Abstract
Metastable ion decompositions, collision-activated dissociation (CAD), and neutralization-reionization mass spectrometry are utilized to study the unimolecular chemistry of distonic ion ·CH2CH2CH−OH (2+·) and its enol-keto tautomers CH3CH=CHOH−· (1 +·) and CH3CH2CH=O +· (3+·). The major fragmentation of metastable 1+·–3+· is H· loss to yield the propanoyl cation, CH3CH2C≡O+. This reaction remains dominant upon collisional activation, although now some isomeric CH2=CH-CH+ OH is coproduced from all three precursors. The CAD and neutralization-reionization (+NR+) spectra of keto ion 3 +· are substantially different from those of tautomers 2+· and 1+·. Hence, 3+· without sufficient energy for decomposition (i. e. , “stable” 3+·) does not isomerize to the ther-modynamically more stable ions 2+· or 1+·, and the 1,4-H rearrangement H-CH2CH2CH=O+·(3 +·) → CH2CH2CH+ O-H (2 +·) must require an appreciable critical energy. Although the fragment ion abundances in the + NR + (and CAD) spectra of 1 +· and 2 +· are similar, the relative and absolute intensities of the survivor ions (recovered C3H6O+· ions in the +NR+ spectra) are markedly distinct and independent of the internal energy of 1 +· and 2 +·. Furthermore, 1 +· and 2 +· show different MI spectra. Based on these data, distonic ion 2 +· does not spontaneously rearrange to enol ion 1 +· (which is the most stable C3H6O+· of CCCO connectivity) and, therefore, is separated from it by an appreciable barrier. In contrast, the molecular ions of cyclopropanol (4 +·) and allyl alcohol (5 +·) isomerize readily to 2 +·, via ring opening and 1,2-H− shift, respectively. The sample found to generate the purest 2 +· is α-hydroxy-γ-butyrolactone. Several other precursors that would yield 2 +· by a least-motion reaction cogenerate detectable quantities of enol ion 1 +·, or the enol ion of acetone (CH2=C(CH3)OH+·, 6 +·), or methyl vinyl ether ion (CH3OCH=CH +·2 , 7 +·). Ion 6 +· is coproduced from samples that contain the —CH2—CH(OH)—CH2— substructure, whereas 7 +· is coproduced from compounds with methoxy substituents. Compared to CAD, metastable ion characteristics combined with neutralization-reionization allow for a superior differentiation of the ions studied.
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Polce, M.J., Wesdemiotis, C. The distonic ion ·CH2CH2CH+OH, keto ion CH3CH2CH=O +·, enol ion CH3CH=CHOH+·, and related C3H6O+· radical cations. Stabilities and isomerization proclivities studied by dissociation and neutralization-reionization. J Am Soc Mass Spectrom 7, 573–589 (1996). https://doi.org/10.1016/1044-0305(96)00053-0
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DOI: https://doi.org/10.1016/1044-0305(96)00053-0