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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Chem.</journal-id>
<journal-title>Frontiers in Chemistry</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Chem.</abbrev-journal-title>
<issn pub-type="epub">2296-2646</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">679750</article-id>
<article-id pub-id-type="doi">10.3389/fchem.2021.679750</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Chemistry</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Insights Into Chemical Reactions at the Beginning of the Universe: From HeH<sup>&#x2b;</sup> to H<sub>3</sub>
<sup>&#x2b;</sup>
</article-title>
<alt-title alt-title-type="left-running-head">Dash et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">First Molecules in the Universe</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Dash</surname>
<given-names>Soumya Ranjan</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="fn" rid="FN1">
<sup>&#x2020;</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1265572/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Das</surname>
<given-names>Tamal</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="fn" rid="FN1">
<sup>&#x2020;</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1142492/overview"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Vanka</surname>
<given-names>Kumar</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1265487/overview"/>
</contrib>
</contrib-group>
<aff id="aff1">
<label>
<sup>1</sup>
</label>Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), <addr-line>Pune</addr-line>, <country>India</country>
</aff>
<aff id="aff2">
<label>
<sup>2</sup>
</label>Academy of Scientific and Innovative Research (AcSIR), <addr-line>Ghaziabad</addr-line>, <country>India</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1079169/overview">Wolfgang Sch&#xf6;fberger</ext-link>, Johannes Kepler University of Linz, Austria</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/122516/overview">Grant James Mathews</ext-link>, University of Notre Dame, United&#x20;States</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/106990/overview">Alejandro Gil-Villegas</ext-link>, University of Guanajuato, Mexico</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Kumar Vanka, <email>k.vanka@ncl.res.in</email>
</corresp>
<fn fn-type="equal" id="FN1">
<label>
<sup>&#x2020;</sup>
</label>
<p>These authors have contributed equally to this&#x20;work</p>
</fn>
<fn fn-type="other">
<p>This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>18</day>
<month>06</month>
<year>2021</year>
</pub-date>
<pub-date pub-type="collection">
<year>2021</year>
</pub-date>
<volume>9</volume>
<elocation-id>679750</elocation-id>
<history>
<date date-type="received">
<day>15</day>
<month>03</month>
<year>2021</year>
</date>
<date date-type="accepted">
<day>07</day>
<month>06</month>
<year>2021</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2021 Dash, Das and Vanka.</copyright-statement>
<copyright-year>2021</copyright-year>
<copyright-holder>Dash, Das and Vanka</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<abstract>
<p>At the dawn of the Universe, the ions of the light elements produced in the Big Bang nucleosynthesis recombined with each other. In our present study, we have tried to mimic the conditions in the early Universe to show how the recombination process would have led to the formation of the first ever formed diatomic species of the Universe: HeH<sup>&#x2b;</sup>, as well as the subsequent processes that would have led to the formation of the simplest triatomic species: H<sub>3</sub>
<sup>&#x2b;</sup>. We have also studied some special cases: higher positive charge with fewer number of hydrogen atoms in a dense atmosphere, and the formation of unusual and interesting linear, dicationic He chains beginning from light elements He and H in a positively charged atmosphere. For all the simulations, the <italic>ab initio</italic> nanoreactor (<italic>AINR</italic>) dynamics method has been employed.</p>
</abstract>
<kwd-group>
<kwd>density functional theory</kwd>
<kwd>ab initio molecular dynamics</kwd>
<kwd>ab initio nanoreactor</kwd>
<kwd>origin of molecules in the universe</kwd>
<kwd>the earliest lewis acid</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<sec id="s1">
<title>Introduction</title>
<p>The way the Universe, and all the elements, came into being is one of the fascinating questions of science. Attempts to answer this question has led to the Big Bang theory, and an understanding of the primeval Universe and the entities that it was made up of (<xref ref-type="bibr" rid="B28">Meyer, 2008</xref>). Further advancement of science and technology has led to greater understanding, which led NASA&#x2019;s Stratospheric Observatory for Infrared Astronomy (SOFIA) to the detection of HeH<sup>&#x2b;</sup> in the planetary nebula NGC 7027, the first molecule formed after the Big Bang (<xref ref-type="bibr" rid="B18">G&#xfc;sten et&#x20;al., 2019</xref>), 94&#xa0;years after its discovery in the laboratory in 1925 (<xref ref-type="bibr" rid="B19">Hogness and Lunn, 1925</xref>).</p>
<fig id="F1a" position="float">
<label>GRAPHICAL ABSTRACT</label>
<graphic xlink:href="fchem-09-679750-g004.tif"/>
</fig>
<p>As the first molecule, the significance of the role of HeH<sup>&#x2b;</sup> in the evolution of other species cannot be overstated. One of these species, and perhaps the most important, is the simplest polyatomic molecule H<sub>3</sub>
<sup>&#x2b;</sup>, which has always intrigued researchers ever since its discovery in 1911 by J.&#x20;J.&#x20;Thomson (<xref ref-type="bibr" rid="B43">Sir Thomson, 1911</xref>). However, the importance H<sub>3</sub>
<sup>&#x2b;</sup> in astrochemistry was realized only after it was detected on Jupiter in the 1980s (<xref ref-type="bibr" rid="B33">Oka, 1980</xref>; <xref ref-type="bibr" rid="B9">Drossart et&#x20;al., 1989</xref>). High abundance of H<sub>3</sub>
<sup>&#x2b;</sup> in the Universe and its ability to donate a proton established this triatomic cation as the interstellar acid of utmost importance for many extra-terrestrial reactions (<xref ref-type="bibr" rid="B53">Watson, 1973</xref>; <xref ref-type="bibr" rid="B32">Oka, 2013</xref>; <xref ref-type="bibr" rid="B35">Olah et&#x20;al., 2016</xref>; <xref ref-type="bibr" rid="B38">Pelley, 2019</xref>). While there are many reports of H<sub>3</sub>
<sup>&#x2b;</sup> formation from doubly ionized organic molecules (<xref ref-type="bibr" rid="B46">Townsend et&#x20;al., 2004</xref>; <xref ref-type="bibr" rid="B8">De et&#x20;al., 2006</xref>; <xref ref-type="bibr" rid="B34">Okino et&#x20;al., 2006</xref>; <xref ref-type="bibr" rid="B23">Kushawaha and Bapat, 2008</xref>; <xref ref-type="bibr" rid="B27">Mebel and Bandrauk, 2008</xref>; <xref ref-type="bibr" rid="B30">Nakai et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B1">Ando et&#x20;al., 2018</xref>; <xref ref-type="bibr" rid="B10">Ekanayake et&#x20;al., 2018</xref>; <xref ref-type="bibr" rid="B37">Palaudoux and Hochlaf, 2019</xref>), our focus is on its origin and the role played by HeH<sup>&#x2b;</sup> on its formation.</p>
<p>H<sub>3</sub>
<sup>&#x2b;</sup> formation was first reported to occur primarily from the combination of H<sub>2</sub>
<sup>&#x2b;</sup> and H<sub>2</sub>, where H<sub>2</sub>
<sup>&#x2b;</sup> would be formed from the ionization of H<sub>2</sub> (<xref ref-type="bibr" rid="B44">Sir Thomson, 1912</xref>; <xref ref-type="bibr" rid="B32">Oka, 2013</xref>). There are other reports which state that H<sub>2</sub>
<sup>&#x2b;</sup> is more likely to be formed from the combination of HeH<sup>&#x2b;</sup> and H (<xref ref-type="bibr" rid="B5">Bovino et&#x20;al., 2011</xref>; <xref ref-type="bibr" rid="B42">Razio, 2014</xref>; <xref ref-type="bibr" rid="B11">Esposito et&#x20;al., 2015</xref>; <xref ref-type="bibr" rid="B14">Fortenberry, 2019</xref>; <xref ref-type="bibr" rid="B16">Gonz&#xe1;lez-Lezana et&#x20;al., 2019</xref>). At the same time, the possibility of HeH<sup>&#x2b;</sup> combining with H<sub>2</sub> to produce H<sub>3</sub>
<sup>&#x2b;</sup> cannot be overlooked (<xref ref-type="bibr" rid="B25">McLaughlin and Thompson, 1973</xref>). Thus, many factors can influence the origin of H<sub>3</sub>
<sup>&#x2b;</sup>, but there have not been any conclusive studies&#x20;yet.</p>
<p>In this work, we have employed the <italic>ab initio</italic> nanoreactor (AINR) method to carry out full quantum mechanical molecular dynamics (MD) simulations on systems containing atoms/ions of helium and hydrogen, and have obtained reaction profiles by varying their mixture ratio and the charge. The <italic>AINR</italic> method, developed by Martinez and co-workers, allows the determination of new reaction pathways and products, without the need of controlling the chemical system (<xref ref-type="bibr" rid="B54">Zimmerman, 2013</xref>; <xref ref-type="bibr" rid="B41">Rappoport et&#x20;al., 2014</xref>; <xref ref-type="bibr" rid="B52">Wang et&#x20;al., 2014</xref>). Our primary goal was to gain insight into the formation of different species from the combination of He and H in the presence of a positively charged atmosphere, as well as their further dissociation and recombination. As the Results and Discussion section will show, our studies provide interesting new insights into HeH<sup>&#x2b;</sup> formation, and shed light on various short-lived intermediates that could have formed <italic>en route</italic> to obtaining H<sub>3</sub>
<sup>&#x2b;</sup>&#x2013; the stable species that was known to exist in the early Universe (<xref ref-type="bibr" rid="B32">Oka, 2013</xref>).</p>
</sec>
<sec id="s2">
<title>Computational Methods</title>
<sec id="s2-1">
<title>
<italic>Ab Initio</italic> Molecular Dynamics Simulations</title>
<p>The AIMD simulations were performed with the TeraChem 1.9 (<xref ref-type="bibr" rid="B48">Ufimtsev and Mart&#xed;nez, 2008a</xref>; <xref ref-type="bibr" rid="B49">Ufimtsev and Mart&#xed;nez, 2008b</xref>; <xref ref-type="bibr" rid="B50">Ufimtsev and Martinez, 2009a</xref>; <xref ref-type="bibr" rid="B51">Ufimtsev and Martinez, 2009b</xref>; <xref ref-type="bibr" rid="B47">Ufimtsev et&#x20;al., 2011</xref>; <xref ref-type="bibr" rid="B21">Isborn et&#x20;al., 2011</xref>; <xref ref-type="bibr" rid="B45">Titov et&#x20;al., 2013</xref>) software package using the Hartree&#x2212;Fock (HF) (<xref ref-type="bibr" rid="B13">Fischer, 1987</xref>) electronic wave function and the 6&#x2013;311&#xa0;g (<xref ref-type="bibr" rid="B4">Binkley et&#x20;al., 1980</xref>) Gaussian basis set, to calculate the Born&#x2212;Oppenheimer potential energy surface. This method has been implemented in TeraChem by Martinez and co-workers. This approach was deemed acceptable because the HF method is well-known for predicting chemically reasonable structures (<xref ref-type="bibr" rid="B12">Feller and Peterson, 1998</xref>). Also, it should be noted that HF was not employed to determine the thermodynamics of the reactions: its only role was in the discovery process. This was also the approach employed by Martinez and co-workers in their original <italic>AINR</italic> paper (employing HF/3&#x2013;21&#xa0;g), where they replicated the results obtained from the Urey&#x2212;Miller experiment, as well as from the interaction of acetylene molecules. The same method (HF) was also employed by us in our previous report (<xref ref-type="bibr" rid="B7">Das et&#x20;al., 2019</xref>) on reaction pathways leading to the formation of precursors of RNA and sugars. Electrostatic interactions were treated using two of the most common methods: the residue-based cutoff and particle mesh Ewald (PME). The PME method has been chosen because it takes care of long-range electrostatic interactions and is the most widely used approach.</p>
<p>The results were obtained from the <italic>AINR</italic> simulations by varying both the He to H ratio, as well as the positive charge of the system. Each simulation was repeated thrice. The system was constrained in a spherical boundary of 4.0 and 2.0&#xa0;&#xc5; radii, so that the atoms resided in a space that alternated between the volumes created by these two radii, and collided with each other. Each <italic>AINR</italic> dynamics was evolved upto 15&#xa0;ps, with a time step of 0.5&#xa0;fs.</p>
<p>Newton&#x2019;s equations of motion were calculated using Langevin dynamics, with an equilibrium temperature of 1,000.0&#xa0;K (also the starting temperature of the dynamics). We have used this high temperature in order to increase the average kinetic energy of the reactants and for faster dynamics (<xref ref-type="bibr" rid="B31">Novotn&#xfd; et&#x20;al., 2019</xref>). We have employed the augmented direct inversion in the iterative subspace (ADIIS) algorithm (<xref ref-type="bibr" rid="B20">Hu and Yang, 2010</xref>) available in TeraChem as an alternative tool for self-consistent field calculations at each AIMD step in which the default DIIS algorithm (<xref ref-type="bibr" rid="B40">Pulay, 1980</xref>) failed to converge. The nanoreactor simulations employ a virtual piston by fluctuating the radius of a spherical boundary, which allows the continuous expansion and compression of the system, thus artificially changing the pressure and the density for the collision cycles, which is necessary for the molecules to collide and also increases the rate of the reactions. Spherical boundary conditions (details in the ESI) were applied to prevent the molecules from flying away, a phenomenon known as the &#x201c;evaporation&#x201d; event. For more details, please check the original paper on the <italic>AINR</italic> by Martinez and co-workers (<xref ref-type="bibr" rid="B52">Wang et&#x20;al., 2014</xref>).</p>
<p>The mechanistic pathways obtained from the <italic>AINR</italic> simulations were then analyzed with full quantum mechanical (QM) calculations. All the structures were optimized with coupled cluster singles doubles (CCSD) (<xref ref-type="bibr" rid="B17">Grotendorst et&#x20;al., 2006</xref>) and with the 6&#x2013;311&#x2b;&#x2b;G (d,<italic>p</italic>) (<xref ref-type="bibr" rid="B26">McLean and Chandler, 1980</xref>) basis set. The Gaussian09 software (<xref ref-type="bibr" rid="B15">Frisch et&#x20;al., 2009</xref>) was employed for the thermodynamic calculations. The complete solution of the H<sub>3</sub>
<sup>&#x2b;</sup> problem requires the consideration of relativistic effects, nuclear motion, and breakdown of the Born-Oppenheimer (B-O) approximation (both adiabatic and non-adiabatic) (<xref ref-type="bibr" rid="B29">Miller et&#x20;al., 2020</xref>). Recent studies have shown that relativistic effects for H<sub>3</sub>
<sup>&#x2b;</sup> are quite negligible (<xref ref-type="bibr" rid="B6">Cencek et&#x20;al., 1998</xref>; <xref ref-type="bibr" rid="B2">Bachorz et&#x20;al., 2009</xref>). Issues of breakdown of the B-O approximation become relevant when considering transition frequencies of H<sub>3</sub>
<sup>&#x2b;</sup>, i.e.,&#x20;in spectroscopic studies. Since the current work is focused on understanding the possible chemical reactivity and thermochemistry of HeH<sup>&#x2b;</sup> and H<sup>&#x2b;</sup>, leading eventually to H<sub>3</sub>
<sup>&#x2b;</sup> formation, the current studies, done within the B-O approximation, are appropriate.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>Results and Discussions</title>
<p>In this section, we will briefly describe the formation of H<sub>3</sub>
<sup>&#x2b;</sup> in the <italic>AINR via</italic> different short lived intermediates. We have taken a fixed composition of the He and H mixture and varied the overall positive charge density of the system (as shown in <xref ref-type="table" rid="T1">Tables 1</xref>, <xref ref-type="table" rid="T2">2</xref>). During the simulations in each case, it was seen that HeH<sup>&#x2b;</sup> was formed at the very beginning of the dynamics as the first molecular species. In our first set of simulations, we have taken a homogeneous mixture of 30 atoms each of H and He. The <italic>AINR</italic> makes them collide with each other at a temperature of 1,000.0&#xa0;K. The simulation with no positive charge in the system does not produce any intermediates and H<sub>3</sub>
<sup>&#x2b;</sup> at all throughout the dynamics. This led us to consider the possibility that a more appropriate set-up would include a positively charged system, which would mimic the collisions between the ionized state of the helium and hydrogen atoms present at the beginning of Universe (<xref ref-type="bibr" rid="B32">Oka, 2013</xref>). A positively charged environment for the formation of H<sub>3</sub>
<sup>&#x2b;</sup> had also been considered by many previous reports, while investigating its origin from different organic molecules (<xref ref-type="bibr" rid="B39">Pilling et&#x20;al., 2007</xref>). Therefore, we have varied the positive charge of the system by even numbers (<xref ref-type="table" rid="T1">Table&#x20;1</xref>) during the <italic>AINR</italic> dynamics. As the dynamics progressed, various short lived species such as He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup> and He<sub>2</sub>H<sup>&#x2b;</sup> (snapshots shown in the <xref ref-type="sec" rid="s9">Supplementary Figures S1&#x2013;S5</xref> in the Supporting Information (ESI) file) were seen to have formed in almost every simulation, though their time of appearance was different in each case. It was also observed that with the increase of the positive charge of the system, the formation of H<sub>3</sub>
<sup>&#x2b;</sup> ions also increased, up to a point. The number of H<sub>3</sub>
<sup>&#x2b;</sup> ions generated was equal to the positive charge in the system, up to a charge of &#x2b;6 (see <xref ref-type="table" rid="T1">Table&#x20;1</xref>).</p>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>
<italic>AINR</italic> simulations with 30 He atoms and 30&#x20;H atoms: different entries represent the variation of the total positive charge of the system&#x2013;by even numbers.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Total charge</th>
<th align="center">First molecule</th>
<th align="center">Intermediate species</th>
<th align="center">Dominant end molecule</th>
<th align="center">No. of H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">0</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td align="left">2</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">2</td>
</tr>
<tr>
<td align="left">4</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">4</td>
</tr>
<tr>
<td align="left">6</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">6</td>
</tr>
<tr>
<td align="left">8</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">7</td>
</tr>
<tr>
<td align="left">10</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">7</td>
</tr>
<tr>
<td align="left">20</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">5</td>
</tr>
</tbody>
</table>
</table-wrap>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>
<italic>AINR</italic> simulations with 30 He atoms and 29 H atoms: different entries represent the variation of the total positive charge of the system&#x2013;by odd numbers.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Total charge</th>
<th align="center">First molecule</th>
<th align="center">Intermediate species</th>
<th align="center">Dominant end molecule</th>
<th align="center">No. of H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">0</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td align="left">1</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">1</td>
</tr>
<tr>
<td align="left">3</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">3</td>
</tr>
<tr>
<td align="left">5</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">5</td>
</tr>
<tr>
<td align="left">7</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">6</td>
</tr>
<tr>
<td align="left">9</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">7</td>
</tr>
<tr>
<td align="left">11</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">7</td>
</tr>
<tr>
<td align="left">21</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>, H<sub>2</sub>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>, HeH<sup>&#x2b;</sup>
</td>
<td align="char" char=".">4</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>However, upon further increase in the positive charge of the system beyond six&#x2013;to eight or ten, the number of H<sub>3</sub>
<sup>&#x2b;</sup> ions formed was not seen to be equal to the total positive charge of the system. Instead of H<sub>3</sub>
<sup>&#x2b;</sup>, the remaining positive charge of the system was balanced by H<sup>&#x2b;</sup> or, in some cases, HeH<sup>&#x2b;</sup>. As shown in <xref ref-type="table" rid="T1">Table&#x20;1</xref>, in case of a positive charge of 10 and after 250&#xa0;fs, we observed only seven H<sub>3</sub>
<sup>&#x2b;</sup> ions remaining with three H<sup>&#x2b;</sup>, which balanced the total charge of the system. The natural population analysis (NPA), or the formal charge analyzed data for all the atoms in several snapshots, has been shown in <xref ref-type="sec" rid="s9">Supplementary Tables S1&#x2013;S4</xref> in the&#x20;ESI.</p>
<p>Similarly, in another set of MD simulations, we have taken 29&#x20;H with 30 He atoms and varied the overall charge of the system by an odd number: one, three, five and so on. These observations have been shown in <xref ref-type="table" rid="T2">Table&#x20;2</xref>. We have observed a similar trend for the formation of H<sub>3</sub>
<sup>&#x2b;</sup> as the only end product up to a certain limit (here, the value is 5) of positive charge and beyond that, the total charge of the system was seen to be balanced by the sum of H<sub>3</sub>
<sup>&#x2b;</sup>, H<sup>&#x2b;</sup> and HeH<sup>&#x2b;</sup>, as seen in the previous section when the positive charge was varied by even numbers.</p>
<p>In short, we can say that in all the cases of AINR dynamics studied, the formation of HeH<sup>&#x2b;</sup> as the first molecule was observed. However, upon varying the total positive charge of the whole system, several short-lived species (He<sub>2</sub>H<sup>&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>
<sup>2&#x2b;</sup>) were observed (<xref ref-type="table" rid="T1">Tables 1</xref>, <xref ref-type="table" rid="T2">2</xref>) after HeH<sup>&#x2b;</sup> formation. At the end of the simulation, H<sub>3</sub>
<sup>&#x2b;</sup> and H<sub>2</sub> were found to be the only stable species left in the reaction mixture.</p>
<sec id="s3-1">
<title>The Timescale of Formation of H<sub>3</sub>
<sup>&#x2b;</sup> and Other Short Lived Molecules</title>
<p>The formation timescale of different short lived species, along with the stable H<sub>3</sub>
<sup>&#x2b;</sup>, has been observed from femtosecond <italic>AINR</italic> simulations. In each and every simulation, HeH<sup>&#x2b;</sup>, which has been proposed to be the first formed molecule, was seen to be formed soon after the beginning of the dynamics. The time of appearance of HeH<sup>&#x2b;</sup> was within 15&#xa0;fs timesteps. Subsequently, other short lived species (He<sub>2</sub>H<sup>&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>
<sup>2&#x2b;</sup>) were formed within the timescale of 0.1&#xa0;ps (shown in <xref ref-type="table" rid="T3">Table&#x20;3</xref>). The observed timescale for the existence of such transient species is around 5&#x2013;10&#xa0;fs. Once these molecules were formed, they quickly dissociated and this ultimately led to the formation of H<sub>3</sub>
<sup>&#x2b;</sup>, which was observed for every case. From the AINR dynamics, we have analyzed the data and found two pathways for the formation of H<sub>3</sub>
<sup>&#x2b;</sup>, starting from He and H in atomic states within the positively charge atmosphere. Both of the pathways involved the well-known roaming hydrogen mechanism (<xref ref-type="bibr" rid="B46">Townsend et&#x20;al., 2004</xref>; <xref ref-type="bibr" rid="B30">Nakai et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B37">Palaudoux and Hochlaf, 2019</xref>). An mp4 file (<xref ref-type="sec" rid="s9">Supplementary Movie S1</xref>) of a movie made of a part of an <italic>AINR</italic> simulation is included in the ESI. The most feasible pathway for H<sub>3</sub>
<sup>&#x2b;</sup> formation is the abstraction of a proton from the first molecule HeH<sup>&#x2b;</sup> by the roaming dihydrogen (shown in <xref ref-type="fig" rid="F1">Figure&#x20;1</xref>).</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>Time (in fs) of the first appearance of different species.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Total charge</th>
<th align="center">HeH<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>H<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">He<sub>3</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">0</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td align="left">2</td>
<td align="char" char=".">4.5</td>
<td align="char" char=".">11.0</td>
<td align="char" char=".">10.0</td>
<td align="char" char=".">25.5</td>
<td align="char" char=".">27.0</td>
</tr>
<tr>
<td align="left">4</td>
<td align="char" char=".">9.0</td>
<td align="char" char=".">25.5</td>
<td align="char" char=".">19.5</td>
<td align="char" char=".">80.0</td>
<td align="char" char=".">45.0</td>
</tr>
<tr>
<td align="left">6</td>
<td align="char" char=".">5.0</td>
<td align="char" char=".">12.5</td>
<td align="char" char=".">16.0</td>
<td align="char" char=".">49.5</td>
<td align="char" char=".">9.0</td>
</tr>
<tr>
<td align="left">8</td>
<td align="char" char=".">4.0</td>
<td align="char" char=".">12.5</td>
<td align="char" char=".">22.5</td>
<td align="char" char=".">53.0</td>
<td align="char" char=".">8.0</td>
</tr>
<tr>
<td align="left">10</td>
<td align="char" char=".">5.5</td>
<td align="char" char=".">18.0</td>
<td align="char" char=".">14.0</td>
<td align="char" char=".">19.0</td>
<td align="char" char=".">7.0</td>
</tr>
<tr>
<td align="left">20</td>
<td align="char" char=".">4.0</td>
<td align="char" char=".">26.5</td>
<td align="char" char=".">14.5</td>
<td align="char" char=".">22.0</td>
<td align="char" char=".">9.0</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>Snapshots of <italic>AINR</italic> simulations showing the progress of the dynamics starting from atomic He and H, leading toward the formation of H<sub>3</sub>
<sup>&#x2b;</sup> from HeH<sup>&#x2b;</sup> and dihydrogen. HeH<sup>&#x2b;</sup> was seen to be formed very early&#x2013;near the beginning of the dynamics (Color: He - peach, H - white).</p>
</caption>
<graphic xlink:href="fchem-09-679750-g001.tif"/>
</fig>
<p>The thermodynamics for this step has been calculated to be &#x2212;32.2&#xa0;kcal/mol (shown in <xref ref-type="fig" rid="sch1">Scheme 1</xref>). In another mechanistic pathway, there is no involvement of HeH<sup>&#x2b;</sup>. Instead of HeH<sup>&#x2b;</sup>, the proton abstraction occurs from a mono-cationic dihydrogen molecule by the roaming dihydrogen. This process is thermodynamically favourable by 27.5&#xa0;kcal/mol.</p>
<fig id="sch1">
<label>SCHEME 1</label>
<caption>
<p>Thermodynamics of H<sub>3</sub>
<sup>&#x2b;</sup> formation.</p>
</caption>
<graphic xlink:href="fchem-09-679750-g005.tif"/>
</fig>
<p>The total number of H<sub>3</sub>
<sup>&#x2b;</sup> molecules formed was also found to be directly correlated with the total charge of the system, as well as the number of He and H atoms taken. Greater charge in the system yielded more short-lived species during the simulations. Most of the intermediate species were found to be formed within 100&#xa0;fs (<xref ref-type="table" rid="T3">Table&#x20;3</xref>) and they were found to exist for only about 5&#x2013;10&#xa0;fs during the <italic>AINR</italic> simulations.</p>
<p>Since it has been postulated that different ratios of helium to hydrogen atoms could have existed in the early Universe (<xref ref-type="bibr" rid="B28">Meyer, 2008</xref>), we have further performed <italic>AINR</italic> dynamics with a 1:3 ratio of helium to hydrogen atoms and varied the total positive charge of the system (see <xref ref-type="table" rid="T4">Table&#x20;4</xref>). Furthermore, in order to investigate the effect of temperature on cosmic reionization (<xref ref-type="bibr" rid="B31">Novotn&#xfd; et&#x20;al., 2019</xref>), we have also carried out <italic>AINR</italic> simulations while varying the temperature (<xref ref-type="table" rid="T5">Table&#x20;5</xref>), with a 1:3 ratio of He:H and a fixed positive charge (8<sup>&#x2b;</sup>). In all such simulations, we have observed trends similar to those discussed in&#x20;the previous sections, like the formation of HeH<sup>&#x2b;</sup> as the first molecule and the subsequent formation of transient species (He<sub>2</sub>H<sup>&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>
<sup>2&#x2b;</sup>), leading eventually to H<sub>3</sub>
<sup>&#x2b;</sup> formation.</p>
<table-wrap id="T4" position="float">
<label>TABLE 4</label>
<caption>
<p>Time of occurrence (in fs) of different species from the <italic>AINR</italic> simulation of 1: 3 ratio of helium to hydrogen while varying total positive charge of the system.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Total charge</th>
<th align="center">HeH<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>H<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">He<sub>3</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">0</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
<td align="center">-</td>
</tr>
<tr>
<td align="left">4</td>
<td align="char" char=".">5.0</td>
<td align="char" char=".">74.5</td>
<td align="char" char=".">14.0</td>
<td align="char" char=".">24.0</td>
<td align="char" char=".">12.5</td>
</tr>
<tr>
<td align="left">6</td>
<td align="char" char=".">9.5</td>
<td align="char" char=".">15.5</td>
<td align="char" char=".">12.0</td>
<td align="char" char=".">27.0</td>
<td align="char" char=".">17.0</td>
</tr>
<tr>
<td align="left">8</td>
<td align="char" char=".">6.0</td>
<td align="char" char=".">54.0</td>
<td align="char" char=".">15.0</td>
<td align="char" char=".">19.5</td>
<td align="char" char=".">21.5</td>
</tr>
<tr>
<td align="left">10</td>
<td align="char" char=".">10.0</td>
<td align="char" char=".">16.0</td>
<td align="char" char=".">14.5</td>
<td align="char" char=".">30.0</td>
<td align="char" char=".">19.0</td>
</tr>
<tr>
<td align="left">12</td>
<td align="char" char=".">6.5</td>
<td align="char" char=".">70.5</td>
<td align="char" char=".">16.0</td>
<td align="char" char=".">23.0</td>
<td align="char" char=".">23.0</td>
</tr>
</tbody>
</table>
</table-wrap>
<table-wrap id="T5" position="float">
<label>TABLE 5</label>
<caption>
<p>Time of occurrence (in fs) of different species from the <italic>AINR</italic> simulation of a 1: 3 ratio of helium to hydrogen while varying the temperature, with a fixed total positive charge of system (8<sup>&#x2b;</sup>).</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Temperature (K)</th>
<th align="center">HeH<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>H<sup>&#x2b;</sup>
</th>
<th align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">He<sub>3</sub>
<sup>2&#x2b;</sup>
</th>
<th align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">3,300</td>
<td align="char" char=".">3.0</td>
<td align="char" char=".">24.5</td>
<td align="char" char=".">10.0</td>
<td align="char" char=".">26.0</td>
<td align="char" char=".">5.5</td>
</tr>
<tr>
<td align="left">2,500</td>
<td align="char" char=".">8.5</td>
<td align="char" char=".">27.0</td>
<td align="char" char=".">14.0</td>
<td align="char" char=".">18.0</td>
<td align="char" char=".">63.0</td>
</tr>
<tr>
<td align="left">2,000</td>
<td align="char" char=".">7.0</td>
<td align="char" char=".">22.0</td>
<td align="char" char=".">16.0</td>
<td align="char" char=".">19.0</td>
<td align="char" char=".">25.0</td>
</tr>
<tr>
<td align="left">1,500</td>
<td align="char" char=".">7.0</td>
<td align="char" char=".">14.0</td>
<td align="char" char=".">11.5</td>
<td align="char" char=".">17.0</td>
<td align="char" char=".">60.0</td>
</tr>
<tr>
<td align="left">1,000</td>
<td align="char" char=".">6.0</td>
<td align="char" char=".">54.0</td>
<td align="char" char=".">15.0</td>
<td align="char" char=".">19.5</td>
<td align="char" char=".">21.5</td>
</tr>
<tr>
<td align="left">500</td>
<td align="char" char=".">8.0</td>
<td align="char" char=".">16.0</td>
<td align="char" char=".">15.0</td>
<td align="char" char=".">20.5</td>
<td align="char" char=".">47.0</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>We have also addressed the speculation on the exact nature of the formed ion He<sub>2</sub>H<sup>&#x2b;</sup>: whether it was formed as [He-H-He]<sup>&#x2b;</sup> or as [He-He-H]<sup>&#x2b;</sup> (<xref ref-type="bibr" rid="B22">Kim and Lee, 1999</xref>), <italic>via AINR</italic> dynamics followed by static CCSD calculations. As shown in <xref ref-type="fig" rid="F2">Figures 2A,B</xref>, two different routes leading to the formation of [He-H-He]<sup>&#x2b;</sup> and [He-He-H]<sup>&#x2b;</sup> were observed during the simulations, generated from the collision of HeH<sup>&#x2b;</sup> and He. The thermodynamics was evaluated and it was found that the formation of the [He-H-He]<sup>&#x2b;</sup> species was exergonic by 32.3&#xa0;kcal/mol, whereas the formation of [He-He-H]<sup>&#x2b;</sup> was only favourable by 4.5&#xa0;kcal/mol. In other words, our calculations indicate that He<sub>2</sub>H<sup>&#x2b;</sup> would have formed predominantly as [He-H-He]<sup>&#x2b;</sup> rather than [He-He-H]<sup>&#x2b;</sup>.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Snapshots of <italic>AINR</italic> simulations revealing the pathway toward the making of He<sub>2</sub>H<sup>&#x2b;</sup>, in the form of <bold>(A)</bold> [He-H-He]<sup>&#x2b;</sup> and <bold>(B)</bold> [He-He-H]<sup>&#x2b;</sup> (Color: He - peach, H - white).</p>
</caption>
<graphic xlink:href="fchem-09-679750-g002.tif"/>
</fig>
<p>In another set of simulations, we have taken a different ratio of helium to dihydrogen and simultaneously varied the total charge of the system. In these cases, due to the high charge density, the dihydrogen quickly dissociated into a proton and atomic hydrogen. Here too, we have observed similar trends: 1) HeH<sup>&#x2b;</sup> is the first molecule to be formed, followed by 2) the formation of other short lived species, leading to H<sub>3</sub>
<sup>&#x2b;</sup>, which remained at the end, along with one or two molecules of HeH<sup>&#x2b;</sup> (shown in <xref ref-type="table" rid="T5">Table&#x20;5</xref>). For the case of 20 He and five H<sub>2</sub> having a total of eight positive charge in the system, for instance, we observed that after a few collisions, there was still one HeH<sup>&#x2b;</sup> molecule present along with one H<sub>3</sub>
<sup>&#x2b;</sup> and that they were in equilibrium with each other, due to the instantaneous proton transfer between HeH<sup>&#x2b;</sup> and H<sub>2</sub>. Similar trends were observed for other simulations where the total positive charge of the system was high (in our simulation conditions, the values were &#x2265;16). It is worth mentioning that in this high positive charge atmosphere with comparatively low H atom density, the number of H<sub>3</sub>
<sup>&#x2b;</sup> that survived after the collisions was either one or two, depending upon the ratio of He to H<sub>2</sub> (shown in <xref ref-type="table" rid="T6">Table&#x20;6</xref>). Also, due to the very high positive charge density and high temperature (1,000.0&#xa0;K) the movement of the light H<sup>&#x2b;</sup> ions was seen to be extremely fast and they repelled each other, going far away. This reduced the propensity toward the formation of H<sub>3</sub>
<sup>&#x2b;</sup> in such simulations.</p>
<table-wrap id="T6" position="float">
<label>TABLE&#x20;6</label>
<caption>
<p>Different ratios of He to dihydrogen while varying the total positive charge of the system.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">No. of He</th>
<th align="center">No. of H<sub>2</sub>
</th>
<th align="center">Total charge</th>
<th align="center">First molecule</th>
<th align="center">Intermediate species</th>
<th align="center">Dominating end molecule</th>
<th align="center">No. of H<sub>3</sub>
<sup>&#x2b;</sup>
</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">20</td>
<td align="char" char=".">5</td>
<td align="char" char=".">8</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">HeH<sup>&#x2b;</sup>, H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">1</td>
</tr>
<tr>
<td align="left">30</td>
<td align="char" char=".">10</td>
<td align="char" char=".">16</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">HeH<sup>&#x2b;</sup>, H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">1</td>
</tr>
<tr>
<td align="left">30</td>
<td align="char" char=".">15</td>
<td align="char" char=".">24</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">HeH<sup>&#x2b;</sup>, H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">1</td>
</tr>
<tr>
<td align="left">30</td>
<td align="char" char=".">15</td>
<td align="char" char=".">26</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">HeH<sup>&#x2b;</sup>, H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">1</td>
</tr>
<tr>
<td align="left">15</td>
<td align="char" char=".">10</td>
<td align="char" char=".">8</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">4</td>
</tr>
<tr>
<td align="left">30</td>
<td align="char" char=".">15</td>
<td align="char" char=".">20</td>
<td align="center">HeH<sup>&#x2b;</sup>
</td>
<td align="center">He<sub>2</sub>
<sup>2&#x2b;</sup>, He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>2</sub>H<sup>&#x2b;</sup>, H<sup>&#x2b;</sup>
</td>
<td align="center">HeH<sup>&#x2b;</sup>, H<sub>3</sub>
<sup>&#x2b;</sup>
</td>
<td align="char" char=".">4</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
<sec id="s3-2">
<title>Formation of Unique Dicationic He Chains</title>
<p>Previously, there have been some reports (<xref ref-type="bibr" rid="B24">Marinetti et&#x20;al., 2008</xref>; <xref ref-type="bibr" rid="B36">Oleksy et&#x20;al., 2010</xref>) with regard to the formation of mono-cationic He ion clusters. Our current <italic>AINR</italic> based dynamics study reveals that there is a possibility of the formation of a dicationic helium chain of up to five He atoms: He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>4</sub>
<sup>2&#x2b;</sup> and He<sub>5</sub>
<sup>2&#x2b;</sup>. This interesting result was obtained when we took a homogeneous mixture of H and He (15 atoms each) in the AINR, with an overall positive charge of 20 for the system. After a certain amount of time had elapsed (1&#xa0;ps), we observed that a chain like structure had formed comprising of up to five helium atoms (shown in <xref ref-type="fig" rid="F3">Figure&#x20;3</xref>).</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Dicationic He chain formation during an <italic>AINR</italic> simulation of 15&#x20;H and 15 He atoms, with an overall positive charge of 20 (Color: He - peach, H - white).</p>
</caption>
<graphic xlink:href="fchem-09-679750-g003.tif"/>
</fig>
<p>An mp4 file (<xref ref-type="sec" rid="s9">Supplementary Movie S2</xref>) of a movie made of a part of such an <italic>AINR</italic> simulation is included in the ESI. We have taken snapshots during the dynamics and carried out natural population analysis (NPA), in order to calculate the charge on the He atoms in the formed linear chain. From the NPA charge analysis (shown in <xref ref-type="sec" rid="s9">Supplementay Table S6</xref> in the ESI), it has been confirmed that all the formed He chains (He<sub>3</sub>
<sup>2&#x2b;</sup>, He<sub>4</sub>
<sup>2&#x2b;</sup>, He<sub>5</sub>
<sup>2&#x2b;</sup>) were dicationic in nature. For further confirmation of the stability of these dicationic He chains, we have done thermodynamics calculations for the formation of the He chain starting from He<sub>2</sub>
<sup>2&#x2b;</sup> (shown in <xref ref-type="fig" rid="sch2">Scheme 2</xref>). The Gibbs free energy values suggest that the formation of the dicationic helium chain up to He<sub>5</sub>
<sup>2&#x2b;</sup> is favourable, but further formation of He<sub>6</sub>
<sup>2&#x2b;</sup> is thermodynamically not feasible. For this reason, we did not observe any He chain beyond five He atoms in our <italic>AINR</italic> simulations.</p>
<fig id="sch2">
<label>SCHEME 2</label>
<caption>
<p>Thermodynamics of helium chain formation (up to He<sub>5</sub>
<sup>2&#x2b;</sup>).</p>
</caption>
<graphic xlink:href="fchem-09-679750-g006.tif"/>
</fig>
</sec>
</sec>
<sec sec-type="conclusion" id="s4">
<title>Conclusion</title>
<p>In our current work, we have tried to shed light on the chemical reactions that might have taken place at the beginning of the Universe. We have focused on how, at the very beginning, simple molecules came into being after the Big Bang. We have investigated how He and H atoms, which were the first atoms formed, collided with each other in a positively charged atmosphere. This has been done by using a fresh computational approach&#x2013;by employing the <italic>ab initio</italic> nanoreactor (<italic>AINR</italic>). The simulations reveal the presence of unique dicationic helium chains of up to five atoms, which should act as a fillip for investigating the possibility of the presence of such species in helium clusters, which have received attention both from experimental and theoretical studies (<xref ref-type="bibr" rid="B3">Bieske and Dopfer, 2000</xref>; <xref ref-type="bibr" rid="B24">Marinetti et&#x20;al., 2008</xref>; <xref ref-type="bibr" rid="B36">Oleksy et&#x20;al., 2010</xref>). Our studies also confirm that HeH<sup>&#x2b;</sup> was indeed the first&#x20;molecule to be formed and that it played a vital role in the origin of H<sub>3</sub>
<sup>&#x2b;</sup>. The preservation of H<sub>3</sub>
<sup>&#x2b;</sup>, as a relatively stable species, in each of the simulations after every collision cycle, also explains the high abundance of H<sub>3</sub>
<sup>&#x2b;</sup> in the early Universe. As such, our work provides interesting computational insights into the origin of unique and interesting molecules at the dawn of the Universe.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/<xref ref-type="sec" rid="s9">Supplementary Material</xref>, further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>SD and TD have carried out the research work and written the manuscript, with the guidance and supervision of&#x20;KV.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>KV is grateful to the Department of Science and Technology (DST) (EMR/2014/000013) for providing financial assistance. TD thanks Council of Scientific and Industrial Research (CSIR) for providing Research Fellowship. SD thanks CSIR-NCL (MLP101026) for providing a Fellowship.</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<ack>
<p>The support and the resources provided by &#x201c;PARAM Brahma Facility&#x201d; under the National Supercomputing Mission, Government of India at the Indian Institute of Science Education and Research (IISER) Pune are gratefully acknowledged.</p>
</ack>
<sec id="s9">
<title>Supplementary Material</title>
<p>The Supplementary Material for this article can be found online at: <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/fchem.2021.679750/full#supplementary-material">https://www.frontiersin.org/articles/10.3389/fchem.2021.679750/full&#x23;supplementary-material</ext-link>
</p>
<supplementary-material xlink:href="Video2.MP4" id="SM1" mimetype="application/MP4" xmlns:xlink="http://www.w3.org/1999/xlink"/>
<supplementary-material xlink:href="Video1.MP4" id="SM2" mimetype="application/MP4" xmlns:xlink="http://www.w3.org/1999/xlink"/>
<supplementary-material xlink:href="DataSheet1.docx" id="SM3" mimetype="application/docx" xmlns:xlink="http://www.w3.org/1999/xlink"/>
</sec>
<sec>
<title>Abbreviations</title>
<p>AIMD, <italic>Ab initio</italic> molecular dynamics; AINR, <italic>Ab initio</italic> nano reactor; DFT, Density functional theory.</p>
</sec>
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