C. Kerst’s research while affiliated with Kiel University and other places

Two primary processes were observed in the Hg-sensitized photolysis of Me 5 Si 2 H: (I) hydrogen abstraction from the Si-H bond with a quantum yield of 0(1) = 0.85, (V) Si-Si bond breaking with 0(V) = 0.04. The hydrogen atoms formed in (/) undergo an H atom abstraction reaction (k(3)), as well as substitution reactions at the Si centers resulting in the formation of dimethylsilane and trimethylsilyl radical (k(4)) or trimethylsilane and dimethylsilyl radical (k(5)). The following branching ratios have been determined: [xxx] The ratio of disproportionation (k(2)) to combination (k(1)) for the pentamethyldisilyl radical has been determined with MeOH as the scavenger for 1-methyl-l-trimethylsilylsilene, 0.046 < k(2)/ A: C1) < 0.071. A mechanism with pertinent rate constants has been proposed which accounts for the results.

The rate constant for the H atom abstraction of trimethylsilyl radicals from pentamethyldisilane (k (4)) was measured relative to the trimethylsilyl combination reaction k (3). A value for k (4)/√K 2() = (8.53+0.08) · 10-11 cm3/2 s-1/2 was obtained. For the dimethylsilyl radical, a smaller value for the corresponding rate constant ratio (5.9 ± 0.2) · 10-11 cm3/2 s-1/2 was measured, and this was attributed to a disproportionation reaction between the dimethylsilyl and the pentamethyldisilyl radical leading to dimethylsilylene.

Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.

The photolysis of pentamethyldisilane, Me5Si2H. is characterized by a large number of decomposition processes of the excited molecule. Quantum yield determinations in the presence and absence of various scavengers support the occurrence of Me2Si elimination (Φ = 0.2) and Si-Si bond breaking (Φ = 0.14) as the two major decomposition processes. Other processes include the elimination of various silaethylenes and MeHSi. The quantum yield of these processes sum up to Φ = 0.16. However, the most important pathway of the excited molecule is collisional deactivation (Φ = 0.5). The material balance for the various silaethylenes is poor in the absence of traps but can be improved greatly in the presence of MeOH and is in agreement with computer simulations. Experiments with SF6 suggest that decomposition occurs mainly from the excited states.

In the Hg-sensitized photolysis of H-2 in the presence of C2H4 the sensitizer Hg is consumed under formation of a metastable compound. The rate of formation and the steady-state concentration of this compound have been studied as a function of Hg, H-2, and C2H4 concentration, absorbed light intensity, temperature, and in the presence of radical scavengers. All these experiments ask for the participation of HgH and C2H5 in the formation of the Hg compound. We therefore suggest that these two radicals combine under formation of HHgC2H5. Further support comes from two color photolysis experiments where HgH and C2H5 radicals have been generated independently. HHgC2H5 decomposes under 206 nm irradiation and much slower in the dark. It is also very easily attacked by radicals.

Mercury-sensitized photolysis of Me 2 SiH 2 yields in the primary step an Me 2 HSi radical and an H atom with a quantum yield of one. The Me 2 HSi radicals undergo a combination reaction [k(2)] as well as two kinds of disproportionation reactions leading to dimethylsilylene [k(3)] and 1-methylsilaethene [k(4)]. The following branching ratios have been determined: k(3)/[k(2)+k(3)+k(4)]=0.64 ±0.10 and k(4)/[k(2)+k(3)+k(4)]0.007. For Me 3 Si radicals the branching ratio for disproportionation was also determined and a value of 0.063 was obtained.

In the mercury-sensitized photolysis of Me3SiH, it was found that the mercury concentration does not remain constant during photolysis, and mercury forms an unidentified compound. The compound decomposes to elemental mercury by radical attack and in the dark by a surface-catalysed reaction. Experiments suggest that mercury atoms in the ground state are involved in compound formation.

The mercury-sensitized photolysis of Me3SiH was studied as a function of the exposure time, substrate pressure and light intensity, and in the presence of the additives McOH and H2. Two primary processes were observed: hydrogen abstraction from the SiCH bond and, to a minor extent, from the CH bond. The sum of the quantum yields of the two primary processes is only 0.8. The main part of the reaction mechanism, which concerns the reactions of the Me3Si radical, can be quantitatively explained by a previous investigation of the direct photolysis of Me4Si (Ahmed et al., J. Photochem. Photobiol. A: Chem., 86 (1995) 33). With the rate constants given by Ahmed et al., the experimental values can be satisfactorily reproduced by computer simulations. In particular, it is confirmed that silaethylene reacts in an almost collision-controlled manner with radicals and the disproportionation reactions of Si-centred radicals leading to an SiC double bond play only a minor role. The ratio of disproportionation to combination of the Me3Si radical was determined to be 0.07 ± 0.01.