RADIATION CHEMISTRY OF CYCLOHEXANE: III. EFFECT OF BENZENE AND IODINE ON THE ISOTOPIC COMPOSITION OF RADIOLYTIC HYDROGEN FROM C₆D₁₂–C₆H₁₂ MIXTURES

Abstract

The isotopic composition of hydrogen evolved in the radiolysis of C B D~T C B H~P mixtures has been measured in the presence of various amounts of benzene and iodine. The yield of the uni~nolecular decomposition and the specihc yield of the bimolecular reaction are reduced in approxin~ately the same proportion as the yield of total hydrogen. I t is concluded that (i) iodine and benzene interact to a comparable extent with both ~~nimolecular and bimolecular decon~position modes, (ii) it is probable that neither of these additives act by scavenging, and (iii) it is probable that they act by quenching a common precursor of the two decomposition modes. INTRODUCTIONWe have shown from measurements of the isotopic composition of hydrogen produced in the radiolysis of C6D12-C6H12 ~~l i x t u r e s(1) that part of the hydrogen is formed in a bimolecular processS n + C G I I~V -+ 1-12 + P PI where the atoms in the H z molecule come fro111 two different cyclohexane molecules, and part is formed in a unimolecular process CGH12 ---3 t I ? + P yield Gl(H)[ 3 I where both hydrogen atoms come from the same cyclohexane molecule. The symbol P represents any product or residue and XE is the reactive intermediate in the biinolecular process.The specific yield of the cleco~~~positionin a large excess of CGI-112 WilS show~l to be 0.25 molecules/100 ev absorbed in the C6DlZ. However, the yield of reaction [3] in pure CGI-112 may amount t o between 25% (2) and nearly 50% (3) of the total yield. I t is conceivable that this large "isotope effect" expresses variations in Gl(D) and Gl(H) with the isotopic con~position of the solvent due to some type of energy transfer or charge transfer effect. In the experinients described in this paper we used the yield of D2 fro111 CGDlZ as an "i~ldicator" of the efficiency of unimolecular processes in benzene and iodine solutions in cyclohexane. We also showed (1) that Gl(D), the yield of reaction [4], was reduced by biphenyl, and slightly reduced by iodine. We concluded that biphenyl could act as a quenchi~lg agent2 on the excited precursors involved in the unilllolecular decomposition.In this paper we describe a series of measurements of the isotopic composition of hydrogen evolved in the radiolysis of CGD12-C61-112 mixtures in the presence of varying amounts of benzene and iodine. We find that, for both these substances, G1(D) is reduced in approxi~llately the same proportioil as the total hydrogen yield, and we infer that the ~~nimolecular and bimolecular decolllpositio~ls are quenched t o approxilllately the salne extent by these substances. Burton and Chang (4)

The Mechanism of Hydrogen Formation Induced by Low-Energy Electron Irradiation of Hexadecanethiol Self-Assembled Monolayers

“…Isotopic labeling provides a clear distinction between unimolecular and bimolecular channels, and allows us to quantify the relative probability of each pathway in the case of multichannel (and multicomponent) systems. This method has been successfully used by Dyne and Jenkinson [30][31][32] to elucidate the mechanism of hydrogen formation in the radiolysis of hydrocarbons; the present work is the first application of this method to the study of resonant processes, and the first in self-assembled monolayers. Our analysis, based on first-and second-order mathematical models of the isotopic composition of the hydrogen evolved expressed as a function of the isotopic composition of the SAM, demonstrates that the formation of hydrogen by DEA in SAMs occurs exclusively via a bimolecular channel.…”

Vapor-Phase Radiolysis of Cyclohexane and Mixtures of Benzene and Cyclohexane

“…T h e interactions causing these low yields are physical; they do not involve the ma1;ing or breaking of chemical bonds. This has been shown in two ways: by complete product analysis (1) and by techniques using isotopically labelled hydrocarbons (2). These isotopic techniques have revealed similar interactions in alkane mixtures (3) which are, however, absent in the vapor phase (4).…”

The Radiation Chemistry of Cyclopentane

“…Although formal recognition of a scavenger-insensitive component of the hydrogen yield was made in steps 1 and 2 of the reaction sequence given above, there is evidence that this portion of the reaction mechanism is more complex and is, in fact, related in some way to the bimolecular hydrogen yield. [23][24][25][26]…”

“…There has been considerable interest in the radiation chemistry of cyclohexane for a number of years. Recently there have been studies of the radiolysis of mixtures of the analogous C6 compound, cyclopentane, with cyclohexane as well as other substances.1,2 However, to date, little effort has been devoted to studies of the radiation chemistry of pure cyclopentane.1 23 Because of its low freezing point (-93°) there are a number of questions which may be investigated using this compound, such as temperature dependence of various processes and possible measurement of activation energies. This paper includes a report of major and minor product yields in the radiolysis of pure liquid cyclopentane and a discussion comparing its radiation chemistry with that of cyclohexane.…”