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|Type:||Artigo de periódico|
|Title:||Gas phase chemistry of the heterocumulene cations O=C=N+=C=O and O=C=C=N+=O|
|Abstract:||The low energy collisional dissociation and ion/molecule chemistry of the heterocumulene cations O=C=N+=C=O 1 and O=C=C=N+=O 2 have been investigated by pentaquadrupole mass spectrometry, and G2(MP2) ab initio calculations applied to interrogate their relative stabilities and dissociation thresholds, as well as those of six other conceivable C2NO2+ isomers 3-8. The calculations show that the acyclic 1 (zero) and 2 (72.4 kcal mol(-1)) are the most stable isomers, whereas both the location of the positive charge mainly at the CO-carbon and the short CO bond lengths characterize their acylium ion structures, Two cyclic isomers, i.e. 7 (131.3 kcal mol(-1)) and 8 (140.0) kcal mol(-1), were also found to be stable, but placed at energy levels considerably higher than 1. Exactly as predicted from G2(MP2) energy dissociation thresholds, low-energy collisions cause dissociation of 1 exclusively by CO loss to yield NCO+ of m/z 42. A more diverse dissociation chemistry is predicted and exhibited by 2, which dissociates mainly by loss of an oxygen atom (C2NO+ of m/z 54), CO (CNO+ of m/z 42) and C2O (NO+ of m/z 30). Both ions are unreactive towards polar [4+2(+)] cycloaddition with isoprene. However, they undergo ketalization with 2-methoxyethanol, and transacetalization with two cyclic neutral acetals, i.e. 2-methyl-1,3-dioxolane and 1,3-dioxane, and these structurally diagnostic ion/molecule reactions confirm experimentally the acylium ion structures of 1 and 2. Cyclic 'ionic ketals', ie. 1,3-dioxonium ions, are formed in these reactions, as evidenced by their MS3 spectra, which show extensive dissociation to re-form the reactant ions. Whereas 1 readily forms a stable and covalently bound adduct with pyridine, 2 reacts mainly by net CN+ and OCN+ transfer via--most likely--the unstable (Py-2)(+) adduct.|
|Editor:||Royal Soc Chemistry|
|Citation:||Journal Of The Chemical Society-perkin Transactions 2. Royal Soc Chemistry, n. 11, n. 2347, n. 2352, 1997.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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