I. Safarik

Rate Constants for Silylene Reactions

It is only since 1985 that the absolute rate constanss have been measured for some reactions of divalent silylene species. In this article the absolute rate constant data reported to date for the reactions of SiH2, SiMe2, SiMePh, SiHCl, SiCl2, SiF2 and SiBr2 are reviewed and, where possible, mechanistic pathways discussed. The reactivity of silylenes is, in general, much higher than had previously been estimated on the basis of relative rate studies.

Dichlorosilylene. The UV spectrum

Abstract The UV spectrum of the SiCl 2 radical has been the subject of some controversy in the literature and earlier claims for the spectrum have been recently refuted. The first absorption band showing a ν′ 2 progression with a maximum at 317.4 nm and assigned to the A 1 B 1 ← X 1 A 1 transition of SiCl 2 from the gas phase flash photolysis of Si 2 Cl 6 is reported. The assignment of the spectrum to SiCl 2 is based on chemical evidence obtained from experiments employing t -butene as a scavenger and using 1,1-dichlorosilacyclobutane as an alternative source of SiCl 2 .

The direct conversion of hydrogen sulfide to hydrogen and sulfur

Abstract The thermal noncatalytic decomposition of H 2 S has been investigated in the temperature range 1000–1200°C, neat and admixed with nitrogen or helium at a total pressure of one atm. It has been found that, contrary to earlier literature claims, the inhibiting effect of the back reaction, the reverse reaction between H 2 and S i ( i = 1–8) molecules to regenerate H 2 S, can be overcome by the use of readily attainable, sufficiently high gas flow rates. In agreement with thermodynamic predictions, the reaction has been shown to be temperature and H 2 S pressure dependent; the experimental conversion was found to increase with rising temperature and declining H 2 S pressure. At 1200 °C and one atm. H 2 S pressure the measured conversion was 35.6% corresponding to 97.5% of the thermodynamic limit when a steady gas flow of 50 mL/min and residence time of 48 seconds were maintained. The highest experimental conversion, 65.8%, was obtained at the highest temperature, 1200 °C, and lowest pressure, 0.050 atm., employed. For optimum conversions under the present experimental conditions it was necessary to use a quartz reactor packed with quartz chips. Apparently, at 1200 °C the chips have no catalytic effect but serve as heat transfer agents.

The thermal decomposition of hydro-carbons. Part 1. n-alkanes (C≥5)

The pyrolysis of the long (C≥5) chain n-alkanes is reviewed with emphasis on the radical chain mechanism and on the reactions of the intermediate alkyl radicals. The effects of the experimental conditions and conversion on the types and distributions of the final products are also evaluated. The Arrhenius rate parameters are summarized for both the elementary steps and the overall decomposition reactions.

Dichlorosilylene: rate constant for reaction with oxygen

Abstract Using the recently detected intense UV absorption spectrum of SiCl 2 (X 1 A 1 ) the absolute rate constant has been measured for its reaction with oxygen. The value of the bimolecular rate constant, k (SiCl 2 + O 2 ), is equal to (3.4 ± 0.7) × 10 9 M −1 s −1 at 25° C.

Dichlorosilylene. Rate constants for reaction with olefins and acetylenes

Abstract Using the recently detected intense UV absorption spectrum of absolute rate constants have been measured for its gas phase cycloaddition to C 2 H 4 , CH 3 CHCH 2 , t -C 4 H 8 , C 2 H 2 and C 2 H 5 CCH. The values at T = 25°C range from 4.3 × 10 7 to 7.9 × 10 8 M −1 s −1 and their variation with substrate structure follows an electrophilic trend similar to the ones established for the reactions of halomethylenes.

THE THERMAL DECOMPOSITION OF HYDROCARBONS. PART 2. ALKYLAROMATIC HYDROCARBONS: ALKYLBENZENES.

The pyrolysis of the n-alkylbenzenes is reviewed with emphasis on the radical chain mechanism and on the reactions of the intermediate radicals. The effects of the experimental conditions and conversion on the types and distribution of the final products are also evaluated. The Arrhenius rate parameters are summarized for both the elementary steps and the overall decomposition reactions.

Hg(3P1) Photosensitization of trimethylsilane in the presence of alcohol

Abstract The reaction between CH 3 OH or C 2 H 5 OH and 1,1-dimethylsilaethene, the disproportionation product of the trimethylsilyl radical, has been studied to establish its efficiency for quantitative measurement of the disproportionation reaction. It has been found that standing in a Pyrex vessel, in mixtures of (CH 3 ) 3 SiH - the usual precursor of the (CH 3 ) 3 Si radical - and CH 3 OH or C 2 H 5 OH, a dark surface reaction takes place giving H 2 , (CH 3 ) 3 SiSi(CH 3 ) 3 and (CH 3 ) 3 SiOR. Using the Hg( 3 P 1 ) photosensitization of (CH 3 ) 3 SiH as a source of (CH 3 ) 3 Si radicals in the presence of 10–20 Torr CH 3 OH or 10 Torr C 2 H 5 OH, the (CH 3 ) 3 SiOR product yield increased linearly with increasing photolysis time but did not vanish when extrapolated to zero time. After minimizing this interfering dark reaction and applying a correction for its contribution we determined the value k d / k c = 0.10±0.01 for the disproportionation-combination reactions of the trimethylsilyl radical.

Dichlorosilylene: Rate constant for the gas-phase reaction with nitric oxide

AbstractUsing the recently detected intense UV absorption spectrum of