Studies of ionic keto/enol equilibria utilizing tandem mass spectrometry

This research focussed upon the keto/enol tautomerization of the radical cation of 3-methyl-2-butanone (3-MB, molecular ion = m/z 86). Loss of a methyl group from opposite ends of the molecular ion yields isomeric ions at m/z 71, an acylium ion and protonated methyl vinyl ketone (MVK). Utilizing tandem mass spectrometry (MS/MS), the fragmentation characteristics of these m/z 71 ions produced from 3-MB (after introduction of neutral keto form into the mass spectrometer) was determined. Performing separate MS/MS experiments on the acylium and MVK reference ions yielded quantitative information about the m/z 71 ions produced from 3-MB itself. Furthermore, MS/MS experiments were performed on m/z 86 of 3-MB (produced from the neutral keto form) to determine the keto/enol composition of the molecular ion. By utilizing the resulting quantitative information obtained for m/z 71 and m/z 86, it was possible to partially deconvolute the mass spectrum expected for the enol ion. The mass spectrum thus obtained agreed well with the mass spectrum of the neutral enol form reported in the literature. A structurally similar compound, 2-pentanone, was also studied. These results represent an application of the linear superposition of MS/MS spectra. For this research, a novel, custom-built hybrid mass spectrometer of QEB geometry and an Ion Trap Mass Spectrometer (ITMS) were utilized. The design and operation of the QEB are discussed, and differences in MS/MS data between the QEB and ITMS are addressed. The acylium ion (m/z 71) rearranged unexpectedly on the ITMS due to the longer time-frame for ITMS MS/MS experiments, thus complicating data interpretation. However, the keto/enol composition of the m/z 86 ions was unaffected.