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 Ea= 8.8 ± 0.4 kJ mol–1(2.1 ± 0.1 kcal mol–1 or 0.092 ± 0.005 eV).