Effects of solutes, deuteration, and annealing on the production and decay of radicals in .gamma.-irradiated 3-methylpentane glasses

Abstract

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-.

Formation and photolysis of alkyl and peroxy radicals in dilute glass matrices

“…This is substantially lower than the total radical yield G, = 3.0 obtained by Perkey and Willard (12). However, the latter value is due to two radical populations; 57% of the total decay by a rapid "composite first-order decay" due to intraspur recombinations, while 43Yc decay by a slower second-order process involving "isolated" radicals (21). It is apparent that oxygen scavenges to a large extent the latter population, but diffusion is not rapid enough to compete with intraspur and other cage processes.…”

“…For this reason we also studied the feasibi!ity of reaction 2 in rigid 3-methyl pentane (3MP) at 77 K, i1v [21 ROz. -+ R. + 0 2 since many alkyl radicals can be identified by their epr hyperfine spectrum when trapped in a solid matrix (7).…”

The √t law in solid‐phase reactions verification of the √t law stability in h‐atom abstraction reactions

“…One can suppose that the nonexponential kinetics of the reaction R' + RH -R'H + R, where RH is a matrix molecule, is associated with a decrease shown in [5]. We carried out experiments to test if law (1) can be related to the matrix structure relaxation.…”

“…In our recent works [l-41 we have managed to find the kinetic law (1) c = coexp (-K&) for H-atom abstraction reactions R' + RH -R'H + R, where RH are matrix molecules. However, the literature offers examples when the manner of preliminary matrix treatment [5] and the temperature of radical formation [6] influence the kinetics of the following solid-phase reactions.…”

.gamma.-Irradiated hydrocarbon crystals. Yields, decay, and photoreactions of radicals: carbanion formation

“…(2) Evidence for intramolecular energy transfer from alkane side chains to aromatic groups,3 for intermolecular energy transfer to solutes with resultant selective bond rupture in the solute molecules,4 and for transfer to toluene solute with sub-sequent luminescence from the toluene.5 (3) G(R•) in glassy branched chain alkanes is uniformly lower (~3.0) than in polycrystalline n-alkanes (~5.0).6a,b (4) G(R•) in deuterated glassy and polycrystalline hydrocarbons in uniformly 30% or more lower than in protiated hydrocarbons,6ab and the localization of bond rupture is different in n-Ci0H22 than n-C10D22.7 (5) Different radicals are formed by 7 irradiation of different solid state phases of some compounds.46 (6) Trapped hydrogen atoms are not formed by the 7 irradiation of hydrocarbon crystals and glasses,66 other than CH4. (7) Optical absorption in the 300-400-nm range,8a,b photostimulated luminescence,8c,d and photostimulated conductivity86•5 indicate carbanion formation and photoionization in hydrocarbon glasses (8) Radicals produced by 7 radiolysis of hydrocarbon glasses show a relatively rapid time-dependent first-order decay (-~50%) resulting from intraspur radical-radical reaction and a slower second-order decay resulting from random encounters.9 (9) Radicals produced by 7 irradiation of polycrystalline hydrocarbons undergo stepwise decay as the temperature is raised to successively higher temperatures, indicating selective softening of crystallites of different sizes, or radical trapping sites of different energies. 10 (10) Different species of radicals in the same polycrystalline pure hydrocarbon matrix show different ESR relaxation times.11 (11) Paired radicals, as well as more widely separated radicals are formed in both polycrystalline7,11 and glassy hydrocarbons,12 the yields of pairs being higher at 4 K than at 77 K.Uc 'd (12) Illumination of trapped radicals in hydrocarbon glasses with 254-nm light accelerates intraspur decay128 and causes a reversible change in the ESR spectrum attributable to isomerization128 or, alternatively,13 to C-C bond rupture.…”