Methyl Migration in the Metastably Decomposing Acetylphenyl Ions, CH_3COC_6H_4^+ (m/z 119), Followed by the Loss of CO

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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, Mw : 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.
2005-04-01