Sequential Reactions of Acetylene with the Benzonitrile Radical Cation: New Insights into Structures and Rate Coefficients of the Covalent Ion Products.

Benzonitrile radical cations generated in ionizing environments such as solar nebulae and interstellar clouds can react with neutral molecules such as acetylene to form a variety of nitrogen-containing complex organics. Herein, we present results from mass-selected ion mobility experiments and coupled-cluster and DFT calculations for the sequential reactions of acetylene with the benzonitrile radical cation (CNH). The results reveal the formation of two covalently bonded adduct ions CNH and CNH with individual rate coefficients of 2.1(±0.4) × 10 cm s and 1.1(±0.9) × 10 cm s, respectively measured at 334.5 K. The direct addition of acetylene onto the N atom of the benzonitrile cation results in the formation of a N-acetylene-benzonitrile radical cation with a calculated collision cross-section of 67.5 Å in perfect agreement with the measured cross-section of 67.5 Å of the CNH adduct. The measured collision cross-section of the second covalent adduct CNH (72.2 Å) is also in excellent agreement with the calculated cross-section (71.2 Å) of the lowest energy isomer of the CNH ion corresponding to the 2-phenylpyridine structure. The formation of the bicyclic 2-phenylpyridine radical cation is explained by the rapid conversion of the classical radical cation CNH into a distonic ion structure that can efficiently cyclize in an exothermic transformation to form the 2-phenylpyridine radical cation. This intriguing mechanism could explain the formation of N-containing complex organics in different regions of outer space. The current results are expected to have direct implications for the search for nitrogen-containing complex organics in space.