Kinetics of the SiH₃ + O₂ Reaction:  A New Transition State for SiO Production

Abstract

The mechanism of SiO formation in the laser photolysis of SiH 4 /O 2 /CCl 4 mixtures was investigated using a laser-induced fluorescence method. Measured rates for the SiO production corresponded to the decay rates of SiH 3 radical and depended linearly on the O 2 concentration. The yield of SiO was estimated on the basis of LIF intensity, and it was found that SiO was one of the major products in the SiH 3 + O 2 reaction. The bimolecular rate constant for the SiO production was determined to be (1.14 ( 0.18) × 10 -11 cm 3 molecules -1 s -1 . Ab initio molecular orbital calculations were performed for various pathways of the SiH 3 + O 2 reaction. Geometries were optimized at the MP2(full)/6-31G(d) level of theory, and relative energies and barrier heights were calculated at the G2(MP2) level of theory. Silyl radical and O 2 react to form SiH 3 OO, which irreversibly decomposes to various excited products. A new transition state for the production of cyclic H 2 SiO 2 (siladioxirane) + H from SiH 3 OO adduct was found. Possible decomposition channels of the vibrationally excited products of the SiH 3 + O 2 reaction to produce SiO are discussed.

Silyl Radicals

“…The reaction of ž SiH 3 radicals with molecular oxygen and the mechanism of SiO formation during the laser photolysis of SiH 4 /O 2 /CCl 4 mixtures have been reported 31 . The bimolecular rate constant for the production of SiO was determined to be 6.8 ð 10 9 M 1 s 1 .…”

“…During this investigation, a new transition state (33) for the formation of siladioxirane 34 was found (Scheme 5). Interestingly, the proposed pathway to 34 involves a rare example of homolytic substitution at silicon by an oxygen-centred radical via a frontside attack 31 . The authors also discuss the various possible decomposition channels leading to SiO.…”

Heterophase combustion of silane near the first ignition limit

“…The chain-branching process (CBP) of oxidation of silane (SiH 4 ) was investigated in [3][4][5][6][7][8][9][10][11][12][13][14][15]. Azatyan et al [5][6][7] studied the reactor surface in contact with silane flame in oxygen.…”

“…Azatyan et al [5][6][7] studied the reactor surface in contact with silane flame in oxygen. Intermediate reaction products (H, O, OH, SiH 3 , :SiH 2 , SiO, Si, SiH 2 O 2 , and SiH 2 O) in gas have been examined by modern methods [8,[11][12][13][14]. The final gas-phase products in this reaction are water and molecular hydrogen [8].…”

“…In [6], the conclusions of [1,3] concerning gas-phase chain branching in silane combustion were extended by the conclusion on heterogeneous chain branching. Later, the fast reaction of SiH 3 radicals with molecular oxygen in gas was studied [8,[11][12][13], and it was assumed in [8] to be the main branching reaction for the gas-phase part of the process. In an early study [4], expansion of the ignition peninsula was found in experiments with mixtures richer in silane and it was inferred that SiH 4 molecules participate in the chain branching reaction.…”