Study on the Fragmentation Mechanism of 1-Substituted Cyclohexane-3,5-diones under Electron Ionization

Abstract

EI mass spectral fragmentation of 1-X-cyclohexane-3,5-diones (X=CH₂, O, S) has been investigated by using MO calculation. It has been revealed that the first bond cleavage occurs at either 2-3 or 5-6 bond corresponding to α-cleavage with respect to carbonyl group and 1-X, followed by two competitive bond cleavage giving [M-CO]^+· or [M-CH₂X]^+·, respectively.
The potential energies of CO or CH₂X elimination from the acyclic molecular ion have been determined by MNDO. In 1-oxa compounds, the CO elimination process is energetically more favorable than the CH₂X elimination process, while the CH₂X elimination process is more favorable in 1-thia compounds.
This result is in good agreement with the relative intensities of fragment ions [M-CO]^+· and [M-CH₂X]^+·.
In addition to the above description, the effect of 1-X on ring-bond cleavage has been discussed.