Weak Interactions in Interstellar Chemistry: How Do Open Shell

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Weak Interactions in Interstellar Chemistry: How do Open Shell Molecules Interact with Closed Shell Molecules? Karuppasamy Gopalsamy, Sorakayala Thripati, and Raghunath Ozhapakkam Ramabhadran ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00208 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Earth and Space Chemistry

Weak Interactions in Interstellar Chemistry: How do Open Shell Molecules Interact with Closed Shell Molecules? K. Gopalsamy, Sorakayala Thripati and Raghunath O. Ramabhadran* Department of Chemistry, Indian Institute of Science Education and Research Tirupati Andhra Pradesh – 517507, India Abstract The interstellar medium (ISM) is home to several open shell and closed shell molecules considered to be unusual or highly unstable on earth. The chemistry of these molecules in the ISM has been widely studied over many decades. However, the concept of weak chemical interactions, which is terrestrially well studied, has not been highlighted much in interstellar chemistry. In this study, we illustrate the wide variety of possible weak interactions in the ISM occurring between a carefully chosen set of open shell (OH, SH, CN, NO, NH2, and HO2) and closed shell molecules (H2O, H2S, HF, HCl, NH3, PH3, HCN, HNC, HCP, CH3OH, and CH3SH) which are important in interstellar chemistry. We expound upon the structural, and energetic features of the weak interactions by employing electronic structure calculations (CCSD(T) and density functional theory). The nature of the weak interactions is further probed by three different techniques, viz. the Atoms-in-Molecules (AIM) method, transfer of spin densities, and the Natural Bond Orbital (NBO) method. The astrochemical implications of the weak interactions are subsequently discussed, and it is suggested that the weak interactions could impact the molecular abundances in the ISM. Key Words – astrochemistry, origin of life, free radicals, hydrogen bonding, halogen bonding, chalcogen bonding, pnicogen bonding, carbon bonding

*To whom correspondence should be addressed. E-mail: [email protected] 1 ACS Paragon Plus Environment

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Introduction The chemistry happening in the interstellar medium (ISM) is widely believed to answer some questions related to the origin of life.[1-6] The dominant inhabitants of the ISM are what are conventionally defined on earth as "small molecules".[7] Molecules so small that, in astrochemistry parlance, even those containing only about 5 or 6 atoms (and containing carbon) are called "complex organic molecules".[7] Vis-a-vis terrestrial chemistry, what the ISM lacks in terms of the size of the molecules is more than compensated by the wide prevalence of both open shell and closed shell molecules in the ISM. Thus, some very rich open shell chemistry often considered to be exotic in terrestrial chemistry is a regular feature in the ISM routinely studied using mass spectrometric, spectroscopic and computational techniques. [8-20] A particularly underappreciated aspect in interstellar chemistry is the concept of weak chemical interactions in the ISM.[21] Weak interactions in chemistry comprises of hydrogen bonding, halogen bonding, chalcogen bonding, pnicogen bonding, and more recently, even carbon (or more generally tetrel) bonding etc.[22-27] They are currently subjects of immense interest in connection to the chemistry occurring on earth, especially for closed shell molecules.[28-32] For weak interactions involving open shell systems, even on earth, there are markedly fewer references.[33] Taken together (both in the ISM and on earth), there is thus tremendous scope for new knowledge on this front. Interstellar reactions occur only in the gas phase or as surface reactions.[34] Weak interactions in the ISM are expected to manifest themselves under both these conditions. In the case of interstellar surface reactions, given that the icy dust grain is made up of about ~ 70% water,[35] the import of weak interactions (mainly hydrogen bonding) is obvious, and accordingly work on hydrogen bonded solids in space has been carried out previously.[36]

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For any chemical reaction to occur, the reacting species should come together in a specific orientation. In the solution phase, besides any weak interaction, the solvent also guides the reactants towards this orientation. But, with gas phase reactions, there is no solvent involved. So, any prevalent weak interaction between the gas phase reactants plays an overriding role in bringing two molecules together in the desired orientation for a subsequent chemical reaction to occur. This is all the more true in the ISM, which possesses a very low number density (relative to earth’s atmosphere).[37,38] Hence weak chemical interactions are important in interstellar gas phase chemistry. Some studies featuring potential energy surfaces of some interstellar/astrochemical gas phase chemical reactions usually report a pre-reactant complex, which facilitates further reaction. Such studies have especially been performed with the OH radical,[39-55] since it is relevant not just in astrochemistry, but also in combustion chemistry. The pre-reactant complex in the case of OH radicals is held together by weak chemical interactions – mainly hydrogen bonding. Thus, the significance of weak interactions in interstellar gas phase reactions has been implicitly recognized previously in the scientific literature in scattered contexts. However, to the best of our knowledge, there are not many focused studies explicitly highlighting all the possible weak interactions in the ISM and their implications.[21, 56] In this scenario, some very interesting questions to ponder over are: What are the different modes of weak interactions possible with molecules in the ISM (hydrogen bonding vs other modes of interactions)? How strong/weak are they? What about open shell molecules interacting with other closed shell molecules? What is the nature of these interactions? Are they electrostatically dominated or is there any significant orbital overlap? What are the astrochemical implications of these weak interactions? Can interstellar weak interactions lead to general ideas on weak interactions in open shell molecules?



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Complementing experimental techniques, quantum chemical calculations have been at the forefront of modelling the chemical reactions occurring in the interstellar medium.[14,15] Precious information related to the geometries of the reactants and products, reaction rates, spectroscopic parameters, and interaction energies can be obtained using theory.14 They are therefore very useful to elucidate weak interactions in the ISM. In this work, we perform quantum chemical computations on a carefully chosen a test-set of open shell molecules (OH,[57] SH,[58,59] CN,[60] NO,[61] NH2,[62,63] HO2[64]) interacting with closed shell molecules (H2O,[65-70] H2S,[71-77] HF,[78-83] HCl,[84-89] NH3,[90-99] PH3,[100-102] HCN,[103-118] HNC,[119-130] HCP,[131] CH3OH,[132] and CH3SH[133-136]), all of which are present in the ISM, and play pivotal roles in interstellar chemistry. These molecules have been particularly chosen for the following reasons: (a) many of these molecules play a critical role in the formation of biomonomers, (b) the molecules present the possibility of studying all the different modes of interactions (eg. Hydrogen bonding with a protic hydrogen vs interaction from the open shell end vs halogen/chalcogen/pnicogen/bonding) (c) with some molecules, comparison can be made with previous studies. The rest of the paper is organized as follows: We first outline the choice of electronic structure calculations used, then expound upon the results obtained for the interaction of the open shell species with the closed shell species, elaborating upon the structural, and energetic aspects as well as the chemical insights obtained. The astrochemical relevance of this study is subsequently described, followed by the conclusions. Electronic Structure Calculations Open shell molecules require a more careful computational handling than closed shell molecules due to the possibility of several low lying excited states.[137,138] We have very carefully chosen the methods employed in this study. Exhaustive details of why the methods


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were chosen, and related details are presented in the supporting information. For the optimizations of the complexes (all the structures presented in this paper are minimum energy structures) we use the UM06-2x (unrestricted version of Minnesota 06 with 54% exact exchange) density functional in conjunction with the aug-cc-pVTZ (augmented correlation consistent polarized valance triple zeta) basis-set.[139-146] The geometries and energies of the ground electronic states (all doublets) of the open shell molecules used in this study (OH - 2Π , SH - 2Π, CN - 2Σ+, NO - 2Π, NH2 – 2B1, HO2 – 2A”) have been compared with those reported in the Computational Chemistry Comparison and Benchmark DataBase (CCCBDB) and found to be very similar to them.[147] To more accurately get the energies, we use the unrestricted version of the coupled cluster method including single and double substitutions with the perturbative inclusion of triple substitutions with the frozen core approximation, i.e, UCCSD(T)(FC)/aug-cc-pVTZ level of theory, based on the UM06-2x/aug-cc-pVTZ geometries.[148-151] Basis-Set superposition errors were alleviated by the use of counterpoise corrections at the UCCSD(T)(FC)/aug-cc-pVTZ level of theory.[152] All the calculations are performed using the Gaussian 09 suite of programs.[153] To validate our methodology for getting the geometries, we have performed additional geometry optimizations and single-point energy calculations on representative systems at various levels of theories, including different density functionals, and using other basis-sets to to get energies at the infinite basis-set limit.[154-160] Overall, we find the geometries and energies to majorly be in close agreement with the level of theory used in the main-text (interaction energies within ~ 1-2 kcal/mol mostly). More details of our careful calibration are provided in the supporting information. The interaction energy (Eint) computed in this work is defined as follows:



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Consider any gas phase open shell molecule “X” and gas phase closed shell molecule “Y” forming a weakly bound complex “XY” in the ISM, i.e, X (g) + Y (g) → XY(g), Eint = E(XY) – E(X) – E(Y). Where E(XY) is the zero point corrected energy of the complex, E(X) that for the open shell molecule, and E(Y) that for the closed shell molecule. The energies come from the CCSD(T) single point calculations, and the zero-point corrections from M06-2x geometries (aug-ccpVTZ basis-set used throughout). The overall interaction energies did not change by much (~ 0.3-0.5 kcal/mol) regardless of whether we used a harmonic potential for the vibrational energy or included anharmonic effects via the literature recommended scale factor of 0.971 for the UM06-2x/aug-cc-pVTZ level of theory. To ascertain the nature of the weak interactions, we adopt three different strategies, and look for corroborations/contradictions amongst them. They are the Bader’s atoms-in-molecules (AIM) method, Mulliken spin density analysis, and the Natural Bonding Orbital (NBO) method.[161-164] Results and Discussions We begin our discussion with the OH radical. Extensive details of how this section is organized, and the terminologies used are given in the supporting information (SI). (a) Weak Interactions with OH Radical The OH radical was detected in the ISM more than five decades ago, in 1964.[57] Besides astrochemistry, it is also important in combustion chemistry.[39-55] Hence a variety of studies featuring OH radical’s interaction, especially with H2O, have been earlier reported.[39-55] The geometries and interaction energies obtained in this work are very much similar to earlier results obtained, thereby validating the present work.


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Figure 1 lists the various modes of interactions wherein OH radical is the hydrogen bond donor. Consistent with prior literature, these were the only modes obtained as energy minimized structures even after multiple attempts. The modes of interaction vary from being linear (with HF in 1a, NH3 in 1c, H2O in 1d, PH3 in1f, HCN in 1g and HNC in 1h), to almost linear (with H2S in 1e, CH3OH in 1j) and bent (with HCl in 1b, HCP in 1i, and CH3SH in 1k). The bent geometry with HCl (1b) likely occurs to maximize all possible weak interactions such that OH simultaneously acts as a hydrogen bond donor and a hydrogen bond acceptor. With CH3SH (1l), the favorable S•••O distance, and the bent geometry clearly indicates chalcogen bonding. Even in case where the weak interaction modes are similar (linear or almost linear), no single factor seems to account for the overall trend obtained in the interaction energies, as can be seen from Figure 1. Two aspects; (a) gas phase proton affinities of the closed shell molecules, and (b) electronegativity of the atom acting as the hydrogen bond acceptor, both impact the interaction energies. Among NH3, HCN, HNC, H2O, CH3OH and HF, the proton affinity trend with minor exceptions mirrors the interaction energies. Both H2S and PH3, containing less electronegative S and P atoms result in lower binding energies. With HCP (1i) the mode wherein the π electron cloud acts as the electron donor is energetically more favorable than another mode (see Figure S1 given in the SI) wherein P (of HCP) is the electron donor (Figure S1). This is in accord with our expected chemical intuition because P is not a highly electronegative atom whereas the C≡P bond is more polarized. The other mode is given in the SI because therein, the interaction energy of OH with HCP is noticeably positive implying that it is thermodynamically unfavorable.



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Figure 1: OH (2Π) radical acting as the hydrogen bond donor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) NH3, (d) H2O, (e) H2S, (f) PH3, (g) HCN, (h) HNC, (i) HCP, (j) CH3OH, and (k) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

In Figure 2, OH radical acts as a hydrogen bond acceptor and the closed shell molecules as the donors (through the H). With the exception of HF (Figure 2a), the interaction energies are uniformly lower in Figure 2, when compared with the corresponding structures in Figure 1. OH radical is clearly a better hydrogen bond acceptor than being a hydrogen bond donor. With HF alone, the enhanced interaction energy when OH is a hydrogen bond donor (Figure 2a vs Figure 1a) clearly mirrors the interaction of HF with OH2, where the F−H···O interaction is the more stable mode of interaction.[165] Further, in comparison with Figure 1, it should also 8 ACS Paragon Plus Environment

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be noted that despite multiple attempts we could not obtain minimum energy complexes of OH (as H bond donor) with water, methanol, and methanethiol. HCP as the hydrogen bond donor results in two modes (Figures 1f vs S2 {in supporting information}). Mode 1f is the more stable mode, likely because in addition to engaging with O (of OH), there is also some contribution from the π electron cloud of C≡P, which engages the H (of OH).

Figure 2: OH (2Π) radical acting as the hydrogen bond acceptor in its interaction with closed shell molecules. (a) HF, (b) HCl, (c) H2S, (d) HCN, (e) HNC, (f) HCP. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

In addition to the structures given in Figure 2, with HCP, NH3 and PH3, even though energy minima are obtained, the interaction energies are positive (structures given in the SI), thereby ruling out their thermodynamic favourability. This point (that is, the interaction energies being positive) is a constant feature with all the radicals in some structures. To avoid repetition, we do not discuss this point with the other radicals hereafter and provide the structures with positive interaction energies in the supporting information.



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These were the only minima we could find with the OH radical. All our attempts to optimize geometries where the O (of OH) would interact with an electronegative atom from the closed shell molecule repeatedly provided only one of the structures shown in Figures 1 and 2. (b) Weak Interactions with SH Radical Whereas the OH radical has been widely studied, the corresponding sulphur analogue SH radical was detected in the ISM only in 2012,[58] and there are very few reports of its weak interaction with other molecules. [166] In Figure 3, we list all the structures where the SH radical acts as the hydrogen bond donor. Given the lesser electronegativity of S (vs O), the interaction energies in Figure 3 are consistently lower than that seen with the corresponding molecules in Figure 1. Most of the weak interactions in Figure 3 adapt a linear mode (barring the following cases – with H2S in 3c, CH3OH in 3j and CH3SH in 3k). This is counterintuitive based on the expectation that SH is a soft species per the HSAB theory, and therefore a larger deviation from linearity (typically associated with enhanced orbital interactions) was anticipated generally. Of the three modes of minima obtained with HCP (Figures 3g vs 3f vs S5 {in the SI}), as noted with OH (Fig. 1), the one with the π-electron involvement is the most stable structure.


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Figure 3: SH (2Π) radical acting as the hydrogen bond donor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HF, (b) OH2, (c) H2S, (d) HCN, (e) HNC, (f) HCP (mode 1), (g) HCP (mode 2), (h) NH3, (i) PH3, (j) CH3OH, and (k) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM062x/aug-cc-pvTZ.

Instead of being a hydrogen bond donor, the “S” of SH radical can also be a hydrogen bond acceptor. This situation is explored in Figure 4. In most of the structures, “H” from the closed shell molecule interacts with the S of SH, and no other interactions are present in these structures. But in 4i, the minima obtained seems to indicate some kind of an SSH•••SCH3SH interaction in Figure 4h (S•••S distance is only 2.85 Å). Taken together with the NBO and spin density analyses (see SI) it is likely that the short S•••S distance is not due to any



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interaction between the two Sulfur atoms, but is in place to perhaps salvage some hydrogen bonding interaction between H (of CH3SH) and S (of SH radical).

Figure 4: SH (2Π) radical acting as the hydrogen bond acceptor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) OH2, (d) HCN, (e) HNC, (f) HCP, (g) CH3OH, and (h) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

As can be seen from Figure 5, we found some minima where the S (of SH) is considerably close to an electronegative atom belonging to the closed shell molecule (well within the sum of their Van der Waals’ radii). At first glance, it may appear that this is due to some kind of an attractive interaction between S (of SH) and the electronegative atoms (from closed shell molecules). However, in most of the structures (Figures 5a-c, 5f) a combination of spin-density analysis and NBO calculations (see SI) indicate that there is no S•••X (X = an electronegative atom from the closed shell molecules chosen) interaction between them. Instead, the hydrogen 12 ACS Paragon Plus Environment

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bonding interactions anchor the structures to adapt a close S•••X interactions. Further support for there being no S•••X interaction stems from the fact that when comparing the structures between Figure 3 (where SH is a hydrogen bond donor) and 5, the structures in Figure 5 always have a thermodynamically more favorable interaction energy (for eg. compare Fig. 3c with Fig. 5a etc).

Figure 5: Figures wherein the S from SH (2Π) is in close proximity with an electronegative atom of the closed shell molecules. The closed shell molecules in the various panels are: (a) H2O, (b) HCN, (c) HNC, (d) HCP, (e) PH3, (f) CH3OH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

In Figure 5d (HCP interacting with SH), the magnitude of the interaction energy is unexpectedly high (about 9 kcal/mol). Upon inspection of the structure, we find that the, the spin density on S (from SH) gets completely transferred to not the P (with which it is interacting), but rather to C (see SI). Accordingly, the C becomes radicaloid, and this aspect is reflected in the bent geometry seen in Fig. 5d. The situation here is one of an almost formed P‒S single bond (experimental bond length – 1.9 Å; ρ(rc) and 2(rc) in SI also support the 13 ACS Paragon Plus Environment


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idea) along with the elongation of the P‒C bond (before binding to SH the P‒C bond length in HCP is ~1.5 Å, after binding it is ~1.6 Å). To ensure that this observation is not an artefact of the level of theory chosen, we performed geometry optimizations using three different density functionals (UB3-LYP, UB2-PLYP, and UωB97X-D) and obtained the very similar results – namely that the carbon becomes radicaloid. Also note that the interaction of PH3 with SH (Figure 5e) leads to a pnicogen bond as supported by spin density analyses and NBO calculations (see SI). (c) Weak Interactions with CN Radical The CN radical was one of the first molecules to be detected in the ISM.[60] Yet, notwithstanding the numerous dynamics, and reactivity studies on CN,[167-172] not much information is available about its involvement in weak interactions.[173,174] In agreement with the conventional Lewis-structure representation of CN radical the spin-density resides on C alone (spin density =1), and N possesses the lone pair of electrons. It thus, presents the interesting case wherein both the C terminal and the N terminal can independently act as hydrogen bond donors and/or participate in other types of weak interactions. The scenario when the N terminus acts as the hydrogen bond acceptor can be apprehended from Figure 6. That, all the modes of weak interaction are either linear/almost linear (barring Fig. 6d – with H2S) in conjunction with spin density, NBO and AIM analyses (see SI) suggests the dominance of the electrostatic effects in the hydrogen bonding herein. In complete agreement to this suggestion, the spin-density on C does not at all change (see SI)) after interaction with these closed shell molecules.


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Figure 6: CN (2Σ+) radical as the hydrogen bond acceptor in its interaction with closed shell via the N end. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) H2O, (d) H2S, (e) HCN, (f) HNC, (g) NH3, (h) PH3, and (i) CH3OH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

The N terminal of CN can also interact with the electronegative atom belonging to the closed shell molecules (Figure 7). Only three structures are presented here. With the rest of the molecules, we either got other minima (not corresponding to the mode of interaction being discussed here) or obtained positive interactive energies (structures shown in SI) or could not attain any optimized structure at all. The HCP•••N (from CN) interactions in 7a is very week. Further, the interactions of N (from CN) with PH3, and CH3SH are accompanied with a reorganization of the spin density on C (From CN). Such partial spin density transfer to S (see SI), optimal interaction energies, favourable P•••N and S•••N (in 7b and 7c) distances, AIM and NBO analyses (see SI), are common interesting features seen in 7b and 7c.



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Figure 7: CN (2Σ+) radical interacting with the electronegative atoms from the closed shell molecules via the N end. The closed shell molecules in the various panels are: (a) HCP, (b) PH3, (c) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-ccpVTZ//UM06-2x/aug-cc-pvTZ.

The C terminal of CN can also engage in weak interaction. Two possibilities are: (a) using it’s lone electron (spin density = 1), it can possibly interact with a hydrogen from the closed shell molecule, (b) or, an electronegative atom from the closed shell molecule could interact with the C (carbon bonding). After repeated attempts, we could never optimize structures reflecting case (a). The second case however leads to interesting results shown in Figure 8. A comparison between Figures 8a and 8e informs us about the significance of non-polar groups in the interaction here. Clearly, the presence of the non-polar CH3 group enhances the interaction energy of carbon bonding. Along the same lines, the more polarizable and softer S in CH3SH in 8f leads to a much higher interaction energy than the harder O in CH3OH (Figure 8e). The same trend can be seen between H2O and H2S (Figure 8a vs 8b). Some other structures leading to positive interaction energies are given in the SI.


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Figure 8: CN (2Σ+) radical interacting with the electronegative atoms from the closed shell molecules via the C end. The closed shell molecules in the various panels are: (a) H2O, (b) H2S, (c) HCN, (d) NH3, (e) CH3OH, and (f) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

(d) Weak Interactions with NO Radical NO (2Π ground electronic state) was detected in the ISM in 1990.[61] In it, the spin density on N is 0.7, and that on O is 0.3. Further, both the N and the O termini have lone pairs of electrons. Thus, a wide variety of interesting weak interactions are possible. In Figure 9, we have NNO acting as the hydrogen bond acceptor. As can be evinced from the interaction energies, the interaction is not strong with any of the closed shell molecules. A compelling reason could well be that N and O are of very similar electronegativities, and this therefore results in the magnitude of the dipole moment of NO to be very small (calculated to be about 0.12 a.u at the UM06-2x/aug-cc-pVTZ level of theory). Consequently, the interaction energies are not high. Note that, since NO is a relatively harder species, its hydrogen bonding interactions are expected to be dominated by electrostatic effects, as supported by spin density analysis (see SI).



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Figure 9: NO (2Π) radical as the hydrogen bond acceptor in its interaction with closed shell via the N end. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) H2O, (d) HCN, (e) HNC, (f) HCP, (g) CH3OH, and (h) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

A very similar situation persists with ONO acting as the hydrogen bond acceptor as well (Figure 10) – magnitudes of interaction energies being small, no change in spin densities (see SI), and orbital interactions not being dominant (see SI). There is again no specific trend in the nonhydrogen bonding modes of interactions (Figures 11 – closed shell molecules interacting with N of NO and 12 – closed shell molecules interacting with O of NO). One prominent common feature though is that, the distance between the weakly interacting atoms in all the structures in Figures 11 and 12 are all slightly larger than the sum of the Van der Waals’ radii of the same atoms. This observation discloses that barring weak electrostatic interactions, there are no covalent forces at work here (supported by NBO and spin density analyses - see SI). 18 ACS Paragon Plus Environment

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Figure 10: NO (2Π) radical as the hydrogen bond acceptor in its interaction with closed shell via the O end. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) HCN, (d) HNC, (e) HCP, (f) CH3OH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

Further, that these interactions are very weak is poignantly illustrated by Figures 11b where, besides an NNH3•••NNO interaction there is also an NNO•••HNH3 interaction. Yet, even the combined effects of these multiple interactions don’t result in larger magnitudes of interaction energies. Note that all the optimized minimum energy structures wherein the NO radical interacts with the electronegative atoms from the closed shell molecules via the O end resulted in positive interaction energies and such structures are given in the SI.



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Figure 11: NO (2Π) radical interacting with the electronegative atoms from the closed shell molecules via the N end. The closed shell molecules in the various panels are: (a) H2O, (b) NH3, (c) CH3OH, and (d) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

(e) Weak Interactions with NH2 Radical Interstellar NH2 radical (2B1 ground state) was detected in 1993 in the molecular cloud Sgr B2.[63] Very little is known about the weak interactions involving NH2.[175-178] We could compute only six optimized minimum energy structures when NH2 acts as a hydrogen bond donor (five of them having a negative interaction energy as shown in Figure 12 and one structure with positive interaction is given in the SI). The interaction energies are in the range of about ‒0.12 to ‒2.00 kcal/mol, following no evident trend. Further, it is interesting to observe that the lone electron on N of NH2 (in all of Figures 12a-e), remains unengaged with any species (based on two facts: (a) that there is no change in the spin density on NNH2 after the interactions (see SI), and (b) NNH2 is not even close to any atom belonging to the closed shell molecules) in all the structures.


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Figure 12: NH2 (2B1) radical as the hydrogen bond donor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HCN, (b) HNC, (c) NH3, (d) PH3, and (e) CH3OH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

In Figure 13, the NH2 radical is a hydrogen bond acceptor. With HF it forms a complex with an interaction energy of about ‒6.8 kcal/mol (Figure 13a). This is the strongest interaction amongst all those seen in Figure 13. Between HF and HCl, HF is clearly the better hydrogen bond donor, as can be rationalized based on the higher electronegativity of F. Similarly, the interaction energy with H2O is more favorable than H2S (Figures 13c vs 13d), and CH3OH over CH3SH (Figures 13h vs 13i). Whenever a comparison can be made between Figures 12 and 13 (Figures 12a vs 13e, 12b vs 13f, and 12e vs 13h) it is no surprise that the interaction energies are throughout higher in 13 than in 12 given that previous molecular dynamics studies have also established that the NH2 radical is a better hydrogen bond acceptor than it a donor.[175] A possible chemical rationale is that, when acting as a hydrogen bond donor, the most reactive site “N” of NH2 bearing the lone electron remains “unquenched” in Figure 13. Note that, only four minimum energy structures could be optimized wherein the N of NH2 interacts with the 21 ACS Paragon Plus Environment


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electronegative atoms from the closed shell molecules (Figure 14) and from them only one leads to a negative interaction energy (rest of the structures given in the SI). Lastly, AIM analysis (see SI) supports the existence of weak interactions. But no specific trend could be spotted here, other than noting that the interaction energies are lower than both the hydrogen bonding modes seen in Figures 12, 13 and 14.

Figure 13: NH2 (2B1) radical as the hydrogen bond acceptor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) H2O, (d) H2S, (e) HCN, (f) HNC, (g) HCP (h) CH3OH, and (i) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.


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Figure 14: NH2 (2B1) radical interacting with the electronegative atoms from the closed shell molecules via the N atom with CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ.

(f) Weak Interactions with HO2 Radical The hydroperoxyl radical (2A” ground state) is a relatively newly detected species in the ISM,[64] although it is an important and well-studied molecule on earth’s atmosphere. There have also been some previous studies on the weak interactions of the peroxy radical with H2O, H2S, HCl and HF(our results concur with these studies, vide infra).[179-183] This molecule can indulge in weak interactions via any of the three atoms (H, O, and O) it possesses, and therefore presents a variety of possibilities. It’s net spin density is shared between the two oxygens – about 0.3 being the spin density on the central oxygen, and 0.7 being borne by the terminal oxygen. In Figure 15, we present the case where HO2 is the hydrogen bond donor. However, with a lot of the closed shell molecules (Figures 15a – HF, 15b - HCl,), the hydrogen atom in the closed shell molecule, concomitantly interacts with the terminal oxygen in HO2, resulting in a ring structure. This points out to the co-operativity between the different types of hydrogen bonds.[184] With the linear closed shell molecules HCN (15e) and HNC (15f) and one mode of HCP (15g) owing to geometric constraints, the H atom in these molecules does not however participate in the weak interactions. However, in another mode with HCP, due to increased CP 23 ACS Paragon Plus Environment


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bond length of 1.53 Å (with respect to CN bond length of 1.14 Å), the P of HCP can engage the terminal O of HO2 via a pnicogen type bonding (15h). The Hs in PH3 also do not exhibit co-operativity, very likely owing to the fact that the PH bond is least polarized (P and H having very similar electronegativities) in comparison with the other closed shell molecules. Overall, the interaction energies in Figure 15 follow no particular trend.

Figure 15: HO2 (2A”) radical as the hydrogen bond donor in its interaction with closed shell molecules. The closed shell molecules in the various panels are: (a) HF, (b) HCl, (c) H2O, (d) H2S, (e) HCN, (f) HNC, (g) HCP (mode 1), (h) HCP (mode 2), (i) NH3, (j) PH3, (k) CH3OH, and (l) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-ccpVTZ//UM06-2x/aug-cc-pvTZ.

Figure 16 lists the situation where HO2 is the hydrogen bond acceptor. In almost all the structures in Figure 16, the interaction energy is lower than the corresponding structures in


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Figure 15. This is easy to understand, given that in a lot of the structures there are two hydrogen bonding interactions in Figure 15 as opposed to only one in Figure 16. HF (Figures16a vs 15a) is a notable exception, though. Despite losing the other hydrogen bond (seen with 15a) in Figure 16a, the magnitude of the interaction energy becomes slightly higher. This is perhaps due to the fact that, the FHO bond angle in 16a is about 1800 but none of the two hydrogen bond angles in 15a is close to being linear. The spin-density analysis (see SI) clearly shows that all the interactions are electrostatically driven. And, the optimal hydrogen bond angle for electrostatically driven hydrogen bonds is 1800. Combining this idea along with the fact that “F” is the most electronegative atom (and thereby shows the maximum propensity favouring electrostatic interactions) therefore, very likely explains the slightly enhanced interaction energy in 16a vs 15a.

Figure 16: HO2 (2A”) radical as the hydrogen bond acceptor (via its terminal oxygen) in its interaction with closed shell molecules. The closed shell molecules in the various panels are: 25 ACS Paragon Plus Environment


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(a) HF, (b) HCl, (c) H2O, (d) H2S, (e) HCN, (f) HNC, (g) HCP, (h) CH3OH, and (i) CH3SH. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM062x/aug-cc-pvTZ.

With HCN and HNC, the interaction energies are more favourable in Figure 15 over Figure 16 despite there being only one hydrogen bonding interaction in both the sets of figures. To understand why, one has to probe the relative abilities of the molecules to be stronger hydrogen bond donor and acceptors - that structure is more thermodynamically stable which involves the better hydrogen bond donor forming a hydrogen bond with the better hydrogen bond acceptor. With both the closed shell molecules (HCN and HNC), owing to a combination of electronegativity and proton affinities, it is straightforward to see that the arrangement wherein HO2 is a hydrogen bond donor is more favoured than the arrangement wherein HO2 is the hydrogen bond acceptor, and this provides a rationale for the pattern noticed with the interaction energies. It is remarkable to note that we could optimize only two structures wherein the central “O” in HO2 participated in the weak interactions. One involving hydrogen bonding with HF (Figure 17), and another involving an O•••Cl interaction (given in the SI). For the rest of the molecules, our multiple attempts invariably resulted in us ending up with only one of the structures which can be seen in Figures 16 and 17.

Figure 17: HO2 (2A”) radical interacting with HF via its middle oxygen. Bond distances are given in Å, and the interaction energy (including zero point energy) is provided below each structure. Level of theory used: UCCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pvTZ. 26 ACS Paragon Plus Environment

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(g) Factors Affecting the Weak Interactions and General Trends: Throughout Figures 1-17, it should be noticed that multiple factors affect the weak interactions discussed in the earlier sections. They include proton affinities, extent of orbital overlap/electrostatic effects (tabulated in the SI) and electronegativity of the participating atoms. The individual aspects of the weak interactions could be succinctly summed up as follows: (a) OH - Only two dominant modes of weak interactions were noticed. One where OH acts as a hydrogen bond donor, and another where OH is a hydrogen bond acceptor. Based on the interaction energies, OH is observed to be a better hydrogen bond donor than a hydrogen bond acceptor. In only one case, chalcogen bonding was observed along with hydrogen bonding. The weak interactions were predominantly found to be driven by electrostatic effects. (b) SH – Three modes of weak interactions were computed for SH. It acting as hydrogen bond donor, hydrogen bond acceptor being two modes, and another involving the S of SH interacting with an electronegative atom belonging to the closed shell molecule reminiscent of chalcogen bonding. Whenever a meaningful comparison was possible, the magnitude of the interaction energies for the SH radical were uniformly lower than that seen with OH, as can be expected based on the lower electronegativity of S (vs O). (c) CN – Only three modes were obtained – (a) hydrogen bonding via the N terminus, hydrogen bonding via the C terminus, and (c) the C terminus participating in carbon bonding. In mode (c), ammonia, hydrogen sulphide and methane thiol exhibited the strongest interactions with the C terminus – interaction energies being about ‒9, ‒8 and ‒14 kcal/mol respectively. The large interaction energies are likely due to enhanced orbital interactions. This is noteworthy given that the magnitude of the interaction energies with the other closed shell molecules are low. In the case of carbon bonding, a substantial amount of orbital overlap is noticed. 27 ACS Paragon Plus Environment


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(d) NO – Both “N” and “O” participated in weak interactions – either as hydrogen bond acceptors, or as part of interactions involving NNo or ONO interacting with an electronegative atom from the closed shell molecule. When comparable, the N terminus of NO is slightly a better hydrogen bond acceptor than the C terminus. With all the modes, the magnitude of the interaction energy is low, and the interactions are dominated by electrostatic effects. (e) NH2 – It is a better hydrogen bond acceptor than a donor. Also participates in weak interactions where N (of NH2) interacts with an electronegative atom from the closed shell molecule. From among these three modes, the hydrogen bonding modes lead to stronger interactions than the one in which where N (of NH2) interacts with the electronegative atom from the closed shell molecule. (f) HO2 – Acts as a hydrogen bond donor, and acceptor. Also interacts via the terminal “O” with an electronegative atom from the closed shell molecule. The interaction energies depend on many factors including the co-operative formation of hydrogen bonds, proton affinity of the closed shell molecule, whether HO2 is a hydrogen bond donor or an acceptor. Only two Structures wherein the central “O” (of HO2) features in weak interactions, were obtained. In all these cases, some interaction energies were positive, which even though is not significant for the chemistry occurring on earth, might well be crucial in interstellar chemistry. In the context of the positive interaction energies, an important note is as follows: From out of a total ~ 160 minimum energy structures optimized as part of this work, ~ 43 of them have a positive interaction energy, despite being fully optimized minimum energy structures. In case, there are systematic errors in the computed interaction energies, they could arise from one of the following three aspects: (a) the UCCSD(T)(FC)/aug-cc-pVTZ method’s accuracy (~ 1kcal/mol), (b) the zero point vibrational energy computation with the UM06-


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2x/aug-cc-pVTZ, or (c) during the optimization of our structures, we could have missed out on more stable isomers/lower lying electronic states. To check if it is point (a) or (b), with our modest computational resources, we performed calculations on representative systems with the more accurate (~ 0.5 kcal/mol) W1U method, and included anharmonic scaling factors also, in our zero point vibrational energy. These measures did not change the interaction energies significantly. To check if it is point (c), we exhaustively attempted to find alternate isomers and electronic states (other than those not reported herein), but ended up getting: (a) either the same optimized structures reported above, or (b) some other minimum energy structures not featuring weak interactions (a full-fledged covalent bond getting formed) or (c) minimum energy structures wherein the interaction energies were again positive or (d) higher order saddle points containing at least one imaginary frequency. Our efforts on this front is summarized in great detail in the SI. Having carefully performed these computations, we believe that we report the appropriate isomer/electronic state, and the correct value of the interaction energies, in this paper. It might however definitely be possible that more accurate measures of the interaction energies can be computed/other more stable isomers or electronic states may be obtained by using much more computationally expensive methods (the kind of which we could not perform with our computational resources). (h) Astrochemical and Terrestrial Implications of the Study The interstellar medium is not a single homogenous place.[34] Rather, it consists of several components including the dense and diffuse molecular clouds (which is the region of interest in this paper), warm ionized medium, the H II region, and the hot ionized medium. The temperature in the molecular clouds can vary from being as low as 10 K to about 107 K in the



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hot ionized medium. Similarly the number density can also vary from being around 105 (per cm3) in the molecular clouds to only about 10-2 in the hot ionized medium. Some fascinating aspects of the interstellar medium (ISM) are: (a) A significantly lower number density than that seen in the earth's atmosphere.[37-38] Because of which, interstellar chemistry is typically what one could call extremely high dilution chemistry. (b) Non-existence of liquids in the ISM[18] - which limits the chemistry occurring to either be gas phase chemistry, or solid phase surface/cluster science. (c) Only some parts of the ISM existing in thermal equilibrium (where the conventional assumption of Boltzman distribution holds good) - due to the molecular clouds in the ISM constantly getting bombarded with various radiations. [185] (d) Due to point (c), favorable kinetics is a much more important factor in driving interstellar chemistry rather than the attainment of thermodynamic stability. Each of these points renders weak chemical interactions to be of great import in the ISM. As mentioned in the introduction, due to the significantly lower number density, the probability of two molecules colliding and leading to a reaction is relatively rarer in the ISM; especially in gas phase interstellar chemistry. So for any two molecules to stick together and subsequently react, some sort of a weak interaction is essential. Surface reactions are major contributors towards chemical evolution in the ISM.[34] Gas molecules in the ISM are known to get adsrobed on to the "cosmic dust grain" majorly composed of carbon, silicon, oxygen, hydrogen, and. These grains facilitate collisions between different molecules to result in chemical reactions. Interestingly gas phase hydrogen bonding involving closed shell molecules was recently shown to impact the relative abundances of the


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various molecules in the ISM based on correlations between surface binding energies and gas phase abundances.[21] The same study further fuelled hypotheses on what putative molecules are likely to be discovered in the ISM.[21] As a natural extension, hydrogen bonding interactions involving

open

shell

systems,

and

the

other

weak

interactions

(such

as

halogen/pnicogen/chalcogen bonding etc) should also affect the relative abundances, given that once molecules are adsorbed on the surface of the grain, there is ample scope for intermolecular weak interactions. A large part of the ISM not being in thermal equilibrium implies that any possible weak chemical interaction in the ISM need not be steered by thermodynamically favorable interaction energies. Molecules could come close enough, and when they adapt a certain orientation could be held together via weak interactions. This is a rather profound and unintuitive feature of the ISM. On earth's atmosphere, weak interactions are possible if only the interaction energies/free-energies/enthalpies are thermodynamically favored. But that is not the case in the ISM. It is for this reason that we presented even those structures wherein the interaction energies were positive (vide supra) and stated that they are possible in the ISM. A few important points in this context are: (a) Say, once some interstellar molecules are bound together via weak interactions, what happens next is something we are not concerned with, in this study. They could dissociate back, or they could result in certain pre-reactant complexes which undergo further molecular reorganization to lead to various interstellar reactions. It depends on the relative rates of the two processes, what radiations the molecules are subject to etc.[185] Further, the lifetime of these complexes depends on their formation mechanisms. In high-density conditions, the formation of molecular complexes is aided by third-body participation (typically the surface of interstellar ice-grain) In low-density conditions, radiative 31 ACS Paragon Plus Environment


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association accompanied with the emission of a photon is another mechanism for the formation of molecular complexes (although for weaker binding, the process is very slow).[186,187] Alternatively, any two molecules adsorbed onto an ice grain surface might have been bound via weak interactions. Then, via radiative desorption, without the weak interaction getting affected, the molecular complex could be desorbed from the surface onto the gas phase. Complexes formed under high-density conditions are likely to be longer-lived than those formed under low-density conditions. This is because the longer formation time also renders the complex becoming more susceptible to radiations. Consequently, the complex could either break down or lead to covalent bond formation. Even if the complexes themselves are short-lived, they are important because, they act as the gateways to the subsequent reaction pathways involving the molecules. Moreover, as shown recently[21] weak interactions in the gas-phase can potentially impact even the relative abundances of the various molecules in the ISM based on correlations between surface binding energies and gas phase abundances. (b) Be it with a negative or positive interaction energy, any weakly interacting complex is feasible in the ISM if only it is permitted by chemical kinetics. However, to calculate the rate constants for the formation of a weakly interacting complex (which are usually formed without a barrier) is very challenging and beyond the scope of this study. (c) In a lot of the cases, the magnitude of the interaction energies we obtained were very small (absolute value < 1 kcal/mol). The method used to calculate the energies is the highly accurate UCCSD(T)(FC), with aug-cc-pVTZ basis-sets. Even with a complete basis-set, the error-bar in the CCSD(T) method is about 1 kcal/mol.[137] The reader should definitely be aware of this aspect. Nonetheless, as far as the ISM is concerned, the exact values of the interaction energies


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themselves are perhaps not the only important aspect, due to the lack of thermal equlibrium in most regions of the ISM. (d) No neutral-neutral molecular complex driven by weak interactions has so far been explicitly detected in the ISM. Although there are strong recommendations made to detect H2-CO, complexes between H2O and CO2, CH3OH and CO2, and H2O and CH3OH.[188-191] (e) There is no necessity that for reactions occurring in the ISM, the ideas of conventional transition state theory should be holding good always. In which scenario, it is not the case that, one pre-reactant complex should be associated with any one given reaction path only. Instead, any given pre-reactant complex featuring weak interactions plays can lead to multiple reaction pathways. The molecular abundances in the ISM is constantly getting updated as: (a) newer molecules are getting detected, (b) more interstellar reaction pathways are determined, (c) more reaction rate constants are obtained, and generally (d) more aspects about interstellar chemistry are unraveled. In this regard, the concept of weak chemical interactions is another important feature in interstellar chemistry which has not been paid due attention to. We urge astrochemists to consider the weak interactions as well, in the chemical models used to update the molecular abundance in the ISM. Terrestrially, there is no question regarding the significance of weak interactions. It is however curious to note that while weak interactions involving closed shell molecules have been exhaustively documented over the last hundred and fifty years, there are markedly fewer references

to

those

occurring

in

open

shell

systems;

restricted

mostly

to

spectroscopic/dynamics studies.[33] Consider two examples which poignantly illustrate the disparity in how much we know about weak interactions in closed shell systems vs how much we are only beginning to know about them in open shell molecules. (a) Water and ammonia 33 ACS Paragon Plus Environment


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are very simple molecules, and the ammonia-water hydrogen bonding has been thoroughly studied since decades.[192] Yet it was only as recent as 2015 that, the nature of the weak interaction of the amino radical (NH2) with water was established.[175] Similarly closed shell N–H•••O═C interactions are commonplace in biochemistry,[193] but it was only in 2016 that the substituent effects in the radicaloid N–H•••O═C bonds have been reported.[194] So, there are lots of new and interesting facets of weak interactions with open shell molecules waiting to be unearthed. Summary In conclusion, this work highlights the potentially crucial role of weak interactions in the ISM. Six open shell molecules (OH, SH, CN, NO, NH2, and HO2) and eleven closed shell molecules (H2O, H2S, HF, HCl, NH3, PH3, HCN, HNC, HCP, CH3OH, and CH3SH) which play a significant role in interstellar chemistry were chosen as illustrative examples to computationally study weak interactions between open shell molecules and closed shell molecules. Hydrogen bonding interactions predominate as can be seen in Figures 1-17. Other modes of interactions noticed include chalcogen bonding, pnicogen bonding, and carbon bonding. No single factor explains the trend in the interaction energies in any of the Figures. A combination of proton affinities, extent of orbital overlap/electrostatic effects and the electronegativities of the atoms affects the interaction energies. AIM calculations were used to corroborate with the computed geometries that week interactions were indeed at work here. Spin density analyses then shed light upon the role of orbital interactions, and NBO computations were used to gauge the local nature of orbital overlap The overall astrochemical significance of this work is highlighting the weak chemical interactions in the ISM which could considerably impact the molecular abundances in the ISM. 34 ACS Paragon Plus Environment

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In the present work, we only considered the open shell molecules to be in their ground state. This need not be the case in the ISM and gives rise to the possibility of probing the weak interactions involving the excited states. Some work on this front has begun in our lab, and we also exhort both theoretical and experimental astrochemists to work on them. Moreover, there is an enormous scope in calculating/experimentally determining the astrochemically relevant vibrational/rotational spectroscopic parameters (which we have not carried out in the present work, since it deserves to be a separate work in its own right) in such weak interactions. On earth, while a lot of the open shell interactions are generally too weak to be useful in practical applications (such as in gas-sensing etc.), they can still have a pronounced effect on the selectivities of chemical reactions wherein these interactions occur (since selectivity is usually proportional to 𝑒 ―∆𝐸). The wide array of applications of weak interactions in chemistry include (but not limited to) main group/transition metals/organometallic chemistry, organic/inorganic chemistry of free radicals, free-radical induced DNA damage, prebiotic & combustion chemistry, and catalysis. These aforementioned areas, can vastly benefit by studies on hydrogen bonding, halogen bonding (and related types of weak interactions), cation−π, anion−π, π−π and other similar interactions in open shell species. Supporting Information Exhaustive details of the computational methods used, details about the organization of the “Results and Discussion” section, NBO, AIM and spin-density analysis including individual spin densities for all the molecules along with the spin contamination values, calibration of the method used in the main text (CCSD(T)(FC)/aug-cc-pVTZ//UM06-2x/aug-cc-pVTZ) with several other theoretical methods UCCSD(T)(FC)/aug-cc-pVTZ//UB3-LYP(D3-BJ)/aug-ccpVTZ,

UCCSD(T)(FC)/aug-cc-pVTZ//UB2PLYP(D3-BJ)/aug-cc-pVTZ,

and

UCCSD(T)(FC)/aug-cc-pVTZ//UωB97X-D/aug-cc-pVTZ.- for both the geometries and 35 ACS Paragon Plus Environment


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interaction energies, details involved in complete basis-set extrapolation, comparing the interaction energies with the WIU method and the CCSD(T) method, our attempts to optimize other isomers/electronic states, the co-ordinates of all the structures given in Figures 1-17, and structures with positive interaction energies are provided in the Supporting Information (SI). This material is available free of cost at www.pubs.acs.org Acknowledgements: K. G, S.T, and R. O. R thank IISER Tirupati, and the Science and Engineering Research Board (SERB), Department of Science and Technology (DST) Government of India for the Early Career Research Award (ECR/2016/000041) to R. O. R funding this work, and a National PostDoctoral Fellowship to K.G (PDF/2018/003926). R. O. R is grateful to Dr. Rajesh Viswanathan for useful comments about the manuscript. We also thank an anonymous referee for insightful technical points. REFERENCES (1) Freeman, D. Origins of Life, 2nd ed.; Cambridge University Press: New York, 1999. (2) Hoyle, F.; Wickramasinghe, N.C. Evolution from space; Simon & Schuster, New York, 1981. (3) Segré, D.; Ben-Eli, D.; Deamer, D. W.; Lancet, D. The Lipid World. Origins. Life. Evol. B 31. 2011, (1–2), 119–145. (4) Oparin, A. I. The Origin of Life, 2nd ed.; Dover Publications: New York, 2003. (5) Historical understanding of life's beginnings, Schopf, J. W. Life's Origin: The Beginnings of Biological Evolution, University of California Press: California, 2002. (6) Miller, S. L. A Production of Amino Acids Under Possible Primitive Earth Conditions. Science. 1953, 117, 528-529. (7) Herbst, E.; Van Dishoeck, E. F. Complex Organic Interstellar Molecules. Annu. Rev. Astron. Astrophys. 2009, 47, 427-480. (8) See references 8-20 for some recent illustrative studies and reviews: Wang, Z.-C.; Li, Y.K.; He, S.-G.; Bierbaum, V. M. The HNO− Radical Anion: A Proposed Intermediate in 36 ACS Paragon Plus Environment

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