of the precursor. For this reason, we have investigated several gas-phase reactions with Møller-Plesset perturbation theory: the aminolyses of TSDE and the decomposition of adducts ((H3C)H2N · SiCl3CH(CH3)BCl2 and (H3C)H2N · BCl2CH(CH3)SiCl3). Our results confirm qualitative chemical intuition: The major primary process in the gas phase is a substitution at the boron center of TSDE. Also, it can be seen that the cleavage of the Si-C or B-C bond in the respective adduct is strongly exothermic but kinetically hindered. The situation in liquid phase has been studied by means of Car-Parrinello molecular dynamics simulations (CPMD). In an excess of CH3NH2, the reaction mechanism of the B-Cl aminolysis changes. Instead of a four-cyclic transition state, a salt-like intermediate occurs. As a result, the reaction includes three minor reaction barriers and becomes highly exothermic. In contrast, the dissociation of the boron adduct is suppressed in an excess of methylamine. A break-up of the Si-C-B unit can therefore not be expected during the first stage of the aminolysis.