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. The experiment was carried out in a static cell at a constant pressure of 700 Torr under highly diluted conditions. Additional experiments were performed by using toluene as a radical scavenger. The species monitored included TMOS, CH2O, 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 x 10(16) exp(-81 200/RT) s(-1) and k(g) = 2.0 x 10(14) 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: TMOS --> CH3OH + (CH3O)(2)SiOCH2 (reaction 2) and CH2OSi(OCH3)(3) - CH2O + Si(OCH3)(3) (reaction 3), which have the following respective first-order rate constants, given in the units of s(-1): k(2) = 1.6 x 10(14) exp(-74 000/RT) and k(3) = 3.8 x 10(14) exp(-60 000/RT). In addition to these new kinetic data, the heats of formation of many relevant SiOxCyHz species computed with the BAC-MP4 method are presented herein.