A PEPICO study of five energy-selected C 6H 12+. isomeric molecular ions. Part 1

Abstract

The breakdown diagrams of five isomeric C 6H 12 ions were obtained by energy-selected molecular ions with photoelectron photoion coincidence (PEPICO) spectroscopy. The breakdown diagrams of cyclohexane, 1-hexene and 3-hexene are quite similar showing that the lower dissociation paths are the losses of C 2H 4 and CH 3 leading to the C 4H 8 +. and C 5H 9 + fragment ions, respectively. The three molecular ions were found to dissociate slowly near the threshold, the time of flight distributions of the C 4H 8 +. ion having been analysed in terms of two exponential decay rates while the time of flight distributions of the C 5H 9 + ion could be fitted considering a single component. The breakdown diagrams of the 2-methyl-2-pentene and 2,3-dimethyl-1-butene molecular ions are identical showing as dominant dissociation channel the elimination of CH 3 . to form the C 5H 9 + fragment ion. The loss of CH 3 . was found to be a fast reaction and accurate dissociation onsets were determined from the crossover energy in the breakdown diagrams. The results indicate that the cyclohexane and the two open chain isomers follow a distinct and more complex dissociation mechanism than the two branched open chain isomers.