Introduction
One of the critical questions of biology is how life originated on earth. Besides the Miller–Urey experiment (Miller, Reference Miller1953), another hypothesis which provides an answer to this question assumes that at-least precursors of biological molecules, particularly precursors of amino acids must have originated in the interstellar medium (ISM) and they have come to earth during the late bombardment period via comets and meteoroids (Abramov and Mojzsis, Reference Abramov and Mojzsis2009; Wickramasinghe, Reference Wickramasinghe2011). Although none of the amino acids has been found in the ISM to date, their presence is expected because most of the molecules involved in the amino acid synthesis (particularly Strecker synthesis) have been already observed in the ISM in the past few decades. Several research groups around the world are looking for the various other possible roots for the amino acid synthesis (Woon, Reference Woon2002, Cecchi-Pestellini et al., Reference Cecchi-Pestellini, Scappini, Saija, Iatì, Giusto, Aiello, Borghese and Denti2004; Andersen and Haack, Reference Andersen and Haack2005; Chiaramello et al., Reference Chiaramello, Talbi, Berthier and Ellinger2005; Mita et al., Reference Mita, Nomoto, Terasaki, Shimoyama and Yamamoto2005; Lattelais, Ellinger and Zanda, Reference Lattelais, Ellinger and Zanda2007).
The carbonaceous chondrite such as Murchison meteorite (Lawless, Reference Lawless1973), Allan Hills meteorite and others (Sephton Reference Sephton2000; Pizzarello and Shock Reference Pizzarello and Shock2010 ) are found to be rich in amino acids and their precursors. These meteoroids are also rich in branched amino acids such as valine, 2-aminoisobutyric acid, etc. Branched amino acids are one of the important constituents of terrestrial biology. Their presence in the meteorites indicates that at least their precursor must have been originated in the ISM. The discovery of several amino acid precursors in ISM provides evidence for this hypothesis (Koch et al., Reference Koch, Toubin, Peslherbe and Hynes2000; Basiuk, Reference Basiuk2001; Woon Reference Woon2001; Singh et al., Reference Singh, Shivani, Tandon and Misra2018).
Propyl cyanide and its branched isomer (hereafter n-PrCN and i-PrCN respectively) belong to the family of interstellar cyanides and are currently the largest and most complex members of this family. Interstellar cyanides are one such family of prebiotic molecules. Cyanides play an important role in the Strecker synthesis since they can easily reduce to their corresponding amine (RCHNH2) (owing to the presence of HCNH2 moiety, amines serve as directed precursors of amino acids) (Chakrabarti et al., Reference Chakrabarti, Majumdar, Das and Chakrabarti2015). Interstellar cyanides are found in a copious amount in interstellar clouds (Jefferts et al., Reference Jefferts, Penzias and Wilson1970; Guélin and Cernicharo, Reference Guélin and Cernicharo1991; Agúndez et al., Reference Agúndez, Fonfría, Cernicharo, Pardo and Guélin2008; Margulès et al., Reference Margulès, Motiyenko, Demyk, Tercero, Cernicharo, Sheng, Weidmann, Gripp, Mäder and Demaison2009; Daly et al., Reference Daly, Bermúdez, López, Tercero, Pearson, Marcelino, Alonso and Cernicharo2013; Belloche et al., Reference Belloche, Garrod, Müller and Menten2014; Cernicharo et al., Reference Cernicharo, Guélin and Pardo2004). Two simplest cyanides viz. HCN and CH3CN were detected in 1971 (Snyder and Buhl, Reference Snyder and Buhl1971; Solomon and Jefferts, Reference Solomon and Jefferts1971) and after their initial discovery, several more were detected in ISM ranging from saturated to unsaturated molecules and radicals (Guélin and Cernicharo, Reference Guélin and Cernicharo1991; Highberger et al., Reference Highberger, Savage, Bieging and Ziurys2001; Cernicharo et al., Reference Cernicharo, Guélin and Pardo2004; Bera et al., Reference Bera, Lee and Schaefer2009). Due to their importance in the chemistry of life, formation mechanisms of cyanides and related reduced species were investigated by both experimental and theoretical means (Schwartz et al., Reference Schwartz, Joosten and Voet1982; Ordu et al., Reference Ordu, Müller, Walters, Nuñez, Lewen, Belloche, Menten and Schlemmer2012; Shivani and Tandon, Reference Shivani, Misra and Tandon2017; Singh et al., Reference Singh, Shivani, Tandon and Misra2018). Currently, the largest member of the family of interstellar cyanides is propyl cyanide (C3H7CN), which is not only the largest but also the only interstellar molecule that can have branched isomer. Propyl cyanide (hereafter n-PrCN; n refers to ‘normal’) was detected in SgrB2 (N) (Belloche et al., Reference Belloche, Garrod, Müller, Menten, Comito and Schilke2009). Since this was the first and only detected molecule which can have a branched structure, the discovery of its branched isomer, isopropyl cyanide (iso-C3H7CN, hereafter i-PrCN), was much awaited. Later, i-PrCN was detected in the hot core of Sagittarius molecular cloud (SgrB2 (N)) along with its linear isomer, n-PrCN with column densities of nearly 7.2 × 1016 and 1.8 × 1017 cm−2, respectively (Belloche et al., Reference Belloche, Garrod, Müller and Menten2014). The detection of i-PrCN is quite significant for both astrochemistry and astrobiology and since owing to the branched structure, i-PrCN can be considered as a precursor for branched amino acids such as leucine (the smallest branched amino acid), isoleucine and valine (the proteinogenic amino acids) and 2-aminoisobutyric acid (non-proteinogenic one).
Both the isomers of propyl cyanide are well studied in the interstellar context. The formation mechanism of n-PrCN was investigated by Belloche et al. (Reference Belloche, Garrod, Müller, Menten, Comito and Schilke2009) using a coupled gas-phase and grain surface model suggested by Garrod (Reference Garrod2008). They proposed that sequential addition of CH2 and CH3 radicals to CN, CH2CN and C2H4CN on the grain surface or direct addition of large hydrocarbon radicals such as C2H5 or C3H7 to CN is the fastest route to n-PrCN. They derived the column density of n-PrCN to be nearly 1.5 × 1016 cm−2 with a local cloud temperature of nearly 150 K. After the detection of i-PrCN, the same group also proposed the formation mechanism of i-PrCN and n-PrCN using chemical kinetics model MAGICKAL (Model for Astrophysical Gas and Ice Chemical Kinetics and Layering) which simulates the time-dependent chemistry of SgrB2 (N) (Belloche et al., Reference Belloche, Garrod, Müller, Menten, Comito and Schilke2009; Garrod, Reference Garrod2013). Their study was focused on the accretion and addition of small radicals at the grain surface resulting in the isopropyl radical. These radicals subsequently add to CN and form i-PrCN. In their study, n-PrCN was formed from the addition of carbon radicals to the pre-existing cyanides. Contrary to the detected results, their model strongly favours the formation of i-PrCN (their proposed abundance ratio for i-PrCN : n-PrCN was 2.2 : 1 against the detected ratio 0.4 : 1). Recently Garrod et al. (Reference Garrod, Belloche, Müller and Menten2017) have studied the chemistry of SgrB2 (N) (with a special focus on the branching of complex molecules) by considering the case of propyl and butyl cyanide in the above-mentioned region using the same chemical model as Belloche et al. (Reference Belloche, Garrod, Müller and Menten2014) but with the updated chemical network. They concluded that the addition of CN to the unsaturated carbon plays an important role in smaller cyanide formations, which then grow towards more complex ones. With these considerations, they successfully obtained the detected abundance ratio between the linear and branched isomers of propyl cyanide.
Both of the previous studies suggest that the growth of less complex small cyanides towards the large one plays an important role in interstellar chemistry. Therefore, we present accurate electronic structure calculations to understand the formation mechanism of both i-PrCN and n-PrCN in the ISM by the reactions of preexisting, less complex cyanides/cyanide radicals such as CH2CN, HCCN, etc. with small organic radicals and unsaturated molecules. These molecules are present in a copious amount in ISM (Feuchtgruber, Reference Feuchtgruber2000; Polehampton et al., Reference Polehampton, Menten, Brünken, Winnewisser and Baluteau2005; Agúndez et al., Reference Agúndez, Fonfría, Cernicharo, Pardo and Guélin2008). The proposed reaction schemes for i-PrCN and n-PrCN formation are listed in Table 1. All of these reaction schemes have also been studied for the catalytic effect of interstellar water ice.
List of all the reaction schemes under study for the formation of linear and branched isomers of propyl cyanide (n-PrCN and i-PrCN, respectively)
a On surface of ice only.
b In gas phase only.
All the reported energies are calculated at the UMP2/6-311++(2d,p)//UM062X/6-311+(2d,p) level of theory and reported in kcal mol−1 relative to the sum of their respective reactants.
Methodology
In the current study, all the quantum chemical calculations were performed using Gaussian 09 software (Frisch et al., Reference Frisch, Trucks, Schlegel, Scuseria, Robb, Cheeseman, Scalmani, Barone, Mennucci, Petersson and Nakatsuji2009). The structures of molecules/radicals/transition states were optimized at the UM062X/6-311+(2d,p) level of theory with tight convergence criteria. The chosen hybrid meta-exchange correlation density functional M062X includes midrange non-covalent dispersion corrections in its definition which are necessary for the current study since we are studying the reactions at the surface of water ice and non-covalent interactions have a large contribution towards the stability of water ice and reactants adsorbed at its surface. In order to improve the accuracy of the energies reported in this study, single-point calculations have been performed at the UMP2/6-311++(2d,p) level of theory on the structures optimized at the UM062X/6-311+(2d,p) level (Frisch et al., Reference Frisch, Head-Gordon and Pople1990; Zhao and Truhlar, Reference Zhao and Truhlar2011). Our approach (optimization at UM062X/6-311+(2d,p) followed by single-point calculations at the UMP2/6-311++(2d,p) level) is referred as UMP2/6-311+(2d,p)//UM062X/6-311+(2d,p) throughout this paper. To infer the nature of the optimized geometry and to calculate the zero-point vibrational energy (ZPVE) corrections, analytical frequency calculations have been performed at the same level of theory as in the optimization. The minima were found to have only positive frequencies and transition structures were found to have only one negative frequency with the vibrations in the proposed reaction coordinates. The intrinsic reaction coordinate calculations were also performed to ensure that the transition structure connects reactants and products (Gonzalez and Schlegel, Reference Gonzalez and Schlegel1989). Only relative energies (including ZPVE) are reported in this study. The sum of the energies of reactants in each step has been taken as the reference for calculating relative energies. Our chosen chemical model viz. UMP2/6-311+(2d,p)//UM062X/6-311+(2d,p) has been benchmarked against the UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ level of theory by studying a reaction CH3CH2CH2 (propyl radical) → [TS] → CH3CHCH3 (isopropyl radical) in the gas phase at both levels of theories (see Fig. 1). This reaction occurs on the doublet reaction surface and produces a radical centre on the secondary carbon which is important for the formation of i-PrCN. Thus, it can represent the reaction systems discussed in the subsequent sections. The benchmarking calculations show that the enthalpy of formation and barrier height of this reaction at UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ is differed by nearly 2 and 1.1 kcal mol−1 only from the corresponding UMP2/6-311++(2d,p)//UM062X/6-311+(2d,p) energy values. This small energy difference is still important in the deep cloud regions of space such a small error in energy is less relevant when the reaction is being studied in the hot cores of molecular clouds such as SgrB2 (N) where temperature can easily go up to 300 K (Schmiedeke et al., Reference Schmiedeke, Schilke, Möller, Sánchez-Monge, Bergin, Comito, Csengeri, Lis, Molinari, Qin and Rolffs2016). Therefore, we urge that the UMP2/6-311++(2d,p)//UM062X/6-311+(2d,p) level of theory is accurate enough to simulate the chemistry in the SgrB2 (N). Hence, all the calculations in the current study are reported at the UMP2/6-311++(2d,p)//UM062X/6-311+(2d,p) level of theory.
Formation of isopropyl radical from propyl radical via proton transfer from the central CH2 to terminal CH2 studied at UCCSD(T)/aug-cc-pVTZ//UMP2/aug-cc-pVTZ (black) and UMP2/6-311++G(2d,p)//UM062X/6-311++G(2d,p) (red).
All the reported reactions were studied in the gas phase as well as in the interstellar ice. The ice model in the current study is built by arranging water molecules near one of the reactants under consideration such that water molecules can form hydrogen bonds among themselves and with the reactant. This initial structure is then optimized at the UMP2/6-311++(2d,p)//UM062X/6-311+(2d,p) level of theory. The second reactant is introduced after the optimization of the first reactant–ice structure. This reactant–ice complex represents the icy grain in which radicals are generated due to the interaction of grain and high energy cosmic rays. The ice complex contains up to 11 water molecules. This ice model is large enough to represent the nearest neighbour interactions between reactants and ice. Only exothermic reactions are reported in the current study since under the energy-starved conditions of ISM no external energy is available for endothermic reaction to be feasible.
Results and discussion
The current study is focused on those reactions in which some previously detected, less complex cyanides react with the small hydrocarbons to form large and more complex cyanides (linear and isopropyl cyanides in the present context). In order to understand the formation of n-PrCN and i-PrCN via the above-mentioned molecules/radicals, the radical–radical and radical–molecule reactions have been studied using CH2, CH3, CN, CH2CN, C2H4, atomic hydrogen, etc. as reactants. All of these molecules are plentiful in the ISM and produced frequently in the interstellar ice when it is exposed to the cosmic rays (see Section ‘Abundance of reactants in the SgrB2 (N)’). Therefore, they are important ice grain species. In the current study, the reactions were first studied in the gas phase and then in the ice in order to understand the catalytic effect of ice. All the proposed reaction schemes are listed in Table 1 and related energy level diagrams are shown in Figs 2–6. The results for the proposed pathways are discussed in separate sub-sections followed by the comparison of their feasibility.
Schematic potential energy surfaces for reaction scheme 1 in the gas phase and in the ice (all relative energies are given in kcal mol−1).
Schematic potential energy surfaces for indicating all the reactants, products and transition states (TS) for reaction scheme 2 in the gas phase and in the ice (relative energies are given in kcal mol−1).
Schematic potential energy surfaces for reaction scheme proposed for the formation of i;-PrCN from vinyl cyanide in the gas phase and in the ice (all relative energies are given in kcal mol−1).
Schematic potential energy surfaces for reaction scheme 3 in the gas phase. Relative energies are given in kcal mol−1. Reaction 3 is not possible in the ice phase due to the reason stated in the sub-section ‘Reaction systems 3 and 4’.
Schematic potential energy surfaces for reaction scheme 4 in the gas phase and in the ice (relative energies are given in kcal mol−1).
Abundance of reactants in the SgrB2 (N)
The reactants used in the current study are abundant in the ISM. Methyl radical (CH3) is an important species found as an impurity in interstellar water ice. Nearly 190 formation pathways for CH3 radical are listed in the UMIST astrochemistry database, the majority of which include either charge transfer from methyl cation (CH3+) by another grain surface species or photo-dissociation of methane followed by the dissociative recombination (DR) (CH4 + hv → CH3+ + H and CH3+ + e− → CH3). Methyl radical (CH3) was first detected by Feuchtgruber (Reference Feuchtgruber2000) towards the galactic centre in Sgr A with a column density of 8 × 1014 cm−2. The methyl cation is not only responsible for the formation of methyl radical, but also for methylene radical (CH2) as it is formed by the DR of CH3+ (Hollis et al., Reference Hollis, Jewell and Lovas1995). Thus, methylene (CH2) has been detected in a similar abundance of CH3+ (Polehampton et al., Reference Polehampton, Menten, Brünken, Winnewisser and Baluteau2005). C2H4 and C2H2 are also formed from the radical recombination of CH2 and H, both of which are present in copious amounts in the ISM (; Betz, Reference Betz1981). Thus, all the hydrocarbon radicals and hydrocarbons involved in the current study can be formed from photo-dissociation of methane or from the sequential addition of atomic hydrogen to a carbon atom present in the cosmic rays. This procedure of hydrocarbon formation is experimentally confirmed by Bennett et al. (Reference Bennett, Jamieson, Osamura and Kaiser2006). Another important radical viz. cyanomethyl radical (CH2CN) is abundant in the circumstellar envelopes of IRC + 10216, TMC-1 and SgrB2 molecular cloud with column density ranging from 2 × 1013 to 1 × 1014 cm−2 in various sources (Agúndez et al., Reference Agúndez, Fonfría, Cernicharo, Pardo and Guélin2008). It is formed by the reaction CH3+ + HCN → CH3CNH+, followed by the DR of the ion CH3CNH+ (Agúndez et al., Reference Agúndez, Fonfría, Cernicharo, Pardo and Guélin2008). Cyanocarbene (HCCN) is a radical of allenic structure which was first detected in IRC + 10216, Orion A and in SgrB2 by Guélin and Cernicharo (Reference Guélin and Cernicharo1991) with a column density nearly 5 × 1012 cm−2. It is an important precursor for the formation of interstellar prebiotic molecules and appears in several other studies in the context of interstellar chemistry (Gupta et al., Reference Gupta, Tandon, Rawat, Singh and Singh2011; Majumdar et al., Reference Majumdar, Das, Chakrabarti and Chakrabarti2013). This radical is mostly formed when small hydrocarbons react with the N(2D) present in the cosmic rays (Hebrard et al., Reference Hebrard, Dobrijevic, Pernot, Carrasco, Bergeat, Hickson, Canosa, Le Picard and Sims2009). Therefore, all the considered reactants are well abundant in the SgrB2 and can be used to study chemistry in SgrB2.
Reaction systems 1 and 2
Reaction system 1 is a radical addition reaction between cyanomethyl radical (CH2CN) and the methyl radical (CH3) while reaction 2 is a radical–molecule reaction between C2H4 and CN radicals. The only product of reaction 1 is ethyl cyanide (CH3CH2CN) while the primary product of reaction 2 is vinyl cyanide (C2H3CN). Ethyl cyanide produces in a single step in reaction 1. The addition of the two radicals is barrierless and exothermic with reaction energy −92.31 and −120.8 6 kcal mol−1 in the gas phase and the ice, respectively (see Fig. 2). This addition step is spontaneous since both the radicals have single unpaired electrons and are therefore very reactive. The sharing of these unpaired electrons results in the formation of a bond between the carbon of CH3 and CH2CN, thus forming CH3CH2CN.
The first step of reaction 2 (addition of C2H4 and CN radicals) is also barrierless and exothermic in the gas and ice phases. The addition of CN radical to the unsaturated hydrocarbon in the gas phase is studied experimentally by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) and Gannon et al. (Reference Gannon, Glowacki, Blitz, Hughes, Pilling and Seakins2007). They also deduce that the addition of multiple bonds to CN is barrierless. Reaction 2 occurs by sharing of an unpaired electron of CN radical with the π electron cloud of C2H4 and yields a (doublet) radical (CH2CH2CN). The reaction energy for this step is −66.39 kcal mol−1 with respect to the separate reactants in the gas phase (see Fig. 3). The gas-phase reaction energy for this addition is calculated by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) and is about −55.502 kcal mol−1. The difference between the reaction energy calculated by us and Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) is due to the difference in the theory level and basis set employed for calculations. Radical CH2CH2CN can further dissociate via hydrogen abstraction (from one of the carbons of CH2CH2 moiety) and yield vinyl cyanide (C2H3CN) as a stable product (step 1 gas phase, Fig. 3). This dissociation has an entrance barrier of about 45.240 kcal mol−1 with respect to CH2CH2CN but this barrier is submerged (about −21.15) when compared to the total energy of the reactants. In study by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000), the gas-phase entrance barrier is about −18.90 for the formation of vinyl cyanide (C2H3CN) and hydrogen atom which is close to our computed value. Due to the submerged barrier, they propose that reaction will take place spontaneously as sufficient energy is available to the radical CH2CH2CN from the internal energy of the reactants to overcome the entrance barrier. According to Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000), a part of radical CH2CH2CN undergo a [1, 2] H atom shift and form CH3CHCN radical with barrier −32.54 kcal mol−1 and reaction energy −68.42 kcal mol−1 with respect to the total reactant energy. CH3CHCN radical quickly reduces to vinyl cyanide (C2H3CN) and a hydrogen atom. When we studied this reaction channel in the gas phase, we observe a submerged barrier of about −33.95 kcal mol−1 and reaction energy of about −74.17 kcal mol−1 with respect to the reactant energy for the formation of CH3CHCN radical. The CH3CHCN radical eventually decays to the cyanide (C2H3CN) and a hydrogen atom.
Now when this reaction (i.e. C2H4 + CN) is studied on the surface of the water ice, the calculated entrance barrier for the formation of vinyl cyanide (C2H3CN) and a hydrogen atom is submerged by −22.59 kcal mol−1 and has reaction energy −39.72 kcal mol−1 relative to the reactant energy. As such, this dissociation is efficient in both the gas and ice phase. We also studied the [1, 2] H atom shift in the radical CH2CH2CN in the ice phase which has a submerged entrance barrier of about −32.57 kcal mol−1, very close to the corresponding gas-phase value. When any of these radical (CH2CH2CN/CH3CHCN) form ice where hydrogen atoms are easily accessible, they can have a chance to be hydrogenated and form ethyl cyanide (see Fig. 3: ice reaction, step 2a/b). Hydrogenation of both these radicals is barrierless and exothermic with reaction energies −96.45 and −108.68 kcal mol−1, respectively. Therefore, the two likely outcomes of reaction 2 in ice are vinyl and ethyl cyanide.
The ethyl cyanide produced via the two above-mentioned reactions can subsequently react with the CH2 radical present either in the gas phase or in ice to produce linear and branched isomers of propyl cyanide. This reaction is initiated by the nucleophilic attack of CH2 on one of the hydrogens of either terminal CH3 group or CH2 group (next to CN) of ethyl cyanide (see Fig. 2, steps 2a and 2b, respectively). The nucleophilic attack will break the CH bond of the CH3 (or CH2) group of ethyl cyanide. Free hydrogen generated from this bond dissociation will add to CH2 and form CH3 radical. This radical will further add to the vacant radical centre created by CH bond dissociation and yield n-PrCN (or i-PrCN). The gas-phase entrance barriers for these radical–molecule reactions are 17.02 and 15.50 kcal mol−1, respectively, while the barriers reduce to 12.05 and 13.81 kcal mol−1 in the ice. Here, reaction energies are ΔH = −93.37 and −96.45 kcal mol−1, respectively, in the gas phase and −96.45 and −108.68 kcal mol−1 in the ice. Therefore, these reactions are more efficient in the ice than in the gas phase. The reason for the greater efficiency in ice can be understood by looking at the partial charge distribution on each atom of the reactants in the gas phase and in ice. Mulliken charges on each hydrogen atom connected to the CH3 and CH2 groups of ethyl cyanide are 0.16 and 0.18 esu in the gas phase, which increase to 0.19 and 0.20 esu, respectively, when ethyl cyanide forms and embeds itself in ice. Thus, the nucleophilic nature of hydrogen atoms of ethyl cyanide increases when it is formed in the ice. At the same time, a large negative charge of −0.32 esu is present on the carbon of CH2 radical. This induces a strong electrostatic interaction of both moieties and hence nucleophilic attack occurs by this CH2 on the hydrogen atoms of CH3 and CH2 groups of ethyl cyanides. This explains the lower entrance barrier of reaction 2: step 2a/b in the ice compared to their gas-phase counterparts. Reaction 2 is efficient in the hot cores of molecular clouds only where temperature can reach up to ~ 200 K (Schmiedeke et al., Reference Schmiedeke, Schilke, Möller, Sánchez-Monge, Bergin, Comito, Csengeri, Lis, Molinari, Qin and Rolffs2016). At this temperature, there is sufficient energy available to overcome the entrance barrier of about 12–15 kcal mol−1 (Woon, Reference Woon2002; Singh et al., Reference Singh, Shivani, Tandon and Misra2018). However, that much energy is seldom present in the deep and cooler regions of any molecular cloud. Therefore, barriers in reaction 2: step 2a/b forbids the reactions in the deep regions of the cloud SgrB2, which could be the reason why no large nitrile has been detected there to date.
On the other hand, vinyl cyanide produced in reaction 2 can form i-PrCN via hydrogenation followed by the barrierless and exothermic CH3 addition as shown in Fig. 4. This route forms the most viable route for i-PrCN formation in ISM from small cyanide. Vinyl cyanide is present in a large quantity of nearly 3.8 × 1016 cm−2 in ISM (Müller et al., Reference Müller, Belloche, Menten, Comito and Schilke2008). It plays an important role in the formation of many other complex interstellar molecules. The hydrogen addition has a barrier of about 15.63 kcal mol−1 in the gas phase and 16.19 kcal mol−1 in the ice phase. This is again a penetrable entrance barrier in hot cores of SgrB2. Due to the importance of vinyl cyanide for the formation of the branched isomer viz. i-PrCN, in the following sub-section; we propose two pathways for its formation in ISM.
Reaction systems 3 and 4
Reaction 3 is a barrierless radical addition between HCCN and CH2. This addition forms vinyl cyanide (C2H3CN) in a single step (Fig. 5). Reaction 3 has been previously studied by Shivani et al. (Reference Shivani and Tandon2014) and Shivani and Tandon (Reference Shivani and Tandon2014) where they also observed the reaction to be barrierless with an enthalpy of formation of about −52.89 kcal mol−1 at B3LYP/6-311++G(d,p) and −51.78 kcal mol−1 at MP2/6-311++G(d,p) levels of theory. At the UMP2/6-311+(2d,p)//UM062X/6-311+(2d,p) level of theory, the enthalpy of formation for reaction 3 is calculated to be −176.958 kcal mol−1. When HCCN adsorbs onto the ice, it quickly gets hydrogenated, and therefore, it is destroyed before reacting further with CH2. Due to this reason, reaction 3 can occur only in the gas phase.
Reaction 4 is a barrierless radical–molecule addition of CN radical to C2H2. The first (addition) step of this reaction produces a radical C2H2CN which, then reduces to the HCCCN after leaving a hydrogen atom. The entrance barrier of hydrogen abstraction is submerged with energy −10.54 kcal mol−1 relative to the total energy of reactants and it is exothermic with reaction energy −43.53 kcal mol−1 (see Fig. 6, step 1, gas phase). Thus primary products of the gas phase reaction are HC3N and H atom. This reaction is previously studied in the gas phase by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) and by Woon and Herbst (Reference Woon and Herbst1997). Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) studied this reaction in the gas phase employing crossed molecular beam experiments and perform some limited quantum chemical calculations at the B3LYP/6-311+G(d,p) level of theory. They also propose a barrierless addition of CN and C2H2 and but the calculate reaction energy for the formation of the HCCCN and H was about −22.49 kcal mol−1 and the height of the activation barrier about −15.55 kcal mol−1. The difference between the values calculated by us and by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) is due to the difference in the theory level employed for calculating the electronic structure. We have employed a comparatively recent and more accurate functional M062X along with the single-point calculations at the MP2 level of theory which provides more accurate energies. According to Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000), a part of initial radical intermediate C2H2CN undergo a hydrogen transfer from central carbon to the terminal carbon and thus forms CH2CCN which also decay to the HCCCN and H. We studied this hydrogen transfer in the gas phase which has an entrance barrier of about 40.945 kcal mol−1 relative to the C2H2CN which is close to the value calculated by Balucani et al. (Reference Balucani, Asvany, Huang, Lee, Kaiser, Osamura and Bettinger2000) viz. 42.35 kcal mol−1. However, our calculated barrier (40.95 kcal mol−1 relative to the C2H2CN) is submerged with energy −17.32 kcal mol−1 with respect to the reactant energy.
According to the study by Bennett et al. (Reference Bennett, Jamieson, Osamura and Kaiser2006), C2H2 can be produced in the ice containing CH4 and thus CN can easily add to C2H2 adsorbed in the ice. Therefore, reaction 4 can occur in the gas phase as well as in the ice phase. The reaction proceeds in the same way as in the gas phase, however, we observe a slight change in the energetics of the ice-phase reaction. The formation of HC3N by hydrogen abstraction of C2H2CN has a submerged entrance barrier of −16.82 kcal mol−1 and reaction energy of about −49.13 kcal mol−1 in the ice phase (step 1, Fig. 6, ice phase). However, in contrast to the gas phase reaction, we could not locate any entrance barrier for the formation of CH2CCN in ice. The reason for the absence of the barrier in our calculations could either the absence of a transition state or the presence of a transition state with a very small entrance barrier. In both cases, the formation of CH2CCN in ice is more favourable than in the gas phase. CH2CCN also eventually decay to the HCCCN and H. Similar to step 2a/b of reaction 2, there is a possibility of hydrogenation of radical C2H2CN as well as radical CH2CCN both of which yields vinyl cyanide (see Fig. 6, step 2a/b). Hydrogenation of both radicals in ice (C2H2CN and CH2CCN) is barrierless and exothermic with the enthalpies of the formation being −165.85 kcal mol−1 for C2H2CN and −88.60 kcal mol−1 for CH2CCN. As such, although the most likely outcome of reaction 4 is HC3N; however, there is still a possibility for the formation of CH2CHCN if the reaction occurs in the ice phase.
Astrobiological significance and conclusions
i-PrCN and its linear counterpart n-PrCN are very complex molecules by astrochemistry standards. The formation of molecules of such complexity is thought to be next to impossible in the ISM to date. But the detection of i-PrCN and n-PrCN tells that our understanding of the chemistry of ISM is severely limited. As such, it is important to understand the chemistry prevails in the ISM which leads to the formation of such complex molecules. In the earlier paper, Belloche et al., (Reference Belloche, Garrod, Müller, Menten, Comito and Schilke2009) proposed the idea of systematic growth of small cyanide towards the more complex one in both gas phase and interstellar ice for the formation of n-PrCN and later for the formation of both i-PrCN and n-PrCN (Belloche et al., Reference Belloche, Garrod, Müller, Menten, Comito and Schilke2009). This, together with his cloud model produces the incorrect i-PrCN : n-PrCN ratio. He suggests less accurate rate constant for gas phase and ice phase reactions as well as fewer reactions in their starting reaction set were to be the primary reason for this inconsistency between calculated versus observed abundance of i-PrCN and n-PrCN. Although Garrod et al. (Reference Garrod, Belloche, Müller and Menten2017) were able to reproduce the observation with the same cloud model, they used an updated reaction network and rate coefficients. This proves the importance of a large and accurate set of reactions in cloud modelling. The quantum chemical approach facilitates us to accurately study the reactions in the interstellar context.
The importance of cyanide chemistry, as well as their astrobiology significance as an amino acid precursor, has already been emphasized in the Introduction. Therefore, it is important to understand their chemistry in the context of the ISM. Our study is dedicated to understanding the formation pathways of the largest cyanides in the ISM. Based on the study of previous research group, our current study utilizes the same approach used by Belloche et al. (Reference Belloche, Garrod, Müller and Menten2014) and Garrod et al. (Reference Garrod, Belloche, Müller and Menten2017) in their research to study the formation of i-PrCN and n-PrCN, i.e. evolution of small cyanides towards the more complex one. However, we have used a different set of formation pathways leading to i-PrCN and n-PrCN and used accurate quantum chemical calculations to study these pathways. As such, our study of new reaction pathways for the formation of i-PrCN and n-PrCN with quantum chemical approach provides information about some additional unexplored roots for the formation of the large and complex molecules in ISM in general and i-PrCN and n-PrCN in particular. The current study also proposed new formation roots for ethyl cyanide, vinyl cyanide as well as the formation of i-PrCN form it. Both ethyl and vinyl cyanide are considered precursors of many other prebiotic molecules. Our research studied four reaction schemes (Table 1) in the gas phase as well as at the surface of interstellar water ice. The following conclusions may be drawn from this study.
1. It is observed that in the hot cores of SgrB2 (N), both n-PrCN and i-PrCN can be formed via nearly all the proposed reactions (from ethyl and vinyl cyanide). However, the formation of n-PrCN and i-PrCN from ethyl and vinyl cyanide has a sufficiently large activation barrier which is penetrable in the hot cores of molecular clouds only, thus forbids the reaction under deep cold cloud conditions.
2. Ethyl and vinyl cyanides are the most important precursors for the n-PrCN and i-PrCN, respectively. Therefore, their abundance is directly related to the abundance of n-PrCN and i-PrCN.
3. Reaction 2 can produce both precursors (ethyl and vinyl cyanides) if a reaction occurs in the ice but will only produce vinyl cyanide in the gas phase; however, reaction 4 can produce vinyl cyanide in the ice only while in the gas phase, it will not contribute towards formation of ethyl and vinyl cyanide.
4. In general, the reactions on the surface of ice are more efficient than their gas-phase counterparts. Also, since most of the ethyl and vinyl cyanide will produce in the ice phase, thus, ice reactions will contribute more significantly towards the formation of both isomers.
Acknowledgement
Financial support to K. K. Singh and P. Tandon from the Indian Space Research Organization (ISRO) under RESPOND project (grant no. ISRO/RES/2/386/15-16) and to A. Misra and Shivani from the Council of Science and Technology, Uttar Pradesh (CST, U.P.) major research project CST/8324, is gratefully acknowledged. We thank Dr Debraj Gangopadhyay, University of Lucknow and D. E. Woon, the University of Illinois Urbana-Champaign for their valuable suggestions. All the ab initio calculations in the current study have been performed with the help of the Central Facility for Computational Research (CFCR) at the Department of Chemistry, University of Lucknow.
