On the structure of the C2H4O2 neutrals (acetic acid versus 1,1-dihydroxyethene) generated from ionized n-hexanoic acid and n-butyl acetate in the gas phase. Implications for the mechanism of the McLafferty rearrangement

Results are reported of CIDI and NRMS experiments which show that the C2H4O2 neutral co-generated with C4H+√8 from the metastable n-hexanoic acid ion and the n-butyl acetate molecular ion is acetic acid and not its stable enol, CH2C(OH)2. This is in marked contrast to the structure of the C2H4O+√2 ion, co-generated with C4H8 from the metastable n-hexanoic acid ion, which was earlier shown to be exclusively the enol form of acetic acid. The implication of this and earlier findings for the mechanism of McLafferty-type processes is discussed and it is suggested that this ubiquitous reaction is more complex than hitherto expected. The reaction may well proceed via long-lived ion/dipole or hydrogen-bridged intermediates

The remarkable dissociation chemistry of 2-aminoxyethanol ions NH2OCH2CH2OH+studied by experiment and theory

Low-energy 2-aminoxyethanol molecular ions NH2OCH2CH2OH+ exhibit a surprisingly rich gas-phase ion chemistry. They spontaneously undergo five major dissociations in the microsecond timeframe, yielding ions of m/z 61, 60, 46, 32 and 18. Our tandem mass spectrometry experiments indicate that these reactions correspond to the generation of HOCH2CH(OH)+ (protonated glycolaldehyde), HOCH2C(O)H+ (ionized glycolaldehyde), HC(OH)NH2+ (protonated formamide), CH2OH2+ (the methylene oxonium ion) and NH4+. A mechanistic analysis of these processes using the CBS-QB3 model chemistry shows that the molecular ions undergo a 1,4-H shift followed by a facile isomerization into the ion–molecule complex [HOCH2C(O)H+][NH3] which acts as the reacting configuration for the five exothermic dissociation processes. Analysis of the D-labelled isotopomer ND2OCH2CH2OD+, in conjunction with our computational results, shows that proton-transport catalysis may be responsible for the partial conversion of the m/z 60 glycolaldehyde ions into the more stable 1,2-dihydroxyethene isomer HOC(H)C(H)OH+

On the formation of C2H5O2+ ions having the structure of hydroxy-protonated acetic acid

Experiments are reported which are best explained in terms of the formation of the long-sought hydroxy-protonated acetic acid, CH3C(O)OH2- This C2 H5O2+ species, generated upon dissociative ionization of 2,4-dihydroxy-2-methylpentane (consecutive losses of CH3. and C3H6), is characterized by a unique collisional activation mass spectrum and an extraordinarily small kinetic energy release for the unimolecular loss of H2O (T0.5<0.15 kJ mol−1). The latter observation is in good agreement with the results of semi-empirical molecular orbital calculations

The loss of NH2O• from the N-hydroxyacetamide radical cation CH3C(=O)NHOH•+: An ion-catalysed rearrangement

A previous study [Ch. Lifshitz, P.J.A. Ruttink, G. Schaftenaar, J.K. Terlouw, Rapid Commun. Mass Spectrom. 1 (1987) 61] shows that metastable N-hydroxyacetamide ions CH3C(=O)NHOH+ (HA-1) do not dissociate into CH3C=O++NHOH by direct bond cleavage but rather yield CH3C=O++NH2O. The tandem mass spectrometry based experiments of the present study on the isotopologue CH3C(=O)NDOD+ reveal that the majority of the metastable ions lose the NH2O radical as NHDO rather than ND2O. A mechanistic analysis using the CBS-QB3 model chemistry shows that the molecular ions HA-1 rearrange into hydrogen-bridged radical cations [O=C-C(H2)-H...N(H)OH]+ whose acetyl cation component then catalyses the transformation NHOH->NH2O prior to dissociation. The high barrier for the unassisted 1,2-H shift in the free radical, 43 kcal mol-1, is reduced to a mere 7 kcal mol-1 for the catalysed tranformation which can be viewed as a quid-pro-quo reaction involving two proton transfers

Formaldehyde mediated proton-transport catalysis in the ketene-water radical cation CH2=C(=O)OH2•+

Previous studies have shown that the solitary ketene-water ion CH2=C(=O)OH2+ (1) does not isomerize into CH2=C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2=O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2C=O+. A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH2C(=O)O(H)-H...OCH2]+ in which the CH2O molecule catalyzes the transformation 1->2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol-1, is reduced to mere 0.6 kcal mol-1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis

The acrylonitrile dimer ion: A study of its dissociation via self-catalysis, self-protonation and cyclization into the pyrimidine radical cation

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH2CHCN+, into their more stable hydrogen-shift isomers CH2CCNH+ or CHCH–CNH+. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/z 54 ions are ions CH2CHCNH+ generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH2CHCNH–C(CN)CH2]+ and [CH2CHCNH–C(H)C(H)CN]+; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH2CCNH+ or CHCH–CNH+ by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/z 53 ions are CH2CHCN+ ions, generated by back dissociation; (iii) the peaks at m/z 79, 80 and 105 correspond with the losses of HCN, C2H2 and H, respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH2CHCN–C(H2)C(H)CN]+; experiments indicate that the m/z 79 (C5H5N) and m/z 105 (C6H6N2) ions have linear structures but the m/z 80 (C4H4N2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule