Reactivity of the Cyano Radical (CN) with Cyanoacetylene (HC3N) and with Vinyl Cyanide (H2CCHCN) in Titan’s Atmosphere: a Combined Crossed Beam and Theoretical Study

Abstract:

Several studies have reported model vertical profiles and abundance measurements for the main nitrile species observed in Titan’s atmosphere including: hydrogen cyanide (HCN), cyanoacetylene (HC₃N), and vinyl cyanide (H₂CCHCN) [1,2]. One of the main mechanisms leading to the formation of these nitriles is the reaction of CN radicals with hydrocarbons via H elimination or H abstraction [3,4]. The main source of CN radicals has been identified as the photolysis of HCN in the upper atmosphere of Saturn’s moon [1,5]. It follows that there exist a continuous supply of CN radicals that can further react with hydrocarbons and nitriles leading to a variety of prebiotic N-containing organic species which, in turn, can act as (some of the) building blocks of life as we know it.

The reactions involving these species are strongly intertwined with each other and hence, the models describing the chemical evolution of Titan’s atmosphere require a detailed knowledge of the chemical network corroborated by quantum-mechanical calculations and laboratory experiments.

Within this context, we have been investigating the dynamics of the CN(X²Σ⁺) + HC₃N reaction and of the CN(X²Σ⁺) + H₂CCHCN reaction by means of the crossed molecular beams (CMB) technique coupled with mass spectrometric detection and time-of-flight analysis and we will report on the primary products from the analysis of our measurements. Additional insight on the micromechanism at play and the interpretation of the scattering results is given by new electronic structure calculations of stationary points and product energetics in the potential energy surface of the investigated systems. RRKM statistical calculations are undergoing to derive the product branching ratios under the conditions of the present experiments and of the atmosphere of Titan. Preliminary results and astrophysical implications will be presented.

ACKNOWLEDGMENTS: The Italian Space Agency (ASI, DC-VUM-2017-034, Grant n° 2019-3 U.O Life in Space) and the Marie Skłodowska-Curie project "Astro-Chemical Origins” (ACO), grant agreement No 811312.

[1] K. Willacy et al. Astrophys. J., 829:79 (2016)

[2] A. E. Thelen et al. Icarus, 319, 417 (2019)

[3] L. C. L. Huang et al. J. Chem. Phys. 113, 8656 (2000)

[4] S. C. S. Ely et al. J. Phys. Chem. A, 117, 12155 (2013)

[5] L. M. Lara et al. Astron. Astrophys., 341, 312 (1999)