Thermal Decomposition of Tetramethyl Orthosilicate in the Gas Phase: An Experimental and Theoretical Study of the Initation Process

Article abstract of DOI:10.1021/j100002a034

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 disappearrance of TMOS without and with toluene can be given by k_g=1.4E16 exp(-81200/RT) s^-1 and k_g=2.0E14 exp(-74500/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 know C/H/O chemistry, identified two rate-controlling reactions: TMOS -> CH3OH+(CH3O)2SiOCH2 (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: k₂=1.6E14 exp(-74000/RT) and k₃=3.8E14 exp(-60000/RT).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.