ORIGINAL RESEARCH published: 29 April 2021 doi: 10.3389/fspas.2021.655405 Interaction of HCO+ Cations With Interstellar Negative Grains. Quantum Chemical Investigation and Astrophysical Implications Albert Rimola 1*, Cecilia Ceccarelli 2 , Nadia Balucani 3 and Piero Ugliengo 4 1 Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, Spain, 2 Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Grenoble, France, 3 Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Perugia, Italy, 4 Dipartimento di Chimica and Nanostructured Interfaces and Surfaces (NIS) Centre, Università degli Studi di Torino, Torino, Italy In cold galactic molecular clouds, dust grains are coated by icy mantles and are prevalently charged negatively, because of the capture of the electrons in the gas. The interaction of the charged grains with gaseous cations is known to neutralize them. In this work, we focus on the chemical consequences of the neutralization process of Edited by: Majdi Hochlaf, HCO+ , often the most abundant cation in molecular clouds. More specifically, by means Université Paris Est Marne la of electronic structure calculations, we have characterized the energy and the structure Vallée, France of all possible product species once the HCO+ ion adsorbs on water clusters holding an Reviewed by: extra electron. Two processes are possible: (i) electron transfer from the negative water David Woon, University of Illinois at cluster to the HCO+ ion or (ii) a proton transfer from HCO+ to the negative water cluster. Urbana–Champaign, United States Energetic considerations favor electron transfer. Assuming this scenario, two limiting Alexis Markovits, Sorbonne Universités, France cases have been considered in astrochemical models: (a) all the neutralized HCO+ is *Correspondence: retained as neutral HCO adsorbed on the ice and (b) all the neutralized HCO+ gets Albert Rimola desorbed to the gas phase as HCO. None of the two limiting cases appreciably contribute albert.rimola@uab.cat to the HCO abundance on the grain surfaces or in the gas. Specialty section: Keywords: DFT, water ice, solvated electron, astrochemical modeling, interstellar medium This article was submitted to Astrochemistry, a section of the journal INTRODUCTION Frontiers in Astronomy and Space Sciences The identification of about 200 molecular species in the interstellar medium (ISM) through Received: 18 January 2021 their rotational (millimeter), rovibrational (infrared), and/or electronic (ultraviolet or visible) Accepted: 05 March 2021 transitions is the proof of the existence of a rich molecular universe in which a multitude of Published: 29 April 2021 chemical processes take place (Caselli and Ceccarelli, 2012; Tielens, 2013; Van Dishoeck, 2014). Citation: Understanding the chemical and physical bases that drive the formation of these molecules in Rimola A, Ceccarelli C, Balucani N the harsh conditions of the ISM is a current challenge to the Astrochemistry community. Many and Ugliengo P (2021) Interaction of of these species are formed in reaction chains taking place in the gas phase (for some recent HCO+ Cations With Interstellar Negative Grains. Quantum Chemical examples, see, e.g., Shannon et al., 2013; Vasyunin and Herbst, 2013; Balucani et al., 2015; Skouteris Investigation and Astrophysical et al., 2017, 2018, 2019; Vazart et al., 2020), but in some cases their production occurs in or Implications. on the icy mantle of interstellar dust grains. Interstellar grains, indeed, can act as a catalyzer Front. Astron. Space Sci. 8:655405. or simply as a concentrator of the reactants, by impeding them to fly apart after non-reactive doi: 10.3389/fspas.2021.655405 encounters. Several models have been developed to account for the chemistry on interstellar ice Frontiers in Astronomy and Space Sciences | www.frontiersin.org 1 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains (e.g., Garrod and Herbst, 2006; Taquet et al., 2012; Agúndez 2) formation of CO adsorbed on the water ice grain surfaces and Wakelam, 2013; Rimola et al., 2014; Penteado et al., 2017; through proton transfer from HCO+ to W − : Loison et al., 2019; Grassi et al., 2020). The role of interstellar grains is especially important for the synthesis of those molecules W− + HCO+ (gas) → HW···CO (2) that cannot be formed in gas-phase reactions, such as the most important interstellar molecule H2 (Vidali, 2013; Wakelam et al., where HW···CO represents CO interacting with the water ice 2017). with an extra, covalently bound H atom; Remarkably, ice-assisted chemical synthesis can be activated 3) formation of CO and H, both adsorbed on the water by UV and high-energy radiation (e.g., X-rays and energetic ice grain surfaces, due to dissociative electron transfer from particles like cosmic rays). The interaction of energetic particles W− to HCO+ : with matter generates many non-thermal secondary low- energy (<20 eV) electrons (LEE), which may themselves induce W− + HCO+ (gas) → H···W···CO (3) chemical reactions (Arumainayagam et al., 2010; Alizadeh and Sanche, 2012). In an astrochemical context, LEE may result where H···W···CO represents CO and H simultaneously from the interaction of cosmic rays with gaseous H2 in dense adsorbed on the water ice. clouds (Oka, 2013) and inelastic collisions of cosmic rays when Finally, we have tried to address the possible astrophysical traversing through the ices (Pimblott and Laverne, 2007). The implications of the present quantum calculations. We remind number density of free electrons in molecular clouds is estimated the readers that HCO is an important intermediate species in to be ca. ne = 10−4 cm−3 (Evans, 1993; Rae et al., 2004; Mason the formation of H2 CO and, subsequently, CH3 OH by CO and Field, 2019). As previously argued, the probability that hydrogenation on ice. In addition, gaseous formyl radical has interstellar dust grains can capture free electrons is high. It has been widely detected in interstellar objects and its formation been long recognized, indeed, that negatively charged grains routes are poorly constrained (see below). New observational are present in the interstellar medium (Draine and Sutin, 1987) results on the gaseous HCO radical have recently been and that their recombination with positive ions dominates their reported, including an analysis of its abundance to elucidate neutralization (Walmsley et al., 2004; Ceccarelli and Dominik, its origin (Bacmann and Faure, 2016). Since the electron–ion 2005). Primary electron attachment can also induce chemical recombination liberates a large amount energy (as large as the transformations because of the production of reactive species ionization potential of the HCO radical, namely, 8.12 ± 0.04 eV), (Boamah et al., 2014; Mason et al., 2014). HCO or its moieties H and CO, once formed on ice, can According to the work by Hatano et al. (2011), LEE are chemically desorb into the gas phase. We will therefore tackle the thermalized within approximately 1 ps via inelastic collisions. possible effects of processes (1)–(3) either in the chemistry of ice When the ice is predominantly formed by water molecules, or in the chemistry of the gas phase. the captured electrons can be treated as metastable solvated electrons. The plausibility of this scenario is demonstrated by METHODS experiments where molecules embedded in water ice irradiated by electrons at 100–400 eV yield the desorption of the molecular Quantum Chemical Calculations species followed by the formation of solvated electrons (Thrower As mentioned in the Introduction, the negatively charged icy et al., 2011). grains have been modeled in this work as water clusters of In the present work, we aim to characterize the role different sizes containing one extra electron. In other words, of fully equilibrated solvated electrons in the chemistry of we consider that the interaction of the additional electron interstellar icy grains by means of quantum chemical calculations occurs only with the water molecules of the icy mantles of complemented by astrochemical modeling. To this end, we have grains. The electronic nature of the solvated electron has been described, as a test case, the interaction of a negatively charged icy extensively studied in physical and quantum chemistry (Turi and grain with the HCO+ ion, the most abundant molecular ion in Rossky, 2012; Young and Neumark, 2012). Quantum chemical molecular clouds (average abundance with respect to H2 of about calculations based on large water clusters converge to the same 10−8 ). Specifically, we have modeled the negative icy grain with a description, that is, the extra electron can be either localized cluster of water molecules containing an extra electron (so-called within the cavity of the water clusters, on the surface of the solvated electron) and its possible interactions with the ion. We clusters in a localized fashion, or delocalized along the cluster have also scrutinized the possible outcomes of such interactions, surface. The preference for those forms depends on the structure which are the following: and size of the water cluster (Kim et al., 1996, 1997; Lee et al., 1) formation of the neutral HCO radical adsorbed on the water 1997; Khan, 2003, 2004, 2005, 2006; Herbert and Head-Gordon, ice grain surfaces through an electron transfer from W− (i.e., the 2005, 2006). To contemplate all the possible arrangements, in water cluster with the extra electron) to HCO+ : this work we have considered three well-described water clusters with one solvated electron (see Figure 1): (i) a 14 H2 O-molecule W− + HCO+ (gas) → W···HCO (1) cluster (Khan, 2006), in which the extra electron is found inside the cavity or localized on the surface cluster (W− − 14 -cav and W14 - where W···HCO represents the radical HCO interacting with loc, respectively); (ii) a 20 H2 O-molecule cluster (Khan, 2003, water ice W; 2005; Herbert and Head-Gordon, 2005, 2006), in which the extra Frontiers in Astronomy and Space Sciences | www.frontiersin.org 2 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains FIGURE 1 | BHYLP/6-31(1+,3+)G(d,p)-optimized geometries of the water clusters with 14 (A), 20 (B), and 24 (C) H2 O molecules with the extra electron (W− 14 , W20 , − and W−24 , respectively) inside the cavity (cav), localized at the surface of the water clusters (loc) and delocalized at the surface of the clusters (deloc). The singly occupied molecular orbitals (SOMO), which represent the localization of the extra electron, are also shown. electron is localized or delocalized on the cluster surfaces (W− 20 - is inside the cavity (xy=cav), is localized on the cluster surface loc and W− 20 -deloc, respectively); and (iii) a 24 H 2 O-molecule (xy=loc), or is delocalized on the cluster surface (xy=deloc). cluster (Khan, 2004; Herbert and Head-Gordon, 2005, 2006), Concerning the calculations including H/C/O, the initial which can present the extra electron within the cavity, localized guess structures were manually built: (i) for the W···HCO on the surface, or delocalized on the surface (W− − 24 -cav, W24 -loc, complexes, the initial guess features HCO+ interacting through and W− 24 -deloc, respectively). For the sake of clarity, along this its O atom end with one H atom of the H2 O cluster ices, (ii) work we will use the “W− N -xy” notation for the water cluster for the HW···CO complexes, the CO molecule is interacting systems, in which W− will refer to water clusters with the extra with one H2 O molecule of the cluster ices, while the H atom electron, N refers to the number of water molecules composing is incorporated into another water molecule of the cluster, the water cluster, and the xy term indicates if the extra electron thus forming an H3 O group, and (iii) for the H···W···CO Frontiers in Astronomy and Space Sciences | www.frontiersin.org 3 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains FIGURE 2 | BHYLP/6-31(1+,3+)G(d,p)-optimized structures for the interaction of HCO+ with the 14-water-molecule clusters with an extra electron inside the cavity (A) and localized at the surface (B). Structures on the left correspond to the electron transfer from W− 14 to HCO+ ; structures in the middle correspond to the H transfer from HCO+ to W− 14 (the transferred H is labeled by an asterisk); and structures on the right correspond to the dissociation of HCO resulting with the adsorption of CO and H on the water ice clusters. Bare values are the calculated reaction energies taking as the zero energy reference the W− 14 + HCO+ asymptote; values in brackets are the relative energies of the different complexes belonging to the same system. These energetics values are calculated at the MP2/6-311(2+,3+)G(d,p)//BHYLP/6-31(1+,3+)G(d,p) theory level. Energy units are in eV, and bond distances are in Å. complexes both CO and H are interacting with different water to calculate the reaction energies of processes (1), (2), and (3). molecules of the cluster ices. Following the work by Herbert It is worth mentioning that more sophisticated treatments on and Head-Gordon (Herbert and Head-Gordon, 2005, 2006), electron capture phenomena are possible, including quantum geometry optimization has been carried out by using the BHLYP effects (Flower and Middleton, 2004). However, our systems are hybrid density functional method with a 6-31(1+,3+)G(d,p) relatively large to be simulated by this methodology (with a basis set, which is the 6-31++G(d,p) standard one with significant increase of the computational cost) and, since the aim two more sets of diffuse s functions on each H atom (with of the work is mainly on the energetics of the studied processes, exponents 1.0843373500 × 10−2 and 3.2660763500 × 10−3 , we consider the used level of computation to be accurate enough. and coefficients 1.00). Single-point energy calculations on the optimized geometries at the MP2 post-Hartree–Fock method Astrochemical Modeling employing a 6-311(2+,3+)G(d,p) basis set [namely, the 6- To assess the implications of our new computations on the 311++G(d,p) standard one including an additional set of diffuse chemical composition of the molecular cloud gas, we used sp functions on each O and C atom (for O, with exponent the GRAINOBLE code (Taquet et al., 2012, 2013). Briefly, 2.5451807230 × 10−2 and coefficient 1.00; for C, with exponent GRAINOBLE is a gas-grain time-dependent astrochemical 1.3192771080 × 10−2 and coefficient 1.00) and the two additional model, which takes into account the grain-surface hydrogenation sets of diffuse s functions on each H atom] have been performed and oxidation plus the multilayered structure of the grain Frontiers in Astronomy and Space Sciences | www.frontiersin.org 4 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains FIGURE 3 | BHYLP/6-31(1+,3+)G(d,p)-optimized structures for the interaction of HCO+ with the 20-water-molecule clusters with an extra electron localized at the surface (A) and delocalized at the surface (B). Structures on the left correspond to the electron transfer from W− 20 to HCO+ ; structures in the middle correspond to the H transfer from HCO+ to W− 20 (the transferred H is labeled by an asterisk); and structures on the right correspond to the dissociation of HCO resulting in the adsorption of CO and H on the water ice clusters. Bare values are the calculated reaction energies taking as the zero energy reference the W− 20 + HCO+ asymptote; values in brackets are the relative energies of the different complexes belonging to the same system. These energetics values are calculated at the MP2/6-311(2+,3+)G(d,p)//BHYLP/6-31(1+,3+)G(d,p) theory level. Energy units are in eV, and bond distances are in Å. mantles. We assumed the standard elemental abundances (see, Analysis of the spin density confirms that the unpaired electron e.g., Agúndez and Wakelam, 2013), a cloud with a density is fully localized on the C atom, indicating that the extra of 104 cm−3 , a temperature of 10 K, and a cosmic ray electron is transferred to form the HCO radical (process 1). ionization rate of 3 × 10−17 s−1 and run the code up to Moreover, in all the analyzed cases, the newly formed HCO group 108 year. interacts with the water ice clusters via hydrogen bond (H-bond) interactions with both the H and O ends acting as donor and RESULTS AND DISCUSSION acceptor, respectively. For the HW···CO complexes resulting after the proton Figures 2–4 show the optimized geometries for the relevant transfer process (2), in all the considered cases, the incorporated species in the (1)–(3) processes. In the same Figures, the H atom remains accommodated in the ice as a H3 O radical reaction energies (bare values) and the relative energies (values species (labeled by an asterisk in the Figures), indicating that it in brackets) are also shown. Different optimized structures were is stable under these conditions. obtained, but only the most stable ones are presented here for the Finally, in the case of the dissociative electron–ion sake of clarity and brevity. recombination (3), the resulting H···W···CO complexes For all the W···HCO cases, the initial HCO+ species (a linear feature the CO moiety interacting with the water ice clusters molecule) becomes bent during the geometry optimization. similarly to the HW···CO systems, but the coproduct H atom is Frontiers in Astronomy and Space Sciences | www.frontiersin.org 5 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains FIGURE 4 | BHYLP/6-31(1+,3+)G(d,p)-optimized structures for the interaction of HCO+ with the 24-water-molecule clusters with an extra electron inside the cavity (A), localized at the surface (B), and delocalized at the surface (C). Structures on the left correspond to the electron transfer from W− 24 to HCO+ ; structures in the middle correspond to the H transfer from HCO+ to W− 24 (the transferred H is labeled by an asterisk); and structures on the right correspond to the dissociation of HCO resulting in the adsorption of CO and H on the water ice clusters. Bare values are the calculated reaction energies taking as the zero energy reference the W− 24 + HCO+ asymptote; values in brackets are the relative energies of the different complexes belonging to the same system. These energetics values are calculated at the MP2/6-311(2+,3+)G(d,p)//BHYLP/6-31(1+,3+)G(d,p) theory level. Energy units are in eV, and bond distances are in Å. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 6 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains not covalently bound to a water molecule but simply physisorbed TABLE 1 | Reaction energies (1E, in eV) calculated at the on water ice clusters. MP2/6-311(2+,3+)G(d,p)//BHYLP/6-31(1+,3+)G(d,p) theory level corresponding to (i) the electron affinity of W24 -cav and HCO+ , (ii) the proton affinity of W− 24 -cav By comparing the energy content of the possible final and CO, (iii) the electron transfer from W− 24 -cav to the HCO cations, and (iv) the + complexes, it is clear that the formation of W···HCO is more H+ transfer from the HCO+ cation to W− 24 -cav. favorable than the formation of HW···CO and H···W···CO by ca. 1.15–1.78 eV and 0.15–0.80 eV, respectively. Even though the Process Reaction 1E system will have enough energy to overcome those differences Electron affinity 24 -cav W24 -cav + e → W− −0.52 because of the large amount of energy liberated in all the three HCO+ + e → HCO −7.67 families of processes (1)–(3), these results imply that HCO+ Proton affinity 24 -cav + H → HW24 -cav W− + −13.01 neutralization on a negative grain is a possible route of HCO CO + H+ → HCO+ −6.51 formation directly on the ice and without the need to surmount a potential energy barrier. This is at variance with HCO formation Electron transfer 24 -cav + HCO → W24 -cav + HCO W− + −7.15 via H addition to CO for which a sizable barrier is present (Woon, Proton transfer 24 -cav + HCO → HW24 -cav + CO W− + −6.50 2002; Marenich and Boggs, 2003; Goumans et al., 2007, 2008; Andersson et al., 2011; Peters et al., 2013; Rimola et al., 2014). In other words, the interaction of HCO+ with a negatively charged transfer is −7.15 eV whereas for the proton transfer is −6.50 eV). grain readily leads to the formation of HCO directly adsorbed on In addition to that, from a dynamical point of view, electron the ice. transfer is naturally favored by the much smaller mass of the Interestingly, all the three situations are below the “W− electron compared to the proton. We can also expect that the + HCO+ (gas)” asymptote and accordingly the three products interaction between HCO+ and the additional electron of the arising from process (1)–(3) can be populated. Nonetheless, water clusters will start at a much longer distance, as the impact the energy liberated in the formation of W···HCO by (1) is parameters and cross sections are usually much larger in the case very large (between −7.0 and −8.0 eV) and it is reasonable to of electron transfer rather than proton transfer (Prigogine and consider that part of this large nascent energy can be used to Rice, 1973). In other words, both energetics and the dynamical break the H–C bond of the HCO radical yielding the formation considerations favor processes (1) + (3) as dominant over of the H···W···CO complexes, which is process (3). We have process (2). calculated the energy barriers for the H–C fission in the W24 - It is worth mentioning that these reaction energies are less cav···HCO, W24 -loc···HCO, and W24 -deloc···HCO cases. Their negative by some amount than the calculated reaction energies values (1.75, 1.35, and 2.01 eV, respectively) are still largely below for the formation of W24 -cav···HCO and HW24 -cav···CO from the asymptote, thus supporting this idea. On the other hand, the interaction of HCO+ with W− 24 -cav (−8.38 and −6.80 eV, the energetics involved in the formation of W···HCO, process respectively), because the W24 -cav···HCO and HW24 -cav···CO (1), is dictated by the difference in the electron affinity of structures account for the stabilizing H-bond interaction between HCO+ and of neutral water ice, while in HW···CO, process (2) the two partners. is dictated by the proton affinity of CO and W-, respectively. The value of the electron affinity for HCO+ and the W24 - ASTROPHYSICAL IMPLICATIONS cav water ice cluster (here W24 -cav is used here as a test case of the water ice) is −7.67 and −0.52 eV, respectively, in our The present calculations only offer a qualitative picture of the calculations (see reactions entitled “Electron affinity” of Table 1), title process, as they do not provide reaction rate coefficients while the calculated proton affinity of CO and W− 24 -cav is, to be introduced in astrochemical models. The generation of instead, −6.51 and −13.01 eV, respectively (see reactions entitled those rate coefficients would require a rigorous treatment of the “Proton affinity” of Table 1). The larger electron affinity of HCO+ energy dissipation between all the degrees of freedom of the compared to W24 -cav indicates that the solvated electron prefers cluster molecules (or better, of the icy mantle of interstellar grain) to be transferred to HCO+ to form HCO, whereas the larger once the energy associated with the electron–ion recombination proton affinity of W− 24 -cav compared to CO indicates that proton (or proton transfer) is liberated. Such a treatment is a daunting transfer to W− 24 -cav to form HW24 -cav is also energetically task adopting state-of-the-art theoretical treatments, even though favorable. Thus, it seems there is a competition between processes the first studies are appearing in the literature (Fredon et al., (1) + (3) and (2), namely, between electron–ion recombination 2017; Fredon and Cuppen, 2018; Pantaleone et al., 2020). This (dictated by electron transfer) and proton transfer. Additionally, is probably one of the main challenges in the characterization for process (2), an energy barrier associated with the H transfer of chemistry occurring on interstellar ice. However, based on the from HCO+ to form the H3 O radical is expected, since the two cited works, a large fraction (≥70%) of the reaction energy is formed H3 O radical is a metastable species in the clusters. Such a absorbed very quickly, in <1 ps, by the water molecules of the ice. stability of the solvated H3 O radical has already been reported by Relevant to this study, Pantaleone et al. (2020) studied the fate of Chulkov et al. (2009). the energy released by the H + CO reaction on the icy surface and By considering the involved energy, the competition between concluded that the 137 kJ mol−1 released by the reaction is largely these processes seems to be favored by the electron–ion absorbed by the icy molecules and that the newly formed HCO recombination (the calculated reaction energies for these only possesses <20 kJ mol−1 , a too low energy for it to break the processes are shown in Table 1 indicating that for the electron bonds with the ice and be liberated into the gas. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 7 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains However, missing specific simulations for the case studied the recombination on the grain surfaces could have a non- here, namely, HCO+ + e− , we consider the two possible negligible role. The results of our GRAINOBLE simulations scenarios. If most of reaction energy is quickly redistributed confirm that, even assuming that all the HCO+ recombined on along all the degrees of freedom of the ice-connected molecules, the grain surfaces is released as HCO in the gas, this contribution then HCO can remain adsorbed on the ice as such; in this same is at most 30% with respect to that derived using the KIDA scenario, we might have the breaking up of the H–C bond, reaction rate coefficients. We, therefore, conclude that, in the but with both moieties still adsorbed on the ice. Alternatively, specific case of HCO+ the neutralization when they meet the the large amount of energy liberated by the electron–ion charged grains does not substantially affect the abundance of recombination will induce the chemical desorption of the whole gaseous HCO. HCO or even of its two moieties H and CO. Finally, we would like to mention that this work exclusively Trying to assess the possible effects of the electron–ion focused on an all-water icy grain model, with the aim to recombination process on interstellar ice without knowing the be a first step toward studying the influence of a captured relative importance of the different scenarios, we will consider electron by an interstellar grain on its chemical properties. two limiting cases: case (a) HCO remains in the grain as such, Thus, accounting for dirty ices (i.e., including other volatile thus becoming available to further hydrogenation up to H2 CO species such as CO, CO2 , NH3 , and CH3 OH) and ice-processing and, eventually, CH3 OH; case (b) HCO survives as such, but it is effects (e.g., cosmic rays’ impact or/and UV photon adsorption) immediately released in the gas-phase by chemisorption. on the attached electron fate is beyond the scope of the work. However, we find interesting to mention a plausible scenario not considered in the (1)–(3) processes. It envisages a Case (a) possible transformation of the negatively charged ice grain, in By employing the GRAINOBLE code (Taquet et al., 2012, 2013), which OH radicals are also present in the grain, as generated, the present authors have demonstrated that, when considering for instance, by cosmic ray impacts. In this case, the OH the energy barriers determined by quantum calculations of radical can scavenge the attached electron to give OH− , as the same kind as those presented here to characterize CO OH− would be more stable than the solvated electron. In hydrogenation (Rimola et al., 2014), the amount of H2 CO this situation, the adsorption of HCO+ does not cause the that can easily be accounted for (in spite of the barriers and electron transfer toward HCO+ , as the OH− stabilized by the thanks to the tunnel effect) is n(H2 CO)/n(H) = 10−6 -10−5 . surrounding water environment will attach to HCO+ to give Now, in the most favorable case, n(HCO+ )/n(H) can be as the neutral HCOOH. In the absence of direct neutralization high as the HCO+ abundance, namely, about 10−8 (see section between HCO+ and OH− , HCO+ will interact with a neutral Introduction). Therefore, even in the limiting case that all HCO+ H2 O molecule, as investigated by Woon (2011), bringing to is systematically converted into HCO adsorbed on water ice, this the barrierless formation of HCOOH and the transfer of process can only account up to about 1% of the total H2 CO the proton, formerly belonging to the reacted water, toward produced on ice. In other words, the title process has a negligible OH− via a proton-reliance mechanism within the grain water effect on the amount of H2 CO produced on interstellar grains. molecules. This scenario is obviously completely different to our (1)–(3) processes. Nevertheless, this alternative route has Case (b) important consequences in the interstellar grain chemistry The formyl HCO radical is one of the first molecules detected because it opens up the possibility of ion–molecule chemistry in space (Snyder et al., 1976) and a rather ubiquitous molecule on grains, traditionally attributed to exclusively occur in the found relatively easily in molecular clouds (e.g., Snyder et al., gas phase. 1985; Schenewerk et al., 1986, 1988; Caux et al., 2011; Frau et al., 2012; Bacmann and Faure, 2016; Spezzano et al., 2017; Rivilla et al., 2019). Its abundance in cold objects is about 10−11 (with CONCLUSIONS respect to H2 ; e.g., Bacmann and Faure, 2016; Rivilla et al., 2019), where the gaseous HCO+ abundance is lower than about 10−9 . The present work provides reliable evidence based on quantum The main formation routes of gaseous HCO are reported mechanical calculations that thermalized low-energy electrons in the astrochemical databases KIDA (http://kida.astrophy. can be easily transferred to cationic molecular species upon u-bordeaux.fr/) and UMIST (http://udfa.ajmarkwick.net/): interaction to form the corresponding radical neutral ones. As neutral–neutral reactions, dominated by H2 CO + OH (Ocaña a test case, this has been investigated for the interaction of the et al., 2017), and ionic routes, dominated by the recombination HCO+ ion with water clusters containing an extra electron. of protonated formaldehyde H2 COH+ + e− (Hamberg et al., Among the possible situations considered, results indicate that 2007). When comparing the theoretical predictions of the spontaneous formation of the HCO radical by electron transfer HCO abundance based on these gas-phase reactions with those is the most favorable one from an energetic point of view. measured in cold objects, both Bacmann and Faure (2016) Accordingly, interstellar grain particles can act as reservoir of and Rivilla et al. (2019) concluded that the ionic route of the electrons, which in turn can trigger electron transfer processes, formation of HCO can account for the measured values. which can be of relevance in the inventory of the interstellar Yet, given the uncertainty in the H2 COH+ + e− reaction molecules and in the molecular evolution of the chemical branching ratio, we verified whether the injection of HCO from diversity of the Universe. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 8 April 2021 | Volume 8 | Article 655405 Rimola et al. HCO+ With Interstellar Negative Grains DATA AVAILABILITY STATEMENT FUNDING The original contributions presented in the study are included This project has received funding from the European Research in the article/supplementary material, further inquiries can be Council (ERC) under the European Union’s Horizon 2020 directed to the corresponding author. research and innovation program (Grant Agreement No. 865657) for the project Quantum Chemistry on Interstellar Grains AUTHOR CONTRIBUTIONS (QUANTUMGRAIN). The authors acknowledge funding from the European Union’s Horizon 2020 research and innovation All authors participated in the initial conception of the program, the European Research Council (ERC) Project the research idea and in the discussion of the results. AR Dawn of Organic Chemistry, Grant Agreement No. 741002, did the quantum chemical calculations, wrote the initial and the Marie Skłodowska-Curie project Astro-Chemical Origins draft of the manuscript, and participated in the editing of (ACO), Grant Agreement No. 811312. MINECO (project the final manuscript. CC did the astrochemical modeling CTQ2017-89132-P) and DIUE (project 2017SGR1323). The and participated in the writing and editing of the final Italian Space Agency which cofunded the Life in Space Project manuscript. NB revised the initial draft and participated (ASI N. 2019-3-U.O) is acknowledged for financial support. in the editing of the final manuscript. PU participated in the writing and editing of the final manuscript. All ACKNOWLEDGMENTS authors contributed to the article and approved the submitted version. AR is indebted to the Ramón y Cajal program. REFERENCES Fredon, A., and Cuppen, H. M. (2018). 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Skouteris, D., Balucani, N., Ceccarelli, C., Faginas lago, N., Codella, C., Falcinelli, No use, distribution or reproduction is permitted which does not comply with these S., et al. (2019). Interstellar dimethyl ether gas-phase formation: a quantum terms. Frontiers in Astronomy and Space Sciences | www.frontiersin.org 10 April 2021 | Volume 8 | Article 655405