Radiolysis of liquid alkanes

During the irradiation of liquids, both ionization and excitation occur and its distribution is strongly affected by the LET value of radiation employed. In liquid alkanes, geminate ion recombination reaction takes place in the time range of one to ten ps [76], leading to the formation of the excited states. The excited states of alkanes have lifetimes of around Ins and decay to give mainly H2 and alkene products [77]. In ion beam radiolysis of liquid alkanes, at ns after... 

The summarized yields of primary C-H and C-C decompositions are nearly constant in the radiolysis of liquid alkanes with a yield of 0.67 0.06 pmol Primary decomposition yields do not contain the yields of molecules decomposed in secondary reactions like H atom abstraction (see reaction (O 23.80)) (GyOrgy 1981 Wojnarovits 1981). 

Katsumura, Y., Yoshida, Y., Tagawa, S., Tabata, Y. 1983. Study on the excited state of liquid alkanes and energy transfer process by means of picosecond pulse radiolysis. Radiat. Phys. Chem. 21(1-2) 103-111. 

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

The Reaction of Nitrous Oxide with Excited Molecules in the Radiolysis and Photolysis of Liquid Alkanes... 

In the radiolysis of liquid n-alkanes both the yields of hydrogen (G 0.5 pmol J ) and C-C bond decompositions ( 0.17 xmol J ) remain essentially constant when the carbon atom number increases from C4 to C17. It should be noted that neither the C-H nor the C-C breaks occur randomly. The frequency of C-H decompositions at the secondary carbon atoms is 3 times more frequent than at the primary carbons. The fragmentation of the carbon skeleton occurs preferentially in its central region, elimination of the methyl group is relatively infrequent (Holroyd 1968 Gyorgy 1981 Wojnarovits and Schuler 2000). 

In the radiolysis of liquid branched alkanes, the C-H decomposition yields are smaller than in the radiolysis of n-alkanes, and this decrease is compensated by the increase in the yields of C-C breaking products. Radical scavenging experiments revealed that in branched alkanes a tertiary C-H bond decomposes 14 times more frequently than a primary C-H bond. The decomposition of a secondary C-H bond is 3 times more frequent than that of a primary C-H bond (Foldiak et al. 1976 Hummel 1992 Wojnarovits and Schuler 2000 Schuler and Wojnarovits 2003). C-C bonds attached to tertiary or quaternary carbons also have high decomposition frequencies. In 2-methylpentane (O Fig. 23.5) the C-C bond between carbon atoms 2 and 3 decomposes 3 times more frequently than the bond between 3 and 4, and 6 times more frequently than the bond between 4 and 5. 

Hobroyd RA (1968) The reaction of nitrous oxide with excited molecules in the radiolysis and photolysis of liquid alkanes. In Gould RF (ed) Radiation chemistry II. Advances in Chemistry Series. American Chemical Society, Washington... 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Early pulse radiolysis studies of alkanes at room temperature showed that the solvated electron absorption begins around 1 pm and increases with increasing wavelength to 1.6 pm for -hexane, cyclohexane, and 2-methylbutane [77]. More complete spectra for three liquid alkanes are shown in Fig. 4. The spectrum for methylcyclohexane at 295 K extends to 4 pm and shows a peak at 3.25 pm [78]. At the maximum, the extinction coefScient is 2.8 x 10 cm The spectrum for 3-methyloctane at 127 K, shown in Fig. 4, peaks around 2 pm. The peak for methylcyclohexane is also at 2 pm at lower temperature. Recently, the absorption spectra of solvated electrons in 2-methylpentane, 3-methylpentane, cA-decalin, and methylcyclohexane glasses have been measured accurately at 77 K [80]. For these alkanes, the maxima occur at 1.8 pm, where the extinction coefScient is 2.7 x 10 cm. ... 

Most publications dealing with the photodecomposition of alkanes discuss the processes in the gas phase several comprehensive works have already been published in this field [14-17]. In the present work, we summarize the results of liquid phase photolytic studies and compare them with those obtained in radiolysis. An early review on liquid alkane photochemistry was published in Ref. 18, a brief overview of the field was given in Ref. 19. 

Busi, F. Labile Species and Fast Processes in Liquid Alkanes. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, Baxendale, J.F. Busi, F., Eds. Reidel Dordrecht, 1982 417 pp. 

It is now agreed, that for the most part, the radiolysis of polar liquids such as alcohol, water, etc. leads to the formation of ions and subsequently radicals rather than excited states. However, the picture is changed as the polarity of the liquid decreases, and in the extreme case of arenes such as benzene only excited states both singlet and triplet are observed. In other liquids such as alkanes, dioxane, etc. both ions and excited states are observed, charge neutralisation of the ions also giving excited states. 

Table I gives data for the yields of excited states and ions observed in the radiolysis of various liquid systems. The yield is stated in terms of the G value or number of molecules of product per 100 eV of energy absorbed by the system. An immediate generalization is possible The radiolysis of nonpolar liquids, arenes, alkanes, etc. produces excited states and sometimes ions. Increasing the polarity of the liquid, e.g., benzene to benzonitrile, benzyl alcohol, phenol, leads to a decrease in the yield of excited states with a concomitant rise in the observed yield of ions. In very polar liquids such as water and alcohols only ions are observed.

Radiolysis of alkane liquids fits the classical picture of radiation chemistry, namely, large yields of ions are initially produced which on recombination give rise to excited states and other products. Much radiation chemistry of alkanes is also interpreted in terms of free radicals. The exact connection between the three reactive regimes of excited states, ions, and free radicals is not always clearly established. 

Radiolysis of low temperature alkane liquid also gives rise to trapped radical ions (23). Thermal annealing of the irradiated samples gives rise to luminescence characteristic of excited states of the solvent and, or, solutes present (23,24). These data conform exactly to those obtained in pulse radiolysis studies. 

Yang J, Kondoh T, Norizawa K, Nagaishi R, Tagushi M, Takahashi K, Rat oh R, Anishchik SV, Yoshida Y, Tagawa S. (2008) Picosecond pulse radiolysis Dynamics of solvated electrons in ionic liquid and geminate ion recombination in liquid alkanes. Radiat Phys Chem 77 1233-1238. 

In this chapter, we examine radiolysis of neat organic liquids. The better studied and most common organic solvents are saturated hydrocarbons and alcohols. By virtue of having low dielectric constant and only C-C and C-H bonds, hydrocarbons represent an ideal medium to examine the fundamental mechanisms of radiolysis in non-polar media. For lack of space, the discussion wUl be limited to paraffins, branched alkanes, and cycloalkanes. 

Early pulse radiolysis studies of alkanes at room temperature showed that the solvated electron absorption begins around 1 pm and increases with increasing wavelength to 1.6 pm for n-hexane, cyclohexane, and 2-methylbutane [77]. More complete spectra for three liquid alkanes are shown in Fig. 4. The spectrum for methylcyclohexane at 295 K extends to 4 pm and shows a peak at 3.25 pm [78]. At the maximum, the extinction... 

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