Methyl Migration in the Metastably Decomposing Acetylphenyl Ions, CH₃COC₆H₄⁺ (m/z 119), Followed by the Loss of CO

Abstract

The metastable ion dissociations of o-, m-, and p-acetylphenyl ions, CH₃COC₆H₄⁺ (m/z 119), generated upon electron ionization from o-, m-, and p-diacetylbenzenes (CH₃COC₆H₄COCH₃, M_w : 162, 1-3), have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry in conjunction with thermochemistry. All these m/z 119 ions, which are generated by the consecutive losses of CH₃ and CO from 1^+·-3^+·, dissociate to only the ion at m/z 91 by the loss of CO. These suggest that in the CH₃COC₆H₄⁺ ions, the CH₃ migration from the acetyl group to the benzene ring occurs prior to the loss of CO. The amounts of the kinetic energy release (KER) accompanying with the dissociation of the m/z 119 ions from compounds 1-3 to m/z 91 ion were larger than those of the corresponding, respective methylbenzoyl ions. In the m-, and p-CH₃COC₆H₄⁺ ions, the charge migrates from the incipient charge site to the ortho position via the so-called hydrogen “ring-walk.” Then CH₃ migrates from the acetyl group to the charge site at the ortho position in the benzene ring prior to the loss of CO. A part of the m/z 91 ions from o-, m-, and p-acetylphenyl ions may be generated via the migration of CH₃ to the incipient charge site, followed by the loss of CO.