Fragmentation and isomerization of [(CH3)2CHCHCH3]+ ions in gas-phase radiolysis

Abstract

A study has been made of the influence of different additives on the radiolysis of gaseous mixtures of hydrogen, 3-methyl-but-1-ene and oxygen. In these systems the [(CH₃)₂CHCHCH₃]⁺ ion is formed by proton transfer from the XH⁺ ion to the olefin. For X = H₂ the secondary pentyl ion is formed with sufficient internal energy to decompose, producing mainly ethylene. When X is CO₂, CH₄, N₂O or CO the secondary pentyl ion partially isomerizes into the tertiary pentyl ion. This isomerization has been investigated in the mixture of xenon, methane, 3-methyl-but-1-ene and nitric oxide between –10 and + 110°C. If it is assumed that the rate of isomerization (internal hydride-ion displacement) is a function only of the temperature, the measured activation energy of the process [(CH₃)₂CHCHCH₃]⁺→[(CH₃)₂CCH₂CH₃]⁺ is E_a= 8.8 ± 0.4 kJ mol^–1(2.1 ± 0.1 kcal mol^–1 or 0.092 ± 0.005 eV).