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, CH2{double bond, long}CHC{triple bond, long}N{radical dot}+, into their more stable hydrogen-shift isomers CH2{double bond, long}C{double bond, long}C{double bond, long}NH{radical dot}+or CH{double bond, long}CH-C{double bond, long}NH{radical dot}+. 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 CH2{double bond, long}CHC{triple bond, long}NH+generated by self-protonation in ion-dipole stabilized hydrogen-bridged dimer ions [CH2{double bond, long}CHC{triple bond, long}N⋯H-C(C{triple bond, long}N)CH2]{radical dot}+and [CH2{double bond, long}CHC{triple bond, long}N⋯H-C(H)C(H)C{triple bond, long}N]{radical dot}+; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH2{double bond, long}C{double bond, long}C{double bond, long}NH{radical dot}+or CH{double bond, long}CH-C{double bond, long}NH{radical dot}+by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/z 53 ions are CH2{double bond, long}CHC{triple bond, long}N{radical dot}+ions, generated by back dissociation; (iii) the peaks at m/z 79, 80 and 105 correspond with the losses of HCN, C2H2and H{radical dot}, 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 [CH2{double bond, long}CHC{double bond, long}N-C(H2)C(H)C{triple bond, long}N]{radical dot}+; 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.

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