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The photolysis of tetramethylsilane at the long wavelength side of the absorption spectrum gives rise to two main primary photochemical processes: A simple Si-C bond-breaking process with a quantum yield of 0.55 & 0.17 and a methane elimination reaction with a quantum yield of 0.22 f 0.07. The radicals disappear by recombination and addition reactions to dimethylsilaethylene. Only an upper limit for the ratio of disproportionation to recombination of 0.1 3 for the trimethylsilyl radicals can be given. Beside the radical addition reaction dimethylsilaethylene also dimerizes. A semiquantitative evaluation of the data shows that the radical addition reaction as well as the dimerisation reaction proceed with a negligible activation energy. Durch Einstrahlung in die langwellige Seite des Absorptionsspektrums zerf$llt Tetramethylsilan im wesentlichen nach zwei Prozessen: Einem einfachen Si -C-Bindungsbruch mit einer Quantenausbeute von 0,55 0.17 und einer Methan-Eliminierungsreaktion mit einer Quantenausbeute von 0,22 f 0,07. Die Radikale verschwinden durch Rekombinationsreaktionen und Addition an Dimethylsilaathylen. Fur das Verhaltnis von Disproportionierung zu Rekombination der Trimethylsilylradikale kann nur eine obere Grenze von 0,13 angegeben werden. Dimethylsilalthylen verschwindet neben Radikaladditionsreaktionen auch durch Dimerisation. Eine semiquantitative Auswertung der Daten zeigt, daB sowohl die Radikaladditionsreaktionen an Dimethylsilaiithylen als auch dessen Dimerisierung rnit einer sehr kleinen Aktivierungsenergie ablaufen.

The reactions of radiolytically generated hydroxyl radicals and H atoms with 1,4- and 1,3-cyclohexadiene were studied by pulse radiolysis and product analysis. Hydrogen abstraction from these substrates by the OH radical yields the cyclohexadienyl radical (ε (310 nm) = 4400 dm3 mol-1 cm-1 from the reaction of the H atom with benzene) with an efficiency of 50% (0.29 ,μmol J-1) in the case of 1,4-cyclohexadiene and 25% (0.15 ,μmol J-1) in the case of 1,3-cyclohexadiene as determined by pulse radiolysis. The remaining OH radicals add to the olefin. In 1.4-cyclohexa- diene the yield of the resulting adduct radicals has been determined in a steady-state 60Co-γ-irradiation experiment by reducing it with added 1.4-dithiothreitol (DTT) to 4-hydroxycyc- lohexene. There are two sites of OH radical attack in the case of 1.3-cyclohexadiene, and only the alkyl radical is reduced quantitatively by DTT (G(3-hydroxycyclohexene) = 0.15 ,μmol J-1). From material balance considerations it is concluded that the allylic radical must be formed with a G value of 0.28 ,μmol J-1 but largelv escapes reduction by DTT (G(4-hvdroxycyclohexene) = 0.03 ,μmol J-1). H atoms add preferentially to the double bonds of 1,4- and 1,3-cyclohexadiene (78% and 93%, respectively), while the O.- radical (the basic form of the OH radical) undergoes mainly H- abstraction (92% and 83%, respectively). The radicals formed in these systems decay bimolecularly (2k = 2.8 x 109 dm3 mol-1 s-1). In their combination reactions the cyclohexadienyl radicals form the four possible dimers in propor­tions such that the dienyl radical moiety shows a 2:1 preference to react from its central (1a) rather than from a terminal carbon atom (1b). Cyclohexadienyl radicals and the OH- and H-adduct radicals also cross-tcrminate by disproportionation and dimerization. Material balance has been obtained for the 1,4-cyclohexadiene system in N2O-Saturated solution (10-2 mol dm-3) at a dose rate of 0.14 Gy s-1, the products (G values in ,μmol J-1) being: benzene (0.085), 4-hydroxycyclohexene (0.25), cyclohexadienyl-dimers (0.144). cvclohexadienyl-OH-adduct- dimers (0.02), OH-adduct-dimers (0.02). Some of the 4-hydroxycyclohcxene is formed in an H-abstraction reaction by the OH-adduct radical from 1,4-cyclohexadiene.

A largely solvent‐separated ion pair 3 is the presumed intermediate in the hydroxylation of cyclohexenes with SO 4⊙⊖. Radical anions like 1 and 2 are formed in a nearly diffusion‐controlled step. They are subsequently hydrolyzed via 3. The axial/equatorial selectivity of the reaction of H2 O with 3 is much greater than that observed in the reaction of OH⊙ with 4‐tert‐butylcyclohexene (90:10 vs. 58:42).