Chemistry of Positive Ions. III. The Radiation Chemistry of Solid n-Hexane at 77°K

The mechanism of the chemical effects of ionizing radiation on solid straight-chain saturated aliphatic hydrocarbons such as polyethylene was studied by use of n-hexane. This hydrocarbon was short enough to allow analysis of the cross-link C12 products and of the hexenes formed. It was thought to be long enough to serve as a legitimate substitute for polyethylene. In any case, the yields of H2, olefines, and cross linkages were very similar in the two systems and we believe the mechanistic implications found here for solid n-hexane at 77°K should apply even to polyethylene. The results indicate: (a) the active species is the original ion radical produced by the ionizing radiation (Co60 gammas); (b) the ion radical reacts before relaxing to redistribute the positive charge or to isomerize in any way, and the products show that there are three primary ions formed corresponding to the first, second, and third carbon atoms in the chain; (c) the ion radical reacts with nearest neighboring groups to bond and release H2, thus forming olefines by reaction with neighboring C atoms in the same molecule four times out of five, but reacting with neighboring chains to form C12 dimers one time out of five; the dimers being only the five hydrocarbons possible by close-neighbor interaction: the 1,1 interaction being n-dodecane; the 1,2 being 5-methylundecane; the 2,2 being 5,6-dimethyldecane; the 2,3 being 4-ethyl-5-methylnonane; the 3,3 being 4,5-diethyloctane and no others; (d) the principal mechanism of the fast interaction of the primary ion is the direct elimination of H2 and formation of a bond due to charge exchange attraction between the primary ion and the neutral group with which it interacts; (e) the second most important mechanism is the abstraction of H to form the electronically saturated analog of CH5+ which when neutralized eliminates H2 and leaves a free radical to combine with the neighboring free radical left by the H abstraction; (f) free radicals seem to play a minor role in the solid hydrocarbon, n-hexane.