P. Potzinger

Photolysis of tetramethylsilane near the absorption onset : mechanism and photophysics

Abstract The excitation of tetramethylsilane (Me 4 Si) into its lowest excited Rydberg state is followed by two main decomposition channels: a simple Siue5f8C bond breaking process with a quantum yield of Φ = 0.45 ± 0.05 and a methane elimination process with the concomitant formation of dimethylsilaethylene ( Φ = 0.17 ± 0.04). Other very minor primary processes occur, with quantum yields of the order of Φ ⩽ 5 × 10 −3 , but their nature could not be identified with certainty. The reactions leading to the stable products are dominated by radical-radical processes and by radical addition reactions to Me 2 SiCH 2 . The addition reaction to the Siue5fbC double bond occurs preferentially at the Si site. Satisfactory material balance was obtained indicating that the products were mostly recovered. A number of relative rate constants were determined. Reactions in the presence of NO, MeOH, GeH 4 and SF 6 were also studied. An explanation of the photophysics by a three-state model was attempted. From the experiments, it was concluded that the two decomposition channels occur from different electronic states. The lack of dependence of the CH 4 quantum yield on the experimental parameters (liquid or gaseous phase, etc.) suggests a decomposition from a strongly predissociating state, which is identified with the lowest excited state, while the Siue5f8C bond breaking process is thought to occur from the triplet state. Molecules which reach the ground state live sufficiently long so that deactivation competes successfully with decomposition.

Stationary and pulsed photolysis of hexamethyldisilane

Analysis of the endproducts of the photolysis of hexamethyldisilane at 206 nm reveals two major decomposition channels: Siue5f8Si bond rupture yielding trimethylsilyl radicals (71%), and the molecular elimination of dimethylsilene (27%). A mechanism is proposed which accounts for the products in the presence and absence of scavengers. Laser pulsed photolysis experiments were carried out to measure the absorption cross-section and the rate constant for combination of the trimethylsilyl radical. The values obtained are σ(260 nm, base e) = (3.7 ± 0.9) × 10−17 cm2 molecule−1 and k = (3 ± 1) × 10−11 cm3 s−1.

Hg-sensitized photolysis of Me3SiH. I: A quantitative approach to the mechanism

Abstract 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 SiCue5f8H bond and, to a minor extent, from the Cue5f8H 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 Siue5fbC 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.

Absorption spectra of trialkylsilyl radicals by a chemical modulation experiment

Abstract The absorption spectra of the trimethyl—, triethyl— and tri— n —propylsilyl radicals, generated by the mercury—sensitized photolysis of the corresponding silanes, were observed in a chemical modulation experiment. The absorption cross-section of the trimethylsilyl radical was measured, the value obtained being σ (260 nm, base e) = 3 × 10 −17 cm 2 per molecule.

Gas-phase photolysis of pentamethyldisilane at 206 nm

Abstract The photolysis of pentamethyldisilane, Me 5 Si 2 H. 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 Me 2 Si 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 SF 6 suggest that decomposition occurs mainly from the excited states.

Photolysis of hexamethyldisilane at 206 nm

Abstract The photolysis of Me 6 Si 2 at 206 nm results in two main decomposition processes: simple Siue5f8Si bond breaking with a quantum yield of Φ = 0.21 ± 0.03, and Me 3 SiH elimination with the concomitant formation of Me 2 SiCH 2 with Φ = 0.18 ± 0.01. There is also a minor decomposition channel with a very small quantum yield, Φ = (5.6 ± 0.2) × 10 −3 , which results in the formation of Me 4 Si and Me 2 Si. The main fate of the excited Me 6 Si 2 molecule produced during photolysis is stabilization by collisional deactivation. The end products observed indicate that the reaction pathways followed by the main intermediates, Me 3 Si and Me 2 SiCH 2 , are the same as those found in the photolysis of Me 4 Si (Ahmed et al., J. Photochem. Photobiol. A: Chem. 86 (1995) 33).

Hg-sensitized photolysis of Me3SiH II. The role of the sensitizer

Abstract In the mercury-sensitized photolysis of Me 3 SiH, 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 Pentamethyldisilane

Abstract 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. Abstract 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)/ k(1) ≤ 0.071. A mechanism with pertinent rate constants has been proposed which accounts for the results.

Reaction of Trimethyl-and Dimethyl-Silyl Radicals with Pentamethyldisilane

Abstract 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 =(8.53 ± 0.08) · 10-11cm3/2s–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.