The homogeneous gas-phase decomposition kinetics of silane has been investigated using the single-pulse shock tube comparative rate technique (T = 1035-1184°K, P~d = 4000 Torr).The initial reaction of the decomposition SiH4 ! + SiHz + H2 is a unimolecular process in its pressure fall-off regime with experimental Arrhenius parameters of logkl (sec-I) = 13.33 f 0.28-52,700 f 1400/2.303RT. The decomposition has also been studied at lower temperatures by conventional methods. The results confirm the total pressure effect, indicate a small but not negligible extent of induced reaction, and show that the decomposition is first order in silane at constant total pressures. RRKM-pressure fall-off calculations for four different transition-state models are reported, and good agreement with all the data is obtained with a model whose high-pressure parameters are logAl (sec-') = 15.5, El(,) = 56.9 kcal, and AE;;, = 55.9 kcal. The mechanism of the decomposition is discussed, and it is concluded that hydrogen atoms are not involved. It is further suggested that silylene in the pure silane pyrolysis ultimately reacts with itself to give hydrogen: 2SiH2 -(SiZHd)* -(SiH3SiH)* -SizHz + Hz. The mechanism of H -D exchange absorbed in the pyrolysis of SiD4-hydrocarbon systems is also discussed.In a prior paper [l], we reported preliminary single-pulse shock tube kinetic results on the silane decomposition. We showed that the initial reaction of the decomposition is molecular Hz elimination [reaction (l)]+ (1)Production of hydrogen atoms in the overall reaction was suggested by large yields of HD found in the pyrolysis of SiD4 in the presence of excess toluene (see Table I). Silylene decomposition [reaction (2)] was postulated to be the source of the D atoms [l]. However, we now believe that this conclusion n + The simple bond rupture process, SiH4 -SiH3 + H, was eliminated as a possible initiation reaction because its high activation energy (93 kcal) would require a chain process with chain lengths in excess of lo6 in order to match observed reaction rates. Such long chains under shock conditions are clearly impossible. Thus one calculates that on average fewer than 50 collisions between silane (0.01%) and product molecules occur in a typical 200 Fsec shock period.

The homogeneous gas-phase thermal decomposition kinetics of germane have been measured in a single-pulse shock tube between 950 and 1060 K at pressures around 4000 torr. The initial decomposition is GeH4 -GeH2 + H2 in its pressure-dependent regime, with log k~~~~( 4 m ) = 13.83 f 0.78 -50,750 f 3570 ca1/2.303RT. RRKM calculations suggest that the high-pressure Arrhenius parameters are log k GeH4(M -a) = 15.5 -54,300 cd2.303RT.Extrapolations to static system pyrolysis conditions (T -600 K, P -200 torr) give homogeneous reaction rates which are much slower than those observed, hence the static system pyrolysis of germane must be predominantly heterogeneous. Shock-initiated pyrolysis reaction stoichiometry is 2 mol H2 per mole GeH4, suggesting that the subsequent decomposition of germylene is essentially quantitative. Investigations of the hydrogen product yields for pyrolysis of GeD4 in @CH3 further indicate that the germylene decomposition reaction is mainly GeHz -H2 + Ge, hut that a small amount of reaction to H atoms may also occur.

Communications to the Editor 5945 sequent attack on aromatic hydrocarbon. Therefore, the kj/kd etermined noncompetitively does not reflect the relative reactivity of toluene over benzene, and thus can not be used to test the Brown selectivity relationship.On the other hand, a kj/k% determined by competitive methods can be used.11 Our preliminary results based upon three competitive runs indicate a kj/k& = 2.5 ± 0.1, together with a toluene isomer product distribution of 45 ± 2% ortho, 6 ± 1% meta, and 49 ± 2% para. These values remained reasonably constant even after the addition of small amounts of H2O. The isomer percentages are also in satisfactory agreement with values obtained by noncompetitive means: %

No abstract

Kinetische Untersuchungen der SiH4‐Zers.