Gas phase chemistry of the heterocumulene cations OCN+CO and OCCN+O

Abstract

The low energy collisional dissociation and ion/molecule chemistry of the heterocumulene cations OCN⁺CO 1 and OCCN⁺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’, i.e. 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.