Simulating the Thermodynamic Landscape of Hydrogen Cyanide-Derived Polymers
Authors: Sharma Siddhant , Sandström Hilda , Izquierdo-Ruiz Fernando , Doğan Rana , Rahm Martin 
Published: September 2022

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&lt;h1 class=&quot;co_mto_htmlabstract-title _color-primary&quot;&gt;&lt;strong&gt;Introduction:&lt;/strong&gt;&lt;/h1&gt;
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&lt;p&gt;Hydrogen Cyanide (HCN) is a ubiquitous and reactive species present in many environments ranging from the interstellar medium to comets, asteroids, and planetary atmospheres. HCN and HCN polymers have a proposed key role in the abiotic origin of life and are suspected to be an essential prebiotic building block for the synthesis of nucleobases, amino acids, and a large variety of polymers (Ruiz-Bermejo &lt;em&gt;et al.,&lt;/em&gt; 2021). HCN is, for example, generated in large amounts by the atmospheric photochemistry of Saturn&amp;#8217;s moon Titan (Molter &lt;em&gt;et al.,&lt;/em&gt; 2016), where it is expected to play a key role in chemical evolution (Cable &lt;em&gt;et al.,&lt;/em&gt; 2012). Despite many decades of research, the complex nature of products derived from HCN polymerization experiments remains largely unknown. Thus, HCN and its reactivity are very important for prebiotic chemistry (Sandstr&amp;#246;m &amp; Rahm, 2021).&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Methodology:&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Figure 1.&lt;/strong&gt; An overview of the methodology for computing Gibbs free energy (&amp;#916;G&lt;sup&gt;&amp;#176;&lt;/sup&gt;&lt;sub&gt;298K&lt;/sub&gt;) content for the polymeric structures in our study.&lt;/p&gt;
&lt;p&gt;We describe how quantum chemical calculations and simulations can garner insight into the possible prebiotic synthesis of macromolecules and polymers. Our evaluation covers several molecules and oligomers, most of which have been discussed in the literature, and ranks them based on thermodynamic preference (Figure&lt;sup&gt;1&lt;/sup&gt;). We sidestep the complexities of experimental reaction mixtures by computationally exploring individual candidate prebiotic molecules and polymers in solution. A method for conformational sampling is combined with Density Functional Theory (DFT) to predict the thermodynamic stability of most HCN-based structures considered in the literature. This data is used to construct a thermodynamic map to evaluate the feasibility of various structures and reaction hypotheses.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Results:&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Our enumeration of thermodynamically plausible reaction products and the reaction routes for the abiotic formation of organic macromolecules starting from simple units of HCN offers extensive insights into the chemical and physical limitations of suspected key prebiotic processes. Some structures and reactions suggested in the literature are here proven unviable on thermodynamic grounds. However, most HCN-self reactions are found to be spontaneous. Polyaminoimidazole, and the nucleobase adenine, are computed to be the most thermodynamically favoured products that can form from HCN. The presented thermodynamics of HCN polymerization coupled with predictions of closely competing reaction kinetics have several consequences for interpreting HCN reactivity in various astrochemical environments and may help explain experimental observations of complex reaction mixtures.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;Ruiz-Bermejo, M. et al. (2021). Processes, 9(4), 597.&lt;/li&gt;
&lt;li&gt;Molter, E.M. et al. (2016) Astron J, 152, 42/1-42/&lt;/li&gt;
&lt;li&gt;Cable, M.L. et al. (2012) Chem Rev, 112, 1882-1909.&lt;/li&gt;
&lt;li&gt;Sandstr&amp;#246;m, H. et al. (2021). ACS Earth Space Chem, 5(8), 2152-2159.&lt;/li&gt;
&lt;/ul&gt;
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