Radiolysis Of Alkanes Research Articles

Pulse radiolysis of alkanes: a time-resolved EPR study—part I. Alkyl radicals

Time-resolved EPR was applied to detect short-lived alkyl radicals in pulse radiolysis of liquid alkanes. Two problems were addressed: (i) the mechanism of radical formation and (ii) the mechanism of chemically-induced spin polarization in these radicals. (i) The ratio of yields of penultimate and interior radicals in n-alkanes at the instant of their generation was found to be ≈ 1.25 times greater than the statistical quantity. This higher-than-statistical production of penultimate radicals indicates that the proton transfer reaction involving excited radical cations must be a prevailing route of radical generation. The relative yields of hydrogen abstraction and fragmentation for various branched alkanes are estimated. It is concluded that the fragmentation occurs prior to the formation of radicals in an excited precursor species. (ii) The analysis of spin-echo kinetics in n-alkanes suggests that the alkyl radicals gain the emissive polarization in spur reactions. This initial polarization increases with shortening of the aliphatic chain. We suggest that the origin of this polarization is the ST − mechanism operating in the pairs of alkyl radicals and hydrogen atoms generated in dissociation of excited alkane molecules. It is also found that a long-chain structure of alkyl radicals results in much higher rate of Heisenberg spin exchange relative to the recombination rate (up to 30 times). That suggests prominent steric effects in recombination or the occurrence of through-chain electron exchange. The significance of these results in the context of cross-linking in polyethylene and higher paraffins is discussed.

Pulse radiolysis of alkanes: A time-resolved EPR study—Part II. Phenolic additives

Time-resolved spin-echo-detected EPR was applied to observed transient radicals formed in pulse radiolysis of phenols. ArOH, in alkanes at 290 K. It is demonstrated that phenoxy radicals ArO are produced in dissociative capture of electrons, alkane holes and via reaction of phenols with hydrogen atoms. Another product of the latter reaction is cyclohexadienyl-type radical H ·ArOH. H atoms add exclusively to the para-position of phenols. A rapid multi-step transfer of singlet spin correlation from the geminate radical ion pairs to the pairs involving ArO · and H ·ArOH radicals is responsible for the early-time A/E polarization in these species.

Spectroscopic evidence on the absence of trapped chemical dimer cations (1) after irradiation of n-pentane in the solid state

Absorption spectra were recorded after irradiation of a number of decane isomers in n-pentane and 3-methylpentane (with CO 2 added as electron acceptor) and in 1,1,2-trichlorotrifluoroethane at 77 K. Absorption bands, with band maxima ranging from 595 to 760 nm were observed in all cases, except after irradiation of 2,6-, 3,5- and 3,6-dimethyloctane and 3,4-diethylhexane in pentane and after irradiation of n-decane and 3,4-diethylhexane in 3-methylpentane. No absorption in the same spectral region was observed after irradiation of pentane with CO 2 or 2-chlorobutane added as electron acceptor. The results specifically rule out the presence of trapped condensation ions, formed by “direct dimerization” (RH + + RH → R + 2 + H 2) or pentane cations with pentane molecules, after irradiation of pentane at 77 K. Arguments against direct “ionic dimerization” in the radiolysis of alkanes are presented and discussed.

The fate of primary cations in radiolysis of alkanes as studied by ESR

The structures and reactions of alkane cations (RH +) have been studied by ESR to elucidate the fate of primary cations in radiolysis of alkanes. Radical cations of prototype alkanes such as C 2H 6, C 3H 8, iso-C 4H 10 and neo-C 5H 12 etc. as well as their partially deuterated analogues were stabilized in irradiated frozen matrices such as SF 6, CFCl 2CF 2Cl and CFCl 3 having a higher ionization potential than that of these alkanes contained as dilute solutes. RH + in SF 6 and in CFCl 2CF 2Cl converts into alkyl radicals by deprotonation probably through bimolecular reactions, whereas RH + in CFCl 3 unimolecularily decomposes into olefinic cations by H 2 and/or CH 4 elimination reactions. It is further found that the electronic structures of propane and isobutane cations in halocarbon matrices are different from those in SF 6 and the difference is drastically reflected in the site preference of their deprotonation reactions. The results are discussed in relation to the mechanisms of pairwise formation of alkyl radicals in low temperature radiolysis of neat alkanes and its suppression by addition of electron scavengers.

Radiolysis of alkanes and olefines in xenon matrices at 4.2 K as studied by ESR: Formation and trapping of hydrogen atoms and their subsequent reactions at cryogenic temperatures

Radiolysis of CH 4, CH 3CH 3, CH 3CD 3, C 3H 8, iso-C 4H 10, and CH 2CH 2 in Xe matrices at 4.2 K gave trapped H(D) atoms with a comparable yield of radicals formed from scission of a C-H(D) bond through energy transfer from Xe to the hydrocarbon additives. H tr and >.;CH 2CD 3 are preferentially formed from CH 3CD 3 with the H tr/D tr ratio of 2.1 and the >.;CH 2CD 3/CH 3>.;CD 2 ratio of 2.3 H tr and CH 2>.;CH are formed from CH 2CH 2 by a rapture of olefinic C-H bond. Upon annealing at around 45 K, H tr is detrapped and mobilized to react with C 2H 6 and C 2H 4 giving >.;C 2H 5 by H abstraction and addition, respectively. Preferential abstraction of H from CH 3CD 3 took place at around 45 K showing a large isotope effect with k H / k D > 9. The reactions of thermal H atoms produced by radiolysis of CH 4 with CO to form H>.;CO have also been observed at around 45 and 18 K in Xe and CH 4 matrices, respectively. These results suggest that methane may not be an exceptional compound to produce hydrogen atoms in the radiolysis of neat alkane at low temperature. Absence of trapped H atoms in irradiated neat alkanes except CH 4 may imply that all the H atoms produced react with alkanes except CH 4. The reactivity of thermal H atoms at cryogenic temperature suggests that not only hot but also thermal H atom reactions are involved in the solid phase radiolysis below 77 K.

ESR studies of irradiated methane and ethane at 4.2 K and mechanism of pairwise trapping of radicals in irradiated alkanes

The spatial distributions of trapped hydrogen atoms and methyl radicals in CH4 and CD4 irradiated at 4.2 K have been studied by ESR. Most of the hydrogen atoms are trapped within ∼16 Å from each parent methyl radical forming distributed radical pairs. This indicates that the thermalization distance of hydrogen atoms produced by 4.2 K radiolysis is ≲16 Å in methane. Evidence has been obtained for the formation of a small amount of ĊH3...ĊH3 pairs, which are supposedly formed from hot hydrogen atom reactions. No evidence has been obtained for the formation of a pair of hydrogen atoms. There has been also obtained the experimental evidence that thermal hydrogen atoms unreactive with methane can abstract a hydrogen atom from ethane at 10–20 K. In the light of these results, the following conclusions are deduced: A part of hydrogen atoms produced in radiolysis of alkanes undergo hot abstraction to form very close alkyl radical pairs whereas the rest is thermalized and immobilized at the place close to each parent alkyl radical. The thermalized hydrogen atoms also abstract a hydrogen atom from the surrounding molecules to form distributed alkyl radical pairs, when the matrix molecule is reactive with thermal hydrogen atoms. On the other hand, the hydrogen atoms are mobile at elevated temperatures and the isolated alkyl radicals are mainly formed.

Selective Hydrogen Atom Abstraction by Hydrogen Atoms in Photolysis and Radiolysis of Alkane Mixtures at 77 K

Abstract A selective hydrogen atom abstraction reaction by H atoms, which are produced at 77K by radiolysis of alkane or photolysis of hydrogen halides, has been found in isobutane, 2,2,3,3-tetramethylbutane (TMB), and cyclopropane matrices as well as in the neopentane matrix. The selective hydrogen atom abstraction reaction is caused by H atoms which have initial kinetic energies in the range from 15 to 67 kcal/mol. The reaction is caused also by D atoms. The competitive reaction between c-C6H12 and HI for H atoms has been studied in the radiolysis and photolysis of neo-C5H12–c-C6H12–HI mixture at 77K. The rate constants of these reactions in the neopentane matrix are quite different from those of a thermal H atom reaction, but similar to those of a hot H atom reaction. The importance of the selective hydrogen atom abstraction reaction by H atoms is pointed out in the radical formation in the radiolysis of pure TMB at 77K.

Excitation transfer in the radiolysis of alkanes in the solid phase at 77 K

The role of excited molecules and excitation transfer in the radiolysis of solid alkanes at 77 K is described. Studies of the effect of electron scavengers in the radiolysis of solid alkanes show that hydrogen formation is mainly due to a nonionic process. Though the yield of hydrogen in the radiolysis of solid isobutane at 77 K is not affected by the addition of N 2O or SF 6, it decreases sharply upon the addition of CCl 4. Similarly, the yield of C 4H 9 radical in the radiolysis of isobutane at 77 K is not changed by the presence of conventional electron scavengers; however, it decreases remarkably upon the addition of CCl 4 or toluene. When 2,3-dimethylbutane (23DMB) containing a small amount of toluene (To) is irradiated with γ-rays or nanosecond pulses of X-rays in the solid phase at 77 K, luminescence from excited toluene is observed. The emission spectrum from 23DMB-To (2 mol%)-N 2O(2 mol%) during γ-irradiation at 77 K is similar to that from 23DMB-To (2 mol%) during U.V.-illumination. The formation of the singlet-excited toluene is not affected by the addition of electron scavengers and hole scavengers. These results are explained by excitation transfer from alkane to additives. The possibility of exciton migration in the radiolysis of solid alkanes is discussed by a simple theoretical treatment.

Formation of excited singlet toluene in the radiolysis of alkane containing toluene in the solid phase at 77 K

When alkane containing a small amount of toluene is irradiated by γ-ray or X-ray in the solid state at 77 K, luminescence from excited toluene is observed. The formation of the excited toluene is not due to direct excitation of toluene by γ-ray or Čerenkov light, but to the rapid energy transfer from the irradiated alkane to toluene. The intensity of luminescence from the excited toluene in the cyclopentane matrix does not correspond at all to the amounts of toluene ions, which are formed by pre-irradiation. The energy transfer is not caused by a free hole and an electron, but by an exciton. Furthermore, the effect of additives on the fluorescence suggests that the Wannier exciton or mobile ion-pair is responsible for the energy transfer in cyclopentane containing toluene in the solid phase at 77 K.

Yields of excited singlet state in the radiolysis of alkane containing toluene in the solid phase at 77 K

When alkane containing a small amount of toluene is γ-irradiated in the solid phase at 77 K, excited toluene is formed by energy transfer from irradiated alkane to toluene. Fluorescence from excited singlet toluene has been measured during γ-irradiation and after nanosecond pulse irradiation. Yields of the excited singlet toluene (S) have been determined for the first time in the solid phase at 77 K and the following G-values were measured: G(S) = 0·8−0·9, 0·7, and 0·7 in the radiolysis of cyclohexane, 2,3-dimethylutane and 3-methylpentane containing toluene (0·2 mol dm −3), respectively. The rate constants for energy migration, obtained experimentally from the kinetics, are of the same order of magnitude as those calculated by the theory of exciton migration.

Effect of Matrix on the Formation of Solvent Radicals in the Radiolysis of Alkanes in the Solid State at 77 K

Abstract The effect of the matrix on the radiolysis of 2,3-dimethylbutane, neopentane. 2,2-dimethylbutane, and isobutane in the solid state has been studied at 77 K by means of ESR spectroscopy. Though only the ·CH2CH-(CH3)CH(CH3)2 radical is formed by the γ-irradiation of 2,3-dimethylbutane in the I crystal, the CH3\overset·C(CH3)-CH(CH3)2 radical is formed by the γ-irradiation of 2,3-dimethylbutane in the II crystal. Though the t-C4H9 and neo-C5H11 radicals are formed by the γ-irradiation of pure neopentane, the formation of radicals is affected appreciably by the addition of a small amount of alkanes. When neopentane containing ethane, etc. is γ-irradiated, the solute radical is mainly formed by energy transfer form the γ-irradiated neopentane to the solute. Most of the solutes have a spherical molecular structure like that of neopentane or are smaller than neopentane. When neopentane containing 3-methylpentane, etc. is γ-irradiated, only the neopentyl radical is formed. The molecular structures of the solutes are very different from that of neopentane. The formation of the solvent radical in the radiolysis of 2,2-dimethylbutane is affected by the addition of 3-methylpentane or methylcyclphexane. The formation of the butyl radical in the radiolysis of isobutane is also changed drastically by addition of alkanes. It is concluded that the formation of solvent radicals in the radiolysis of solid alkane is appreciably affected by the conditions of the matrix.

The simple exponential distribution for initial electron-positive ion separation distances as seen in the γ radiolysis of alkanes

A numerical solution of the Smoluchowski equation for diffusion, reaction, and neutralization of ion pairs in a dielectric medium in the presence of a scavenger has been applied for analysis of recent electron scavenging experiments in alkanes. It is shown that these experiments can be quantitatively understood in terms of a simple exponential distribution of initial electron-positive ion separation distances. The credibility of this distribution is considerably enhanced when it is employed, in conjunction with Onsager's solution for diffusion of an ion pair in the presence of an external field, to calculate ion yields as a function of field strength. The agreement with experiment is once again excellent. The attenuation parameter which characterizes our initial exponential distribution is related to the liquid structure factor and gas phase molecular scattering cross sections through the multiple scattering theory of Lax. The wave nature of electron motion is explicitly considered in a discussion of the physical meaning of ionization in liquids.

The Migration of the Exciton in the Radiolysis of Alkanes

The Migration of the Exciton in the Radiolysis of Alkanes

Charge Separation and Similarities of Ionic Processes in the Radiolysis of Alkanes and of Water

The charge-pair separation distribution function, d lnn / d lnR = − 2.7 is consistent with results from the rate of correlated ion-pair recombination, as well as the temperature and field dependence of the electrical conductivity of n-hexane under x irradiation for data reported by Allen et al. The T dependence derives from (1) and Rescape = e2 / εkT. The combined T and E dependence fits the description Rescape = e2 / ε(kT + gλ′Ee) where g is a geometric factor and gλ′5 × 10−7 cm (which implies λ′∼102 Å for energy gained from the field), while (1) and (2) together account for the E dependence. The integral distribution function is P(R) = (R / Rm)−2.7 for separation ≥R, with P = 1 at Rm≅100 Å. It is shown that many electron reactions in organic liquids and glasses fit a common concentration dependence, which is predictable from (3). It is also shown that in the region of ∼ 10−4 − 10−1 M solute in water, the yield of chemical reduction obeys the same concentration dependence as liquid alkanes. This effect is attributed to a population of electrons which are not trapped and do not become hydrated. Consequently their electrical behavior depends upon the high-frequency dielectric constant, accounting for similar behavior in cyclohexane and in water.

Role of O− in the Gas Phase Radiolysis of Alkane–Nitrous Oxide Mixtures

WHEN an alkane gas is irradiated in the presence of nitrous oxide, nitrogen is formed. This has been shown to result from the dissociative capture of electrons by nitrous oxide1 At constant dose rate the yield of nitrogen, G(N2), obeys the expression expected for competition between electron capture and ion recombination. G(N2)∞ is a constant, for a given hydrocarbon, and represents the yield of nitrogen when all electrons are captured. The fact that G(N2)∞ is greater than the yield of electrons, Ge , calculated from the W value3, has been ascribed to the subsequent reaction of O− with nitrous oxide1,2. Evidence has been found, in the mass spectrometer, for reactions (2) and (3) (ref. 4). The relative rates of these reactions are not, however, reported. On the basis of the ethylene dosimeter* (G(C2H4→H2) = 1.313), (G(N2)∞ for the C2, C3 and C4 alkanes is found to be 1.55 Ge (to be published). This can be explained by assuming a rate constant ratio, k2/k3, = 1.22.

Radiation chemistry of hydrocarbons

Fundamental aspects of production of free radicals from organic molecules by ionizing radiations and the potential reactivity of such excited states for large polyatomic molecules are discussed. An outline is given of the quantitative theory of molecule-ion reactions, with particular emphasis on the rates and energetics of the unimolecular and bimolecular processes that lead to the formation of molecules, uncharged free-radicals, and secondary ions. The historical development of radiation chemistry is also considered as well as the shifting interests and interpretations that have marked the growth of this subject. The concept of the G value is discussed and a consideration of theories developed to explain primary radiation reactions leads to the conclusion that reaction mechanisms based on neutral free radicals alone can often provide a superficial explanation of the gross results without revealing the more subtle and hidden factors that operate. Present knowledge of elementary reactions is discussed with special consideration of the formation and dissociation of polyatomic ions, ionmolecule reactions, neutralization of ions, and electronic excitation processes. Basic criticisms are directed at the Eyring quasi- equilibrium unimolecular rate theory: first, that the assumption as to rapid energy randomization may not apply strictly to molecules with less than about 6more » atoms, for then the electronic levels will tend to be more discrete; secondly, that energy exchange between vibrational modes of different symmetry properties may be forbidden. For quantitative discussion on the applicability of the theory, the simple classical form of the unimolecular rate constant is used. Experimental data and interpretations relating to radiation reactions in gas, liquid, and polymeric phases are reviewed. From a consideration of data from radiolysis of alkanes, it was concluded that consideration of the possible dissociation paths of the alkane parent ion, and of the ion-molecule reactions of fragment ions, serves as a useful guide in accounting for product distribution. Postulated mechanisms involved in crosslinking and chain-breaking reactions in irradiated polymers are reviewed and evaluated. (TCO)« less

Detection and Identification of Free Radicals in the Radiolysis of Alkanes and Alkyl Iodides

Detection and Identification of Free Radicals in the Radiolysis of Alkanes and Alkyl Iodides