Dissociation of protonated 2,2,6-trimethylcyclohexanone and related compounds upon high and low energy collisional activation

Abstract

Both energetic and entropic barriers are important factors in controlling the accessible dissociation channels of gas phase ions upon collisional activation. The accessible fragmentation pathways are dependent on the kinetic energy of the selected precursor ion and the number of inelastic collisions that it undergoes with the neutral target. A major fragmentation process of protonated 2,2,6-trimethylcyclohexanone after collisional activation under certain reaction conditions results in the protonated acetone ion. The product ion structure is demonstrated using three methods: (1) deuterium labelling, (2) sequential fragmentation experiments in a hybrid mass spectrometer in which a comparison is made between the consecutive daughter spectrum for protonated 2,2,6-trimethylcyclohexanone and the daughter spectrum of protonated acetone, and (3) by comparison of daughter spectra and breakdown curves for model ions of m/z 59 generated from different precursors in an ion trap mass spectrometer. Furthermore, the dissociation channels for protonated 2,2,6-trimethylcyclohexanone and its daughter ions are characterized using the multiple activation (MS n ) capabilities of an ion trap mass spectrometer. Additionally, the product ions from high energy collisional activation of the title ion are examined as a function of scattering angle in a hybrid mass spectrometer. The reaction in which protonated acetone is formed is of particular interest because it involves complex structural reorganization via two 1,2-methyl transfers and a ring contraction. A study of this complex rearrangement/fragmentation sequence with respect to the effects of collision energy (electrovolt range) and number of collisions reveals that it is favored only at high energies and pressures. Large enthalpic and entropic barriers are indicated, and such processes have seldom been studied in any detail.