Alkane radiolysis

From the oxygen-containing compounds much attention was paid to the radiolysis of alcohols. Their relative permittivities are between those of the hydrocarbons ( 2) and water ( 80). The behavior of alcohols in radiation chemistry is in many respects similar to that of water. The electrons ejected from alcohol molecules can be trapped in solid alcohol matrices and can be observed by optical absorption techniques. In Uquids, pulse radiolysis measurements confirmed the presence of solvated electrons, for instance in methanol = 630 nm, Smax = 1,700 m moP, G(esoiv ) 0.5 pmol in the picosecond time range. On the other hand, as main product of decomposition, H2 gas forms with a yield comparable to the yield observed in liquid -alkane radiolysis (G(H2) 0.5 pmol J ) (Freeman 1970, 1974 Spinks and Woods 1990). 

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). 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

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. 

The quantum yields and G values of H2 elimination for a larger group of alkanes are eolleeted in Table 4 the values were mostly determined by using Eq. (10). Because in the photolysis and radiolysis of the -alkanes shown in the table various kinds of radicals are produced simultaneously (e.g., -propyl and ec-propyl from propane), the weighted averages of several k /kc values were used in Eq. (10). The ratios can be determined by suppressing the radiolytic alkene and dimer yields in the presence of radical scavenger (e.g., I2) ... 

FORMATION AND YIELD OF ALKANE EXCITED STATES IN RADIOLYSIS... 

Some of the cations may fragment or undergo ion-molecule reactions before neutralization. As shown in the older literature, the fragmentation has higher yield in the radiolysis of the smaller and/or highly branched alkanes such as neopentane or isooctane. Ion-molecule reactions, such as the H transfer, may also reduce the Si yield ... 

Figure 8 Connection between primary C-H and C-C bond ruptures during radiolysis and photolysis. Alkanes (1) propane, (2) w-butane, (3) -pentane, (4) -hexane, (5) w-heptane, (6) n-octane, (7) w-decane, (8) isobutane, (9) neopentane, (10) 3-methylpentane, (11) 2,2-dimethylbutane, (12) isooctane, (13) cyclopentane, (14) cyclohexane, (15) cycloheptane, (16) cyclooctane, (17) cyclodecane, (18) methylcyclopentane, (19) methylcyclohexane, (20) ethylcyclohexane, (21) 1,1-dimethylcyclohexane, (22) cis-l,2-dimethylcyclohexane, (23) fraw5-l,2-dimethylcyclohexane, (24) cis-1,3-dimethylcyclohexane, (25) trarw-l,3-dimethylcyclohexane, (26) cw-l,4-dimethylcyclohexane, (27) trawi-l,4-dimethylcyclohexane. (From Refs. 18, 29, 91, 92, 99, 100, 108, 110, 111, 113, 114, and 160.)...

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