The distonic ion .CH 2CH 2CH +OH, keto ion CH 3CH 2CH=O +., enol ion CH 3CH=CHOH +., and related C 3H 6O +. radical cations. stabilities and isomerization proclivities studied by dissociation and neutralization-reionization

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.