Gas-Phase Radiolysis of n-Pentane. A Study of the Decompositions of the Parent Ion and Neutral Excited Pentane Molecule

Abstract

The direct and inert‐gas‐sensitized radiolyses of equimolar n‐C₅H₁₂–C₅D₁₂–NO mixtures have been investigated in the absence and presence of an applied electrical field. It is shown that the partially deuterium‐labeled products remain essentially unchanged upon the application of an electrical field, while the yields of the fully deuterated and nondeuterated species increase indicating that the former entirely originate from ionic processes. Comparison of the ion‐pair yields of these partially labeled products, which can be ascribed mainly to hydride ion transfer from pentane to carbonium ions, with the ion‐pair yields deduced from the 70‐eV mass spectral pattern shows that, in the direct radiolysis, those fragment ions which are formed with a high activation energy are produced in a lower yield than in the mass spectrometer and are also reduced most strongly by an increase in pressure. In all experiments, both the propyl and butyl ions formed in the decomposition of the pentane ion acquire the secondary configuration either prior to or during reaction. In the inert‐gas‐sensitized radiolysis, the fragment ions which are formed with a high appearance potential are seen to increase with an increase in the recombination energy of the added inert gas. From the applied‐field experiments, it was possible to determine the modes of decomposition of neutral excited pentane molecules formed by electron impact. These were found to be analogous to those deduced from vacuum‐ultraviolet photolysis experiments carried out at 1470 or 1237 Å. On the basis of the information gained from these experiments, and a detailed isotopic analysis of the products formed in the radiolysis both in the presence and absence of benzene, a value of 0.3±0.05 could be deduced for the number of neutral excited molecule decompositions per ion pair. Neutral excited pentane molecules are also seen to be formed in the inert‐gas‐sensitized radiolysis.