T.N. Bell

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 .

Excess and interaction second virial coefficients for seven binary gaseous systems

Abstract Excess and interaction second virial coefficients have been measured over the temperature range 293–321 K for the systems He-N 2 , He-CO 2 , He-N 2 O, He-CH 4 , He-SF 6 , He-CF 4 and Ar-SF 6 . The excess second virial coefficients B E are estimated to have an accuracy of 0.3 cm 3 mol −1 .

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.

Rate parameters for the reactions of the bromomethyne radical

Abstract Absolute rate parameters were measured for the gas phase reactions of CBr( ∼ X 2 Π) produced in the flash photolysis of CHBr 3 with a variety of paraffins, olefins, O 2 and NO. The rate of cycloaddition to unsaturated bond increases with alkyl substitution, pointing to the electrophilic nature of CBr. In comparison with published data on CCI and CH the reactivity follows the trend CBr ≈ CCl

Rate constants for the reaction of the bromomethyne radical with alkynes

Abstract Absolute rate constants were measured for the gas phase reactions of the CBr( ∼ X 2 Π) radical produced in the flash photolysis of CHBr 3 with a series of alkynes. The rate of addition to the triple bond followed the trend established for addition to double bonds and a linear correlation between log k and ionization potentials reflects the electrophilic nature of the reaction. In comparison with published data on CCl, S( 3 P 2 ) the reactivity follows the trend CBr ≈ CCl ≈ S( 3 P 2 ) > O( 3 P 2 ).

Rate constants for the reactions of CCl(X2II) with a series of silanes

Abstract Rate constants for the gas phase reactions of CCl generated by the flash photolysis of CHBr 2 Cl with a series of silanes have been obtained by kinetic absorption spectroscopy. In general, the rate constants are very high, and range from (4.8 ± 0.5) × 10 8 (SiH 4 ) to (6.4 ± 0.34) × 10 9 for Si 2 H 6 . CCl does not insert into the SiC or primary CH bonds of silanes and its rate of reaction with tertiary SiH bonds is 600 times greater than with tertiary CH bonds. CCl reacts slowly with the SiSi bond. k H / k D varies from 1.9 to 1.0 on going from primary to tertiary SiH bonds. The electrophilic character of CCl is manifested, on a per SiH bond basis, by excellent correlations between the rate constants and the hydrilic character of the SiH bond, and between log k and the ionization potential.

Rate constants for the reaction of the chloromethyne radical with alkynes

Abstract Absolute rate constants were measured for the gas phase reactions of the CCl(X 2 Π) radical produced in the flash photolysis of CHBr 2 Cl with a representative series of alkynes. The rate of addition to the triple bond follows the trend established for the addition CCl(X 2 Π) to olefinic double bonds and the acetylenic triple bond by the latter reagent. A linear correlation between log k and ionization potentials of the alkynes reflects the electrophilic nature of the reaction.

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.

The pressure and composition dependence of mutual diffusion in the system helium-nitrogen at 300 K

Abstract Diffusion coefficients for the system helium-nitrogen have been measured as a function of pressure at 300 K from 1 to 12.5 atmospheres. These measurements include composition dependences at 1, 3, 5 and 7 atmospheres. The results show reasonable agreement with the Thorne-Enskog theory for moderately dense gases.