A PEPICO STUDY OF 5 ENERGY-SELECTED C6H12-DOT ISOMERIC MOLECULAR-IONS- PART 1(CENTER)

The breakdown diagrams of five isomeric C6H12 ions were obtained by en ergy-selected molecular ions with photoelectron photoion coincidence ( PEPICO) spectroscopy. The breakdown diagrams of cyclohexane, I-hexene and 3-hexene are quite similar showing that the lower dissociation pat hs are the losses of C2H4 and CH3. leading to the C4H8+. and C5H9+ fra gment ions, respectively. The three molecular ions were found to disso ciate slowly near the threshold, the time of flight distributions of t he C4H8+. ion having been analysed in terms of two exponential decay r ates while the time of flight distributions of the C5H9+ 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 identic al showing as dominant dissociation channel the elimination of CH3. to form the C5H9+ fragment ion. The loss of CH3. was found to be a fast reaction and accurate dissociation onsets were determined from the cro ssover 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 isome rs. (C) 1997 Elsevier Science B.V.