Thermal decomposition of tetramethyl orthosilicate in the gas phase. An experimental and theoretical study of the initiation process

The thermal decomposition of Si(OCH₃)₄ (TMOS) has been studied by FTIR at temperatures between 858 and 968 K. Additional experiments were performed by using toluene as a radical scavenger. The species monitored included TMOS, CH₂), CH₄, and CO. According to these measurements, the first-order global rate constants for the disappearance of TMOS without and with toluene can be given by k_g = 1.4 × 10¹⁶ exp(-81 200/RT) s^-1 and k_g = 2.0 × 10¹⁴ exp(-74 500/RT) s^-1, respectively. The noticeable difference between the two sets of Arrhenius parameters suggests that, in the absence of the inhibitor, the reactant was consumed to a significant extent by radical attacks at higher temperatures. The experimental data were kinetically modeled with the aid of a quantum-chemical calculation using the BAC-MP4 method. The results of the kinetic modeling, using the mechanism constructed on the basis of the quantum-chemical data and the known C/H/O chemistry, identified two rate-controlling reactions. In addition to these new kinetic data, the heats of formation of many relevant SiO_xC_yH_z species computed with the BAC-MP4 method are presented herein.