Reactivity of N(2D) atoms and of the cyano radical (CN) with vinyl cyanide (H2CCHCN) in Titan&#8217;s atmosphere: a combined crossed beam and theoretical study
Authors: Marchione Demian , Mancini Luca , Liang Pengxiao , Vanuzzo Gianmarco , Casavecchia Piergiorgio , Skouteris Dimitrios , Rosi Marzio , Balucani Nadia 
Published: July 2021

Several&#160;studies have reported model vertical profiles&#160;and/or abundance measurements for the main nitrile species observed in Titan&#8217;s atmosphere&#160;including: hydrogen cyanide&#160;(HCN),&#160;cyanoacetylene&#160;(HC3N), and&#160;vinyl cyanide&#160;(H2CCHCN) [1-4]. One of the main mechanisms leading to the formation of these nitriles&#160;(molecules bearing the CN group)&#160;is the reaction of CN radicals with hydrocarbons (molecules with general formula&#160;CxHy)&#160;involving H elimination (e.g.&#160;CN + C2H4&#160;= H2CCHCN + H) or H abstraction (e.g.&#160;CN + CH4&#160;= HCN + CH3) [5-9].&#160;In this regard,&#160;it is worthwhile to highlight the recent spectroscopic detection of H2CCHCN, obtained using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), because this species is the best candidate molecule&#160;to&#160;form&#160;stable&#160;cell membranes/vesicle structures of potential&#160;astrobiological&#160;importance&#160;in Titan&#8217;s hydrocarbon-rich lakes and seas [10-12].&#160;

In the upper atmosphere of Saturn&#8217;s moon,&#160;photolysis of&#160;N2&#160;and&#160;HCN&#160;leads to a continuous supply of reactive neutral species including&#160;N(4S,&#160;2D) atoms and CN&#160;radicals&#160;[2,&#160;13-16] that can further react with hydrocarbons and&#160;nitriles leading to two possible outcomes: 1) these reactions can directly contribute to the growth of molecular complexity, leading to&#160;a variety of N-rich organic species&#160;which, in turn, can&#160;act as&#160;(some of&#160;the) building blocks of life as we know it; 2) break the molecules into smaller but more stable fragments.&#160;&#160;

A detailed knowledge of the relevant potential energy surface for each reactive systems on the basis of quantum-mechanical calculations and laboratory experiments is of pivotal importance in identifying the nature of the primary products and estimating their branching ratios, and hence providing the&#160;modellers&#160;with solid ground to&#160;describe and predict&#160;the rich, complex&#160;chemical evolution of&#160;Titan&#8217;s&#160;planetary atmosphere.&#160;

Within this framework, we will report&#160;on two elementary chemical processes: 1)&#160;N(2D) + H2CCHCN&#160;and 2) CN(X2&#931;+) + H2CCHCN&#160;with emphasis on&#160;the&#160;N addition or&#160;CN addition&#160;(followed by&#160;H elimination)&#160;reactive channel&#160;by&#160;means of the&#160;crossed molecular beams (CMB) technique&#160;coupled&#160;with mass spectrometric detection and time-of-flight analysis, and&#160;we&#160;will report on the primary products from the analysis of our measurements. Additional insight on the&#160;micromechanism&#160;at play and the interpretation of the scattering results is&#160;given&#160;by new electronic structure calculations of stationary points and product energetics in the potential energy surface of the investigated systems. RRKM statistical calculations&#160;for the&#160;CN(X2&#931;+) +&#160;H2CCHCN&#160;system&#160;allows us&#160;to derive the product branching ratios&#160;and rate constants&#160;under the conditions of the present experiments and of the atmosphere of Titan.&#160;Preliminary results and astrophysical implications will be presented.&#160;

ACKNOWLEDGMENTS:&#160;&#160;

This research was supported by the Italian Space Agency (ASI, DC-VUM-2017-034, Grant n&#176; 2019-3 U.O Life in Space) and the Marie Sk&#322;odowska-Curie project "Astro-Chemical Origins&#8221; (ACO), grant agreement No 811312.&#160;

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