Understanding of the mechanisms of radical production and decay in -irradiated hydrocarbon glasses, and in -irradiated and photolyzed solutions of alkyl halides in such glasses, has been extended through studies of yields and decay rates in 3-methylpentane (3MP) and 3MP-di4, with and without charge scavengers, using ESR as the detection technique. About half of the C6Hi3 radicals produced by irradiation of pure 3MP decay at 77°K by relatively fast intraspur radical-radical reaction with time-dependent firstorder kinetics. The remainder decay with second-order kinetics (k = 1.6 X 10-25 cm3 molecule-1 sec-1), by reaction following random diffusion (D e¿ 9.0 X 10-2°cm2 sec-1). Changes in resolution of the ESR spectra of trapped C6Hi3 radicals during decay and during and following photolysis reflect changes in spacing and geometry. The G value for radical production (radicals per 100 eV absorbed) in 3MP at 77°K is 3.0 ± 0.3, while GiCgDis) from the radiolysis of 3MP-di4 is 2.0 ± 0.4. GiCgHis) in 3MP-hi4 and G(CeDi3) in 3MP-di4 are lowered by electron scavengers, implying that part of the radical formation in the pure hydrocarbons results from electron capture by cations (unless the additives are unexpectedly effective as energy scavengers). Scavengeable electron yields are higher in CeDu glass than in CeH^, which is an unexpected matrix isotope effect. G(CgHi3) and GlCgDis) from radiolysis of 3MP-hi4 and 3MP-£¿i4 glasses are increased by the presence of 1 mol % hydrogen halide, implying that hot H atoms produced from the halides by dissociative electron capture abstract H (or D) from the matrix. The energetics require capture of electrons with greater than thermal energy. In 3MP-1% HI and 3MP-1% HBr systems, ~70% of the radicals decay by the CgHia + HX -* CV.Hn + X reaction, the estimated rate constant and HI diffusion coefficient for the HI system being 8.5 X 10-5 M-1 sec-1 and 1.6 X 10-19 cm2 sec-1, respectively. HC1 forms a stable complex with C6Hi3 radicals (presumably CgHis • HC1), which can be photochemically decomposed regenerating the original radicals. Alkyl radicals produced in 3MP-<¿44 by dissociative electron capture by alkyl chlorides decay faster than identical radicals produced from bromides and iodides, whereas the rates in 3MP-h44 are independent of the geminate halide ion, consistent with recent evidence that the decay mechanisms of CH3 are different in the two matrices. Photolysis of CH3I in 3MP-hi4 produces CgHis (by abstraction of H by hot CH3), but no trapped CH3 radicals. The photolysis of CH3I in 3MP-di4 yields [CHsJ/tCgDis] = ~0.02. CH3 produced photolytically in 3MP-d44 decays faster than CH3 produced by dissociative electron capture, implying that geminate recombination with I is faster than with I-.
Articles you may be interested inGeometrical structure of solvated electrons in γirradiated 3methylpentane glass at 77 K from an analysis of the modulation of the electron spin echo decay envelopeThe isotopic methane yields from y irradiation of 3-methylpentane, 3MP-I %CH 3 I, 3MP-d I4 -1 %CH 3 I, and 3MP-dI4-1 %CD 3 I glasses confinn recent evidence from decay kinetics that thermal CH 3 radicals can abstract H from C-H bonds in hydrocarbon glasses at 77 OK (plausibly by tunneling) and show that similar abstraction does not occur from C-D bonds. Also, in 3MP-d 14 the CH 3 (fonned by the CH3I+ e--> CH3+I-process) does not decay by radical combination processes (CH 3 +CH 3 or CH 3 +C.D 13 ), implying that decay is by geminate recombination with the 1-partner. In the y irradiation of 3MP-d I4 -1 %CH 3 I irradiated at 77 OK, G(CH 3 D) from hot abstraction by CH 3 is 0.7; thennaI CH 3 and CD 3 radicals are able to abstract H from low concentrations of C-H bond isotopic impurities; and the yields of scavengeable electrons and of radicals are substantially different than in 3MPh 14 •
Quantum yield studies of the borazine-ammonia and borazine-methyl bromide photochemical reactions at 1849 Á have revealed aspects of the two systems which may be general features of borazine photochemistry at this wavelength. The quantum yields for formation of B-monoaminoborazine and H2 increase with an increasing ammonia to borazine ratio and reach limiting values at approximately an equimolar mixture. The limiting value for 2 is 1.1 ± 0.1. In the borazine-CHsBr system, the quantum yield for methane production increases with the ratio of methyl bromide to borazine to a maximum of about 0.75 ± 0.1 at a 30-fold excess of methyl bromide, and at higher ratios decreases slowly. Studies of these reactions with added inert gas indicate that at high borazine/reactant ratios the reacting intermediate is a vibrationally excited borazine molecule. In experiments with high reactant/borazine ratios corresponding to predominant reactant absorption, radical production results in abstraction or replacement of hydrogen atoms on the borazine ring. The extent of the photodecomposition of borazine is also a considered factor.
Bei der Photolyse mit 1849 Ä‐Strahlung wird H2, Borazanaphthalin, Diborazinyl und ein nicht flüchtiges Polymerprodukt gebildet.
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