Electron Attachment and Solvation in Gamma Irradiated Organic Glasses at - 196°

A mechanism is suggested for the radiolysis of hydrocarbons. Both ions and excited ~nole-cules are produced in the initial excitation. Ion-electron recombination leads to the formation of excited molecules which have similar properties to thosc formed initially; all subsequent decon~position proceeds through these excited molecules. 111 ~nixtures of hydrocarbons, ions and n~olecules may undergo positive charge exchange and additives reduce the hydrogen yield by capturing electrons or reacting with the positive ion. T h e mechanism provides a ratio~lalization for a variety of observatio~ls. This paper presents a mechanism for part of the radiolytic decomposition of hydrocarbons in which electron capture and charge exchange play a dominant role. There is no novelty in postulating these reactions a s mechanistic steps. The significance of electron capture has been discussed on inany occasions by I-Iamill and his colleagues (see, for instance, refs. 1 , 2, 3). Charge exchange was proposed by Hardxirick (4) as a inechanisn~ for the reactivity transfer observed in mixtures of aliphatic hydrocarbons and has also been suggested by Hamill. The crux of this proposed mechanism lies in the combination of two hypotheses, first, that the initial act of excitation leads to the formation of both ions and excited molecules and secondly, that, in pure hydrocarbons, the recapture of the electron by the positive ion also leads to the formation of excited inolecules which are, for practical purposes, identical to these forined in the initial excitation. The mechanism provides an explanation for a variety of phenomena; viz., the rapid inhibition of part but not all of the hydrogen yield by solutes, reactivity transfer in mixtures of aliphatic and alicyclic hydrocarboils, and the effects of additives on the isotopic composition of radiolytic hydrogen produced from lnixtures containing deuterated hydrocarbons. Tlze iWeclzanism( 1 ) Radiolysis of a hydrocarbon A produces, initially, positive ions, an equivalent nuinber of electrons, and excited, un-ionized molecules A*.The yields of ions and excited inolecules are comparable and probably 3/100 eV absorbed.( 2 ) The recapture of the electron by the positive ion produces an excited species which is, for the purposes of the present discussion, identical with A* produced in [lb].

Section: Dyne: Chargb Transfer and Electron Capturementioning

confidence: 99%

Section: The Effect Of Hydrogen Halides On the Radiolysis Of Hydrocarmentioning

confidence: 99%

Charge Transfer and Electron Capture in the Radiolysis of Aliphatic Hydrocarbons

Dyne¹

1965

“…This inobile electron will have one of three fates; it may siinply be retrapped in reaction [2], or be recaptured by the positive ion after therinalization or if biphenyl is present, it may be captured by the biphenyl to give 4 2 -(reaction [3]). …”

Section: Mechamistic Steps In Photobleachingmentioning

confidence: 99%

“…Figure 4 shows the quantum efficiency for bleaching electrons and forming biphenyl anions. This could only occur if, in the absence of biphenyl, there was a coinpetition between capture by positive ions and retrapping in the glass (reaction [2] If PN and PT are the probabilities of neutralizing and trapping, then the measured quantum efficiency for bleaching, an, is related to @4, the quantum efficiency for initial release from the traps by the relation:…”

Section: Tlze Bleaclzing Of the Electron Bandmentioning

confidence: 99%

Photochemistry of Electrons Trapped in Glasses of Methyltetrahydrofuran at 77°k

Dyne¹,

Miller²

1965

I n the radiolysis of methyltetrahydrofuran glass a t 77 "I< electrons are produced with a high yield. These are immobilized in traps in the glass and do not recombine with positive ions. They can be released from their traps by illumination with visible and infrared light and may then diffuse to positive ions and be captured. The quantum efficiencies for bleaching have been measured as a function of wavelength. This quantum efficiency falls as bleaching proceeds and depends on the fraction of the electrons bleached and not on their concentrations. About half the electrons are trapped close to positive ions and, on release, have a high probability of diffusing to the positive ion. The remaining half are trapped hon~ogeneously. On release from their traps, few of these electrons reach positive ions although they diffuse over long distances before being retrapped in the glass. This behavior is compared to that discussed by Samuel and Magee for the thermalization and capture of electrons in liquid water.The rate of recombination of an electron and positive ion in a condensed phase is an alinost unltnown quantity and the uncertainties which remain about this process lie a t the root of inany fundainental problems of radiation chemistry. Important advances, permitting a direct study of the process under special conditions, have, however, recently been inade by W. H. Hamill who has extended to radiolytic reactions the techniques of studying the photocheinistry of organic glasses which were pioneered by G. N. In an organic glass a t low teinperature reactive interinediates forined in radiolytic or photolytic decoinposition are held iininobile in a rigid solvent cage. They have long lifetimes and can be observed, a t leisure, by conventional optical techniques. Ronayne, Guarino, and Hamill (2) (henceforth R. G. and H.) found that, on radiolysis of glasses of 2-methyltetrahydrofuran (i\lITHF), a strong optical absorption is observed in the near infrared kvhich is due to electrons trapped in the glass. In glasses containing biphenyl as a solute the intensity of the electron band is reduced and a new absorption due to the biphenyl anion 4~ appears.'The electron band is photosensitive, being bleached by visible and near infrared light. If biphenyl is present in a bleaching experinlent the intensity of the biphenyl anion band increases as the intensity of the electron band is diminished. Electrons released froin their traps in the the glass inay be captured by positive ions or by biphenyl molecules.In a sense this technique allows one to observe, in slow motion, the recombination of electrons and positive ions. This paper describes a study of the photocheinistry of the bleaching of trapped electrons in i\lITHF glasses which shows that thermal electrons have a high mobility in the glass. While few electrons recoinbine with the positive ion before being trapped, they prefer trapping locations in the vicinity of positive ions. E X P E R I M E N T A LThe experimental techniques for purifying M T H F , making optical cell...

“…absorption of trapped electrons in polar glasses, such as alkaline ice (1 -3), alcohols (4)(5)(6)(7)(8)(9)(10)(11)(12), water-alcohol mixtures (13)(14)(15), and methyltetrahydrofuran (16)(17)(18)(19)(20)(21)(22)(23)(24)(25). One objective of scavenger studies has been to produce new anions from the reaction of the mobile electron with the scavenger molecule.…”

mentioning

confidence: 99%

Reactivity of Organic Halides toward the Mobile Electron in Methyltetrahydrofuran Glass

Kato¹,

Fueki²,

Kuri³

1972

The relative reactivity of a number of organic halides with the mobile electron produced in y-irradiated methyltetrahydrofuran glass at 77 "K has been studied by a scavenger competition kinetics. It has been found that for the organic halides there exists a correlation between the relative electron capture cross section in the glassy matrix and the'heat of reaction for dissociative electron capture estimated from gas phase data.Nous avons ktudit la reactivite relative de certains halogtnures organiques vis-a-vis l'tlectron mobile produit en irradiant par des rayons-? le mtthyltttrahydrofuranne vitreux a 77 OK, au moyen d'une cinetique de competition de pikge. Nous avons trouvC qu'il existe, pour les halogenures organiques, une correlation entre la surface relative de capture d'tlectron dans la matrice vitreuse et la chaleur de reaction dans le cas de la capture dissociative d'electron, estimee a partir des rksultats en phase gazeuse.Canadian Journal of Chemistry, 50,2702Chemistry, 50, (1972 The effect of a number of different electron scavengers have been studied on both the optical and e.s.r. absorption of trapped electrons in polar glasses, such as alkaline ice (1 -3), alcohols (4-12), water-alcohol mixtures (13-15), and methyltetrahydrofuran (16-25). One objective of scavenger studies has been to produce new anions from the reaction of the mobile electron with the scavenger molecule. The second objective has been to study the relative reactivity of the radiation produced mobile electron with various scavenger solutes.In this note we report the relative cross sections of various organic halides for capturing mobile electrons (e,) produced in y-irradiated methyltetrahydrofuran (MTHF) glass at 77 OK. We also show that for the organic halides there exists a correlation between the electron capture cross section in the glassy matrix and the heat of reaction for dissociative electron capture estimated from gas phase data.MTHF was purified by passage through a column of activated alumina, and was degassed by the freeze-pump-thaw technique and dried over calcium hydride under vacuum. MTHF was further dried with a sodium mirror prior to use. Biphenyl was purified by recrystallization from n-hexane. Organic halides as solutes were the purest grade commercially available. They were alkyl iodides (CH31, C2H,I, n-C3H71, i-C3H71, n-C4H91), alkyl bromides (C,H,Br, i-C3H7Br, n-C4HgBr, t-C4H9Br), alkyl chlorides (n-C3H7CI, i-C3H7Cl, n-C4H9Cl, i-C4HgCl, t-C4HgCl), and benzyl bromide and chloride. y-Irradiations of samples containing 0.13 mol% biphenyl and the halides at various concentrations were carried out at 77 OK in the dark to a dose of 1.04 x lo4 rad. The optical absorption band of biphenyl anion with a peak at 408 nm was measured on the y-irradiated samples at 77 OK in the dark.The efficiencies of electron capture by the halides were determined by making use o f competitive reactions between biphenyl (4,) as the reference substance and the halides (RX) as second solutes (reactions 1 and 2).A simple competition kinetic...