Mass spectrometric fragmentation of the tautomers of 1,3‐diketones. A gas chromatographic/mass spectrometric study

Abstract

The diketo and ketoenol tautomers of aliphatic 1,3-diketones can be easily separated by gas chromatography. The mass spectra of tautomers of pentane-2P-diones substituted at C(1) and C(3), separated in this way, have been obtained. The fragmentation mechanisms are discussed. The mass spectra of the tautomers are quite different, and the main fragmentation pathways can be easily linked to the structures of the (non-interconverting!) tautomeric molecular ions. Furthermore, isomers differing by the position of the substituent can also be identified by their mass spectra.

Application of mass spectrometry for the analysis of organic tautomeric compounds

“…The results reported in (Masur et al, 1987) refuted the much-discussed hypothesis (Maquestiau & Flammang, 1982;Kingston, Bursey, & Bursey, 1974;McAdoo, McLafferty, & Parks, 1972;Vajda et al, 1981) that Mf' ions of keto and en01 tautomers may interconvert to a considerable extent in the gas phase. Moreover, ICR ex-* According to their data, tautomeric M" ions differ in EF by 67. periments (Hass et al, 1972) showed that isomerization of M+' ions of the enol form of cyclopentanone does not occur at ion lifetimes of 5 4 -1 0 -~ -1 -s. It was only observed at ion lifetime of 5 -For ions possessing a high internal energy, the possibility of keto/enol tautomerization cannot be totally excluded.…”

“…The above conclusions, which were largely intuitive at first, were later substantiated by a brilliant GC-MS study of 1-and 3-substituted acetylacetones (Masur et al, 1987). Mass spectra of individual keto and enol tautomers were obtained.…”

“…The enolic M+' ions fragmented by the "allylic" mechanism to form F5 and F6 ions. The results reported in Masur et al (1987) indicated that keto structures 1 are more stable as neutral species and less stable as radical ions when compared to the enol structures 2. Importantly, this conclusion was confirmed by independent quantum chemical calculations (Bogolyubov et al, 1975;Bouchoux, 1988;Turecek & Cramer, 1993, as well as the data on thermochemistry, IEs, AEs, and enthalpies of formation of these structures (Holmes & Lossing, 1980;Siretskaya, 1985;Takhistov, 1990;Frenking et al, 1982).…”

Gas-phase tautomerism in hydroxy azo dyes - from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol

“…in the study of various heterocyclic tautomers, by Allegretti et al 9. in the investigation of the tautomerism of some amides and thioamides, by Masur et al 10. in the case of keto/enol tautomerism in a series of 1‐ and 3‐substituted acetylacetones, and by Borgerding et al 11.…”

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

“…In rare cases, orthogonal separation of tautomers prior to MS analysis could be afforded by gas chromatography [4], but the vast majority of cases involve MS analysis of convolved mixtures of tautomers. As such, the probing of tautomeric systems with MS has consisted of observing the relative ratios of tautomers and their fragment ions by adjusting ionization conditions [5], adjusting the energy of a tandem mass spectrometry (MS/MS) event [6], using diagnostic ion/molecule reactions [7][8][9][10][11][12], probing with ion spectroscopy [13,14], or through the assistance of computational modeling to confirm the presence and behavior of a given tautomer [15].…”