Some new reaction pathways for the formation of cytosine in interstellar space – A quantum chemical study

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Advances in Space Research 51 (2013) 797–811 www.elsevier.com/locate/asr

Some new reaction pathways for the formation of cytosine in interstellar space – A quantum chemical study V.P. Gupta ⇑, Poonam Tandon, Priti Mishra Department of Physics, University of Lucknow, Lucknow 226007, India Received 19 July 2012; received in revised form 26 September 2012; accepted 2 October 2012 Available online 10 November 2012

Abstract The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth. Small N-heterocycles, including pyrimidine, purines and nucleobases, have been extensively sought in the interstellar medium. It has been suggested theoretically that reactions between some interstellar molecules may lead to the formation of cytosine, uracil and thymine though these processes involve significantly high potential barriers. We attempted therefore to use quantum chemical techniques to explore if cytosine can possibly form in the interstellar space by radical–radical and radical-molecule interaction schemes, both in the gas phase and in the grains, through barrier-less or low barrier pathways. Results of DFT calculations for the formation of cytosine starting from some of the simple molecules and radicals detected in the interstellar space are being reported. Global and local descriptors such as molecular hardness, softness and electrophilicity, and condensed Fukui functions and local philicity indices were used to understand the mechanistic aspects of chemical reaction. The presence and nature of weak bonds in the molecules and transition states formed during the reaction process have been ascertained using Bader’s quantum theory of atoms in molecules (QTAIMs). Two exothermic reaction pathways starting from propynylidyne (CCCH) and cyanoacetylene (HCCCN), respectively, have been identified. While the first reaction path is found to be totally exothermic, it involves a barrier of 12.5 kcal/mol in the gas phase against the lowest value of about 32 kcal/mol reported in the literature. The second path is both exothermic and barrier-less. The later has, therefore, a greater probability of occurrence in the cold interstellar clouds (10–50 K). Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Cytosine; Interstellar space; Reaction mechanism; Density functional theory; Chemical reactivity; QTAIM

1. Introduction Nitrogen containing cyclic organic molecules (N-heterocycles) play an important role in terrestrial biology, for example, as nucleobases in genetic material. Nucleobases can be divided into two groups according to their molecular structure: the purine derivatives (adenine, guanine, xanthine and hypoxanthine) and pyrimidine derivatives (cytosine, thymine and uracil). A number of theories propose that RNA, or an RNA-like substance played a role ⇑ Corresponding author. Tel.: +91 9451387136/5222735874.

E-mail addresses: [email protected], [email protected] (V.P. Gupta), [email protected] (P. Tandon), pritimishra60@ gmail.com (P. Mishra).

in the origin of life. In the great majority of these theories, Watson–Crick pairing of A with U and of G with C is retained as the basis of genetic template recognition. These suggestions presume that the bases adenine, cytosine, guanine and uracil were readily available on the early Earth (Joyce et al., 1987; Spach, 1984; Shapiro, 1999; Orgel, 1986; Cherny et al., 1993; Nielsen, 1993; Pitsch et al., 1995; Ehrenfreund and Charnley, 2000). Some of the biologically important molecules and their precursors have been detected in circumstellar envelopes, interstellar molecular clouds, comets and meteorites. The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth (van der Velden and Schwartz, 1977; Stoks and Schwartz, 1979, 1981). Carbonaceous chondrites appear to contain

0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.10.005

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several classes of N-heterocyclic compounds including purines, pyrimidines, quinolines etc. However, some of the stable isotopes of these compounds that may determine their extra terrestrial origins are yet to be reported (Peeters et al., 2005). One of the major ongoing questions in current science is whether nucleic acid bases and biomolecules were originally produced terrestrially or extra-terrestrially. Small N-heterocycles, including pyrimidines, purines and nucleobases, have been extensively sought in the interstellar medium. To be considered as prebiotic, a synthesis should be based on chemicals and on experimental conditions that are reasonably coherent with those prevailing at the time of origin of life. The prebiotic synthesis of cytosine reported so far involves the reaction of cyanoacetylene and its hydrolysis product cyanoacetaldehyde (Ferris et al., 1968; Robertson and Miller, 1995a,b) with cyanate, cyanogens and urea. The prebiotic availability of cyanate is undetermined. It can be produced by the hydrolysis of cyanogen (Peeters et al., 2005) and related nitriles. Cyanogen is a simple substance that may be widely distributed elsewhere in the universe, as it has been detected in the atmosphere of Titan (Sagan et al., 1992). Sanchez et al. (1966) reported that aqueous solution of OCN and HCCCN produced cytosine, a component of DNA. Since OCN ion can be made through the reaction H2O + HCCCN, the formation of cytosine would not be unreasonable in this mixture. The cyanate ion (OCN) is also a stable radiation product in any Titan region having both nitriles and H2O-ice (Hudson and Moore, 2004). Amino acids or cytosine made at too low temperatures by either cosmic rays bombardment or far UV photons would be available for injection into the gas phase or, in the case of comets, delivery to the early Earth (Hudson and Moore, 2004). Saladino et al. (2004) report the formation of cytosine and 4-hydroxy pyrimidine from formamide (HCONH2) when heated to 160o C in the presence of Zeolite (of Y-type) which acts as catalyst. Construction of purines and pyrimidine scaffolds were probably generated in situ by formamide decomposition. Since 4-hydroxypyrimidine is found in the Murchison meteorite in appreciable amounts (Folsome et al., 1973) it is believed that it was formed from formamide. The work of Saladino et al. (2004) appears to be the first example of the prebiotic synthesis of cytosine starting from one carbon atom precursor as simple as formamide. While the hitherto reported sources of cytosine formation involve reaction between cyanoacetylene and cyanoacetaldehyde with cyanate, cyanogen, guanidine and urea, the presence of several reactive molecules like NH, NH2, isocyanic acid (HNCO) and propynylidyne (CCCH), formamide (HCONH2) etc. in the interstellar space open new possibilities of formation of cytosine. Chemically, the NH2 radical is of considerable interest for testing the production pathways of nitrogen bearing molecules. The NH2 radical was first detected by van Dishoeck and Jansen (1993) in the interstellar cloud towards Sgr B2 (N) and Sgr

B2 (M). It may also lead to the formation of the interstellar molecule NH. In the interstellar medium, NH was discovered in the diffuse clouds towards the Zeta Per and HD 27778 interstellar clouds; the NH columns densities are 9.0  1011 and 2.7  1012 /cm2, respectively. Wagenblast et al. (1993) have suggested that within diffuse clouds H + N!NH + e is a major mechanism of formation of NH with rate constant 1.0  109. The HNCO molecule, isocyanic acid, is the simplest chemical species to posses the four main biogenic elements. It has been observed in cometary coma (Bockele´e-Morvan et al., 2000), interstellar medium (Buhl et al., 1973) and in external galaxies (Nguyen-Q-Rieu et al., 1991). It is likely to be a parent of OCN ion associated with the 4.62 lm absorption feature in interstellar ices (Hudson and Moore, 2004; Grim and Greenberg, 1987). HNCO is observed in Sgr B2 with a tentative detection in W51 (Snyder and Buhl, 1972, 1973) and in Orion A (Johansson et al., 1984). The propynylidyne (C3H) radical is an important reaction product in the prototypical reaction implicated in the growth of carbon chains in interstellar clouds and circumstellar envelopes. The linear C3H was first produced and studied by microwave spectroscopy in the laboratory (Gottlieb et al., 1985) and subsequently identified in the space the same year (Thaddeus et al., 1985). In the circumstellar envelopes of evolved stars, C3H is found to exist near the envelope edge (Lucas et al., 1995). Deep-space is known to have ultracold temperatures between 10 and 20 K while the hot molecular cores have temperatures between 100 and 200 K. In an effort to understand the deep-space glycine formation in ice-water dust mantles, Rimola et al. (2010) calculated the relative free energies in different reaction paths at 10, 100 and 200 K by DFT methods. It was noted that at higher temperatures the computed energy barriers and reaction energies increase slightly by about 2 kcal/mol at the most. This was explained as being mainly due to entropic effects since the computed relative enthalpy values differ only by 0.1– 0.3 kcal/mol with respect to the zero-point vibrational energy (ZPVE) corrected values. A similar situation might be expected in the formation of cytosine. In a recent study, Wang and Bowie (2012) explored from theoretical considerations the possibility of formation of cytosine and uracil in the interstellar region and considered reactions between CCCNH and CCCO with urea and monosolvated urea. Though exothermic, these reactions involve quite significant potential barriers between 32 and 39 kcal/mol; the barrier with monosolvated urea being higher than with urea. Very low particle density in combination with very small rate coefficients for activation energies P1 eV at very low temperatures (about 10 K) may not allow these reactions in the interstellar media to happen. Herbst (2001) has pointed out that the processes in gaseous phase obeying Arrhenius law that do not occur efficiently include all endothermic reactions and most exothermic reactions and rules out all possible processes with an activation energy. Park and Woon (2004a,b) have, however,

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shown that within a matrix of water ice certain reactions having gas phase barriers of 30 kcal/mol can also occur. As such, there is a need to search for other viable reactions for the formation of cytosine which may either be barrierless or have very small barriers not exceeding a few kcal/ mol. In the present communication we are reporting the results of density functional theory (DFT) calculations used to explore the possibility of cytosine formation in the interstellar space starting from some of the simpler molecules detected there. Radical–radical and radicalmolecule interaction schemes have been adopted in the study. Barrier-less reactions and those having very low activation energy are being reported. 2. Computational details Computations were performed based on the B3LYP density functional theory with the 631G** and 6311+G** basis set. All calculations were carried out with the Gaussian 09 W program. Total energies, zero point vibrational energy (ZPVE), electronic energies (EE) and reaction energies of all molecules/radicals involved in the formation of cytosine were calculated. Electronic energies corrected for zero point vibrational energies (ZPVE) and the reaction energies and thermodynamic parameters are reported in Tables 1 and 4 and Appendices A and B. All of the relative energies are corrected by the ZPVE and correspond to the classical reaction barriers (i.e., without proton tunneling). In agreement with Rimola et al. (2010), our preliminary studies show that temperature has very small affect on relative enthalpy and Gibbs free energy to influence our findings. The values of these parameters in Tables 1 and 4 are, therefore, given for room temperature. The reaction barriers in these tables are defined as the difference in sum of electronic and zero-point energies of the reactant complex and transition state and the relative free energy DG is defined as difference between the sum of electronic

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and thermal free energies of reactant and product. Selfconsistent reaction field (SCRF) calculations using the Polarizable continuum model (PCM) as implemented in the Gaussian 09 program was used to include the bulk solvation effect of the medium such as water ice. The PCM “bulk solvent medium” is simulated as a continuum of the dielectric constant e (=78.5) which surrounds a solute cavity, defined by the union of a series of interlocking spheres centered on the atoms. Instead of using single point calculations at the gas phase optimized geometries as reported in the literature (Roy et al.,2007), the polarizable continuum model (PCM) was used to reoptimize the gas phase geometries to obtain ZPVE-corrected reaction energies and thermodynamic parameters. This provides a proper reference to condensed phase reactions. Harmonic frequency calculations were conducted to identify the stable and transition states and the intermediate reaction coordinate (IRC) calculations were used to verify reaction pathways. In order to understand the reaction mechanism and for selecting the most probable pathway of reactions, the global and local reactivity indices such as global philicity, hardness and softness, and local Fukui functions (f k+, f k, f k0) and local philicity (xk+, xk, xk0) were calculated in Koopman´s approximation by standard formula given by Chattaraj et al. (2003, 2006) and Padmanabhan et al. (2007a,b). The Fukui functions (f k+, f k, f k0) and local philicity (xk+, xk, xk0) (Padmanabhan et al., 2007a,b) are local reactivity descriptors that indicate the preferred regions where a chemical species will change its density when the number of electrons are modified. It, therefore, indicates the propensity of the electron density to deform at a given position upon accepting or donating electrons. To obtain these indices, we realized single-point calculations of the anion and cation at the optimized geometry of the neutral molecule. For the ionic structures, the single-point calculations were conducted using the open-shell theory to obtain results without spin contamination. The

Table 1 Computed relative energies DE0 (corrected for ZPVE), reaction energies and activation barriers for the formation of cytosine from propynylidyne (CCCH) in gas phase and in water solvent (PCM). Molecule numbers (in bold letters) correspond to Fig. 2. All energies are in kcal/mol. S. No.

Reaction steps

Gas phase

1 2 3 4 5 6 7 8 9 10

HCCC + NH2 ! 1 1 + NH ! 2 2 + HNCO ! TS1 TS1 ! 3 3 ! TS2 TS2 ! 4 4+H!5 5 + NH ! 6 5 + OH ! 7 5 + NH2 ! 8

DH*

Relative energies DE0

DG*

PCM

Gas phase

631G**

6311+G**

631G**

631G**

6311+G**

Gas phase 631G**

6311+G**

102.2 108.7 10.2 0.3 5.7 39.5 79.4 61.9 89.7 78.1

101.7 106.5 12.5 0.2 7.2 37.3 78.1 59.6 86.3 75.2

96.9 106.5 0.3 3.5 11.0 39.5 74.9 63.0 100.7 75.8

104.2 110.0 9.7 0.5 5.2 39.9 79.2 61.9 90.7 79.1

103.9 107.6 11.8 0.4 6.7 37.8 77.9 59.4 88.8 72.8

92.5 99.3 20.2 0.4 6.6 39.5 79.0 54.8 80.5 68.6

92.0 97.2 22.7 0.5 8.2 36.9 78.3 54.1 79.1 65.0

* DH is defined as difference between the sum of electronic and thermal enthalpies and DG as difference between the sum of electronic and thermal free energies of reactant and product.

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Table 2 Calculated global hardness (g), global softness (S) and global philicity index (x), and local reactivity descriptors (fk+, fk, f 0k, xk+,xk and x0k) for reacting species in cytosine formation from propynylidyne (CCCH) using Mulliken population analysis. Molecule numbers (in bold letters) and atomic numbers correspond to Fig. 2. Molecules

HNCO C1 O2 N3 H4 HCCC C1 C2 C3 N4 1 C1 C2 C3 N5 2 C2 C3 N8 TS 1 C1 C2 C3 N8 C 11 N 12 3 C1 C2 N8 C 11 N 12 TS 2 C1 C2 N 12 4 C1 C2 C3 C 11 N 12

Global descriptors

Local descriptors

g eV

S eV

x eV

fk+

4.1625

0.2402

1.9791

0.3742 0.1951 0.500 0.3808

0.2310 0.3114 0.3350 0.1225

0.3026 0.2533 0.1925 0.2516

0.7406 0.3861 0.0989 0.7536

0.4573 0.6164 0.6631 0.2424

0.5989 0.5013 0.3810 0.4980

0.6481 0.3860 0.3018 0.3018

1.2191

0.8202

10.7657

0.3715 0.2255 0.2042 0.1988

0.4180 0.1220 0.2929 0.1671

0.3948 0.1738 0.2485 0.1829

3.9894 2.4217 2.1921 2.1344

4.4884 1.3096 3.1454 1.7943

4.2389 1.8657 2.6687 1.9644

0.1210 0.1290 0.1939 0.1859

1.9868

0.5033

2.7832

0.2756 0.0397 0.2497 0.0617

0.3094 0.0618 0.2566 0.0511

0.2925 0.0507 0.2532 0.0564

0.7669 0.1105 0.6949 0.1717

0.8612 0.1720 0.7143 0.1422

0.8141 0.1412 0.7046 0.1569

0.0100 0.1714 0.3725 0.4988

2.7781

0.3560

1.7873

0.0872 0.1407 0.1788

0.2385 0.0301 0.1557

0.1629 0.0854 0.1673

0.1558 0.2514 0.3196

0.4264 0.0537 0.2782

0.2911 0.1526 0.2989

0.4148 0.0234 0.4791

2.2666

0.4412

2.8226

0.0716 0.0570 0.0570 0.0002 0.0902 0.1823

0.1583 0.0591 0.1650 0.0792 0.0504 0.0105

0.1149 0.0581 0.1110 0.0397 0.0703 0.0859

0.2021 0.1610 0.1609 0.0006 0.2547 0.5144

0.4467 0.1668 0.4656 0.2235 0.1422 0.0297

0.3244 0.1639 0.3133 0.1120 0.1985 0.2424

0.0079 0.4597 0.0529 0.4843 0.6016 0.6051

1.5165

0.6594

3.9818

0.1791 0.0320 0.0610 0.0406 0.0048

0.0722 0.0481 0.0559 0.0990 0.2894

0.1256 0.0080 0.0026 0.0698 0.1471

0.7132 0.1275 0.2428 0.1615 0.0190

0.2874 0.1914 0.2224 0.3940 1.1521

0.5003 0.0320 0.0102 0.2777 0.5856

0.0241 0.4040 0.4577 0.5982 0.6508

1.7868

0.5597

3.7765

0.1425 0.0731 0.0031

0.0165 0.1127 0.2535

0.0630 0.0929 0.1283

0.5380 0.2760 0.0118

0.0622 0.4256 0.9572

0.2379 0.3508 0.4845

0.4778 0.1384 0.6745

1.8894

0.5292

2.1439

0.1381 0.0842 0.1161 0.0461 0.0177

0.1325 0.2301 0.1299 0.0466 0.0056

0.1353 0.1571 0.1230 0.0464 0.0061

0.2961 0.1804 0.2490 0.0988 0.0380

0.2840 0.4934 0.2785 0.1000 0.0120

0.2900 0.3360 0.2638 0.0994 0.0130

0.4693 0.1866 0.0309 0.7292 0.5917

fk

Charge densities fk0

xk+

xk

xk0

Table 3 Geometrical, topological and energy parameters at the bond critical points (BCPs) of molecules and transition states involved in cytosine formation from propynylidyne (CCCH) based on QTAIM calculations. Molecule numbers correspond to Fig. 2. 0

Molecule

Bonds

˚ Bond lengths in A

q(b)

$2q(b)

k1

k2

k3

Eint

3

C2–N12 N8–C11 C2–N12 C11–N8 C1–N12

2.7256 1.5555 2.985 1.8771 2.2767

0.01652 0.20367 0.00973 0.09758 0.0441

0.05233 0.08931 0.03178 0.06607 0.10116

0.0118 0.37 0.0062 0.1384 0.0472

0.01 0.3547 0.0049 0.1364 0.0448

0.07407 0.3674 0.04284 0.34087 0.19316

3.0999 91.208 1.7006 27.203 9.513

TS 1 TS 2

Abbreviations: q(b) – electron density at BCP (a.u); k1, k2, k3 – eigenvalues of the Hessian matrix, and $2q(b) – Laplacian at BCP (a.u), Eint – interaction energy at BCP in kcal/mol.

V.P. Gupta et al. / Advances in Space Research 51 (2013) 797–811 Table 4 Computed relative energies DE0 (corrected for ZPVE), enthalpies and Gibbs free energies for the formation of cytosine from cyanoacetylene in gaseous phase and in water solvent (PCM) based on B3LYP/631G** calculations. Molecule numbers (in bold letters) correspond to Fig. 4. The energies are in kcal/mol. S.No.

1 2 3 4 5 6 7

Reaction steps

HCCCN + OCN! 1 1!2 2!3 3!4 4!5 5!6 6 ! 7 Cytosine + H2

Relative energies DE0

DH*

DG*

Gas phase

Gas phase

Gas phase

PCM

20.0

4.5

20.8

15.5

24.4 105.9 59.3 72.8 37.6 89.6

28.5 100.1 63.7 69.4 38.8 44.1

24.2 107.2 59.2 73.4 37.3 88.2

25.0 96.4 58.6 71.5 37.6 92.1

*

DH is defined as difference between the sum of electronic and thermal enthalpies and DG as difference between the sum of electronic and thermal free energies of reactant and product.

results of calculations are given in Tables 2 and 5. These tables also contain Mulliken charge densities. Quantum theory of atoms in molecules (QTAIM) (Bader, 1990), which is a generalization of quantum mechanics to open quantum systems, has been used to investigate the possible weak bonding features in reaction complexes and transition states and to decide about the possible reaction pathways. Electron charge density q(b) at the bond critical points, eigenvalues (k) of the Hessian matrix at the critical points and the associated Laplacian r2 q(b) were calculated using AIM2000 software (Bader and Cheeseman, 2000). Geometrical as well as topological parameters are useful tools to characterize the strength of hydrogen bonds as well as weaker interactions. The possible bonding features in the transitions state have been shown in molecular graphs in Fig 2. 3. Results and discussion Reactions involving molecules and radicals are mainly of two types; viz. (a) unimolecular reactions like fragmentation and rearrangement and (b) bimolecular reactions between radicals and molecules like addition, displacement, atom abstraction etc. The reactions leading to the formation of cytosine in the interstellar space involve several simple neutral molecules and radicals such as HNCO (Bockele´e-Morvan et al., 2000; Buhl et al., 1973; NguyenQ-Rieu et al.,1991), cyanoacetelyne HCCCN (Bockele´eMorvan et al., 2000; Ziurys, 2006; Coustenis et al., 1999), propynylidyne CCCH (Lucas et al., 1995), NH2 (van Dishoeck and Jansen, 1993), NH (Wagenblast et al.,1993), OCN (Hudson and Moore, 2004) etc. Two reaction pathways starting with interstellar molecules CCCH and cyanoacetelyne as given in the Schemes 1 and 2 are being considered presently.

801

3.1. Reaction of propynylidyne (C3H) with Isocyanic acid (HNCO) Electronic energies corrected for zero point vibrational (ZPVE) energies of molecules, radicals and transition states involved in the formation of cytosine from propynylidyne (CCCH) in the gas phase and in water icy-grains (PCM) based on B3LYP/631G** and B3LYP/6311+G** level calculations are given in Appendix A. The computed relative energies, activation barriers and reaction energies are listed in the Table 1 and the sketch map showing the reaction pathway is given in Fig. 1. The geometries of the reactants, transition states and the products are given in Fig. 2. A number of studies have been reported in the literature on the structure of propynylidyne (CCCH) in the ground electronic state. Thus from a combined experimental and theoretical study on the formation of interstellar C3H-isomers, Kaiser et al. (1996) have reported the existence of both the cyclic and linear forms for the molecule. The most stable isomer on the doublet C3H potential energy surface is a cyclic structure with C2V symmetry. Takahashi (2000) performed QCISD/631G** level calculations and predict three possible isomers for C3H – cyclic (C2V,2B2), linear (C1V, 2p) and an isomer having C2V symmetry with 2A2 electronic ground state. They found that although the cC3H is isoenergetic with l-C3H, the third isomer is much less stable and reported that the cyclic and linear isomers of C3H have almost the same abundance in the interstellar space. The c-C3H isomer is energetically more stable than l-C3H by 2.15–3.11 kcal/mol (Kaiser et al., 1996; Takahashi and Yamashita, 1996; Ochsenfeld et al., 1997). On the basis of ab initio calculations and vibrational and rotational analysis of the gas phase electronic spectrum in the visible region, Ding et al. (2001) concluded that l-C3H may have a linear/bent structure. Using CASSCF calculations with aug-cc-pVDZ/ aug-cc-pVTZ/augcc-pVQZ basis sets and RCCSD (T) calculations with TZP/cc-pVQZ/cc-pVTZ basis sets), they found that l-C3H has a bent geometry at lower basis set but tends to be linear with the use of larger basis sets. Takahashi and Yamashita (1996) and Ochsenfeld et al. (1997) have found on the basis of high level ab initio calculations that the ground state geometry of l-C3H is slightly bent with a rather small barrier to linearity. These authors suggest a slightly distorted linear structure for l-C3H having C–C–C bond angle of about 174°, C–C–H bond angle of about 156°, and the barrier to linearity of 100–200 cm1. The lowest vibrational frequency of C3H is only about 208 cm1 (Kaiser et al. (1996) indicating extreme floppiness of the bent isomer. The present DFT calculations at B3LYP/631G** and B3LYP/6311+G** levels also suggest a bent structure for l-C3H. The angles C–C–C and C–C–H at the B3LYP/6311+G** level are 176.2° and 160.7°, respectively. Thus, our results using density functional theory are in agreement with the findings of Ding et al. (2001),

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Table 5 Calculated global hardness (g), global softness (S) and global philicity index (x), and local reactivity descriptors (fk+, fk, f 0k, xk+, xk and x0k) for reacting species in cytosine formation from cyanoacetylene (HCCCN) using Mulliken population analysis. Molecule numbers (in bold letters) and atomic numbers correspond to Fig. 4. Molecules, involved in reaction pathway OCN C1 O2 N3 HCCCN C1 C2 C3 N4 H5 1 C1 C2 C3 N4 H5 C6 O7 N8 2 C1 C2 C3 N4 H5 C6 O7 N8 H9 3 C1 C2 C3 N4 H5 C6 O7 N8 H9 N10 H11 4 C1 C2 C3 N4 H5 C6 O7 N8 H9 N 10 H 11 H 12 5 C1 C2 C3 N4 H5 C6 O7 N8 H9

Global descriptors

Local descriptors fk +

fk 

Charge densities fk0

xk+

x k

xk0

g eV

S eV

x eV

4.8416

0.2065

2.8072

0.5678 0.1632 0.2690

0.3102 0.3288 0.3611

0.4390 0.2460 0.3150

1.5939 0.4581 0.7551

0.8708 0.9230 1.0134

1.2324 0.6906 0.8843

0.3248 0.6428 0.6820

3.451

0.2897

3.5516

0.2229 0.1558 0.2603 0.1949 0.1661

0.3351 0.1554 0.2016 0.1742 0.1337

0.2790 0.1556 0.2310 0.1845 0.1499

0.7917 0.5533 0.9246 0.6921 0.5898

1.1902 0.5520 0.7159 0.6186 0.4748

0.9910 0.5526 0.8203 0.6553 0.5323

0.4813 0.6530 0.0905 0.4717 0.2095

2.1428

0.4666

0.8226

0.0725 0.2029 0.1417 0.1572 0.1397 0.1769 0.1084 0.0007

0.1313 0.2465 0.1286 0.1720 0.1189 0.0983 0.1028 0.0017

0.1019 0.2247 0.1351 0.1646 0.1293 0.1376 0.1056 0.0012

0.0597 0.1669 0.1165 0.1294 0.1149 0.1455 0.0891 0.0006

0.1080 0.2028 0.1058 0.1415 0.0978 0.0809 0.0845 0.0014

0.0838 0.1848 0.1112 0.1354 0.1063 0.1132 0.0868 0.0010

0.0544 0.1987 0.1941 0.6032 0.0385 0.5491 0.4858 0.4898

1.1172

0.8950

4.1819

0.1209 0.0562 0.1096 0.1423 0.1469 0.1981 0.1164 0.0229 0.1325

0.1304 0.0959 0.1042 0.1487 0.1306 0.1584 0.1229 0.0162 0.1252

0.1256 0.0761 0.1069 0.1455 0.1388 0.1783 0.1197 0.0195 0.1288

0.5054 0.2350 0.4582 0.5949 0.6145 0.8286 0.4867 0.0957 0.5543

0.5452 0.4011 0.4358 0.6217 0.5460 0.6623 0.5140 0.0676 0.5234

0.5253 0.3180 0.4470 0.6083 0.5803 0.7454 0.5004 0.0817 0.5388

0.0209 0.1972 0.2098 0.6155 0.0245 0.4683 0.5126 0.3856 0.0291

1.4696

0.6804

0.1811

0.1300 0.1234 0.1264 0.1645 0.1516 0.0928 0.0737 0.0177 0.0802 0.0319 0.1072

0.1049 0.0844 0.1144 0.1466 0.1189 0.1066 0.1126 0.0059 0.0660 0.0483 0.1032

0.1175 0.1039 0.1204 0.1556 0.1352 0.0997 0.0932 0.0118 0.0731 0.0082 0.1052

0.0236 0.0224 0.0229 0.0298 0.0275 0.0168 0.0134 0.0032 0.0145 0.0058 0.0194

0.0190 0.0153 0.0207 0.0266 0.0215 0.0193 0.0204 0.0011 0.0120 0.0088 0.0187

0.0213 0.0188 0.0218 0.0282 0.0245 0.0181 0.0169 0.0021 0.0132 0.0015 0.0191

0.0516 0.1025 0.1520 0.5922 0.0586 0.5955 0.6430 0.5038 0.2246 0.6195 0.1737

1.9191

0.5211

0.4064

0.1446 0.0734 0.1636 0.1025 0.1356 0.0378 0.0677 0.0209 0.0747 0.0113 0.0717 0.0961

0.0350 0.0468 0.0717 0.0829 0.0773 0.0694 0.1596 0.0067 0.0640 0.2185 0.1031 0.0650

0.0898 0.0601 0.1177 0.0927 0.1064 0.0536 0.1137 0.0138 0.0694 0.1149 0.0874 0.0805

0.0588 0.0298 0.0665 0.0416 0.0551 0.0154 0.0275 0.0085 0.0304 0.0046 0.0292 0.0391

0.0142 0.0190 0.0291 0.0337 0.0314 0.0282 0.0649 0.0027 0.0260 0.0888 0.0419 0.0264

0.0365 0.0244 0.0478 0.0377 0.0433 0.0218 0.0462 0.0056 0.0282 0.0467 0.0355 0.0327

0.0134 0.2200 0.0793 0.7233 0.0657 0.5493 0.6519 0.4751 0.2299 0.6392 0.1620 0.1968

1.2109

0.8258

5.1341

0.1568 0.0035 0.0683 0.1174 0.1608 0.0798 0.0901 0.0296 0.0661

0.1249 0.0585 0.0478 0.1399 0.1505 0.0479 0.0905 0.0010 0.0646

0.1409 0.0310 0.0580 0.1286 0.1556 0.0638 0.0903 0.0143 0.0653

0.8050 0.0178 0.3505 0.6025 0.8256 0.4097 0.4624 0.1521 0.3393

0.6414 0.3003 0.2453 0.7183 0.7724 0.2458 0.4646 0.0053 0.3315

0.7232 0.0574 0.1590 0.1651 0.2979 0.4189 0.6604 0.7128 0.7990 0.0012 0.3278 0.6658 0.4635 0.6122 0.0734 0.5295 0.3354 0.2261 (continued on next page)

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Table 5 (continued) Molecules, involved in reaction pathway N H H H

Global descriptors g eV

S eV

Local descriptors x eV

10 11 12 13

6 C1 C2 C3 N4 H5 C6 O7 N8 H9 N 10 H 11 H 12 H 13 H 14 7 C1 C2 C3 N4 H5 C6 O7 N8 H9 N 10 H 11 H 12 H 13 H 14 H 15

fk +

Charge densities fk 0

fk 

xk+

x k

xk0

0.0004 0.0827 0.0803 0.1243

0.0093 0.0603 0.0825 0.1223

0.0045 0.0715 0.0814 0.1233

0.0018 0.4246 0.4124 0.6383

0.0478 0.3097 0.4238 0.6278

0.0230 0.3671 0.4181 0.6330

0.5979 0.2191 0.1376 0.0085

1.8940

0.5279

4.4263

0.0348 0.0133 0.0038 0.1550 0.0850 0.0950 0.0869 0.0153 0.0916 0.0260 0.1062 0.3323 0.1055 0.2419

0.1930 0.0202 0.1159 0.0346 0.1510 0.0468 0.0919 0.0063 0.0654 0.0307 0.0703 0.0715 0.1075 0.0481

0.1139 0.0034 0.0598 0.0602 0.1180 0.0709 0.0894 0.0108 0.0785 0.0024 0.0882 0.2019 0.1065 0.1450

0.1542 0.0588 0.0168 0.6859 0.3761 0.4204 0.3847 0.0679 0.4056 0.1149 0.4699 1.4707 0.4669 1.0708

0.8542 0.0892 0.5129 0.1530 0.6682 0.2070 0.4069 0.0280 0.2893 0.1358 0.3111 0.3167 0.4756 0.2129

0.5042 0.0152 0.2649 0.2665 0.5221 0.3137 0.3958 0.0480 0.3475 0.0105 0.3905 0.8937 0.4712 0.6418

0.1658 0.1221 0.2473 0.6644 0.0356 0.6417 0.6212 0.5273 0.2159 0.5799 0.2167 0.2002 0.0200 0.2146

2.6713

0.3743

2.3859

0.1327 0.0039 0.0880 0.0411 0.1393 0.0465 0.0945 0.0178 0.0786 0.0904 0.0536 0.0670 0.1189 0.0604 0.0743

0.0432 0.0883 0.0173 0.0395 0.0965 0.0631 0.2197 0.0462 0.0691 0.1027 0.0661 0.0474 0.1093 0.0466 0.0772

0.0880 0.0461 0.0526 0.0403 0.1179 0.0548 0.1571 0.0320 0.0739 0.0966 0.0598 0.0572 0.1141 0.0535 0.0758

0.3167 0.0094 0.2098 0.0980 0.3324 0.1110 0.2255 0.0424 0.1876 0.2157 0.1279 0.1599 0.2838 0.1442 0.1774

0.1031 0.2107 0.0413 0.0941 0.2302 0.1505 0.5242 0.1102 0.1650 0.2451 0.1576 0.1130 0.2608 0.1113 0.1841

0.2099 0.1100 0.1255 0.0961 0.2813 0.1307 0.3749 0.0763 0.1763 0.2304 0.1427 0.1365 0.2723 0.1277 0.1807

0.1708 0.1633 0.5129 0.6267 0.1279 0.6857 0.5128 0.5439 0.2787 0.5639 0.0438 0.2908 0.0928 0.2712 0.0639

Ochsenfeld et al. (1997), Takahashi (2000) and Takahashi and Yamashita (1996). Although, the cyclic and linear isomers of C3H have been reported to have almost the same abundance in the interstellar space, the l-C3H alone is found to participate in the cytosine formation. The first step in the reaction process involves the carbene centre of: CCCH interacting with an NH2 radical to form 1-amino-prop-2-yl (NH2CCCH) (molecule 1) .This reaction is exothermic and may be taken to be barrier-less; the calculated value of the reaction energy at B3LYP/ 6311+G** level after correcting for the zero-point vibrational energy (ZPVE) is 101.73 kcal/mol and the free energy of 92.03 kcal/mol (Table.1). CCCH has a large value of global philicity x = 10.7657 eV (Table 2) indicating that it is a very reactive electrophile. The carbon atom C1 of CCCH has a Fukui function f k of 0.4180 and local philicity xk of 4.4884 eV (Table 2) and shows that it is a suitable site for the addition of an amino radical (NH2) leading to the formation of 1-amino-prop-2-yl (molecule 1). Global hardness g = 1.9868 eV and philicity x = 2.7832 eV of molecule 1 shows that it is also an electrophile which may react with NH in an exothermic reaction with reaction energy 106.47 kcal/mol and DG = 97.17 kcal/mol (Table 1) to form molecule 2. The addition of NH having the 3R ground state to molecule

1 can occur at sites C1 or C3. While the site C1 has f k and xk values of 0.3094 and 0.8612, these parameters for site C3 have values 0.2566 and 0.7143, respectively. The addition of NH to site C3 of molecule 1 leads to the formation of the cytosine molecule. The addition reaction between molecule 2 and isocyanic acid (HNCO) leading to pyrimidine ring 4 proceeds in several steps (Fig. 1). First, a transition state TS 1 having a potential barrier of 12.49 kcal/mol and internuclear dis˚ is formed. This then leads to the tance N8–C11 of 1.877 A formation of an intermediate molecule 3. It may be noted from Table 2 that the global philicity of HNCO (x = 1.9791) is more than that of molecule 2 (x = 1.7873) making it to be a more reactive electrophile. The carbon atom of HNCO has f k+ and xk+ values of 0.3742 and 0.7406 eV, respectively, suggesting that it is more prone to a nucleophilic attack. This carbon can form a bond with the nitrogen atom N8 of molecule 2. The presence of positive atomic charge 0.6481 at the carbon atom of HNCO and a negative charge 0.4791(Table 2) at N8 atom of molecule 2 may also explain the formation of a N8. . .C11 bond in the molecule 3. As the molecular graph of 3 (Fig. 2) shows, a bond path exists between the nitrogen atom N8 and 0 the carbon atom C11 (inter-atomic dis˚ ). This bond passes through a bond critical tance = 1.555 A

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V.P. Gupta et al. / Advances in Space Research 51 (2013) 797–811

Scheme 1.

Scheme 2.

point (BCP) having electron density q(b) of 0.2037 a.u. and Laplacian $2q(b) of 0.0893 a.u. (Table 3). The interaction energy between the two interacting atoms is 91.208 kcal/mol. These parameters indicate that the interaction between atoms N8 and C11 is covalent in nature. A positive value of $2q(b) indicates that the bond is polar in nature. A rearrangement of atoms of molecule 3 in the formation of the pyrimidine ring 4 passes through a transition state TS 2 which offers a potential barrier of 7.22 kcal/mol (B3LYP/6311+G**). Molecule 3 has a very large value of global philicity x = 3.9818 eV corresponding to a good electrophile. While the carbon atom C1 of 3 has f k+ and xk+ values of 0.1791 and 0.7132, respectively, and is more prone to an nucleophilic attack, the nitrogen atom N12 has large f k – and xk values of 0.2894 and 1.1521 eV and is more prone to an electrophilic attack. This suggests the possibility of a ring closer through a C1. . .. . .N12 bond. The same inference may be drawn from the reactivity parameters of the transition state TS2 which has global hardness g = 1.7868 and global philicity x = 3.7765 eV. The f k+ and xk+ values 0.1425 and 0.5380 at the carbon atom C1 and f k and xk values 0.2535 and 0.9572 eV at the nitrogen atom N12 are also indicative of a ring closer leading to the formation of the six membered pyrimidine ring 4. In the molecular graph of the transition state TS2 based on AIM calculations (Fig. 2), a bond path connected

through a critical point (CP) exists between C1and N12 ˚ . The electron atoms having interatomic distance 2.2767 A density q(b) 0.0441 and Laplacian $2q(b) 0.1012 a.u. at the BCP indicate a weak bond between these atoms. This leads to the formation of C1–N12 bond in 4. Though presenting a maximum activation barrier of 12.49 kcal/mol to form the pyrimidine ring 4, the reaction between 3-iminoprop-2-yl-1-amine, 2 and HNCO is exothermic with reaction energy of -17.82 kcal/mol. The path of this reaction was also verified by intermediate reaction coordinate (IRC) calculations. The addition of hydrogen atom at the unsaturated carbon atom C2 of molecule 4 having electronic charge of 0.1866 and fk value 0.2301 results in the formation of molecule 5. We explored the reaction of molecule 5 with NH, OH and NH2 radicals and found that they may participate in proton abstraction at the nitrogen atom N12 of the intermediate compound 5 to give the final, stable products 6 (cytosine + NH2), 7 (cytosine + H2O) and 8 (cytosine + NH3) (Fig. 2), respectively. All these three processes are barrier-less and exothermic with reaction energies of 59.59, 86.34 and 75.19 kcal/mol, respectively (Fig. 1, Table 1). For the reaction process shown in Fig. 1 to be effective as interstellar synthesis, it needs to be as efficient as possible, both from kinetic and thermodynamic points of view.

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Fig. 1. Sketch map showing the steps of formation of cytosine from Propynylidyne (CCCH) based on B3LYP/6311+G** calculations.

All the reactions in Fig. 1 are exothermic with negative reaction energies. The barriers offered by the transition states TS1 and TS2 in the gas phase are 12.49 and 7.22 kcal/mol, respectively. These are much lower than the barrier of about 32 kcal/mol for reaction between CCCNH and urea which has been suggested by Wang and Bowie (2012) as a viable reaction for interstellar synthesis of cytosine. Herbst (2001) has noted that the reaction processes in gaseous phase must ideally be exothermic and barrier-less for occurrence in the interstellar space. However, the processes involving H tunneling at low temperature and reaction on interstellar ice, where the possibility of solvation of a reactant might reduce the barrier to transition state, may allow the reactions with small barriers also to occur. 3.2. Reaction of cyanoacetylene with cyanate (OCN) ion Electronic energies corrected for the zero point vibrational energy (ZPVE) of molecules, radicals and transition states involved in the formation of cytosine by reaction between cyanoacetylene (HCCCN) and cyanate anion (OCN) in the gas phase and in water–ice mantles treated as bulk solvent medium of dielectric constant 78.5 in the PCM model are given in Appendix B. The computed relative energies, activation barriers and reaction energies are listed in the Table 4 and the sketch map showing the reaction pathway is given in Fig. 3. The geometries of the reactants and products are given in Fig. 4.

Nitriles and nitrile chemistry are found in a variety of astrochemical environments and have been used to understand many observations. Cyanopolyne series HCnCN (n = 0,2,4, . . . 10) is found in the interstellar medium (Bell et al., 1998) and has been proposed as precursor to cytosine in the prebiotic synthesis on Earth (Horn et al., 2008) and as precursor to extra-terrestrial pyrimidines (Dickens et al., 2001). The OCN ion is known to be an interstellar molecule (Hudson et al., 2001) and is produced in large abundance through photo-processing and thermal processing of HNCO in the presence of NH3. Hudson and Moore (2004) report the formation of cyanate ion from either radiolysis or photolysis of nitriles, such as cyanogen (NC–CN), cyanoacetylene (H–C„C–CN), methyl nitrile and ethyl nitrile in the presence of water. Thus, the formation of OCN in nitrile rich astronomical environments having water has been suggested. Based on quantum chemical studies, Park and Woon (2004a,b) have demonstrated that HNCO can spontaneously transfer a proton to NH3 in a barrier-less reaction to yield OCN and NH4+. Present calculations reveal that the addition of cyanate anion (OCN) to cyanoacetylene (HCCCN) leads to the formation of molecule 1 in a barrier-less and exothermic reaction with reaction energy 19.99 kcal/mol (Table 4, Fig. 3). The values of global hardness g = 3.451 eV and global philicity x = 3.5516 eV of HCCCN (Table.5) indicate that this molecule is more stable and electrophilic in nature than the cyanate anion with x = 2.8072. The carbon atom C1 of HCCCN has f k+ value 0.2229 indicating that it is an

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Fig. 2. Geometries and molecular graphs of reactants, transition states and products in the formation of cytosine from propynylidyne (CCCH). Critical points are shown as green dots. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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807

Fig. 2. (continued)

electron deficient site and is more likely to be attacked by nucleophilic site of OCN species. The OCN anion has two heteroatoms oxygen and nitrogen having Fukui function f k values 0.3288 and 0.3611, respectively. The corresponding values of local philicity xk are 0.9230 and 1.0134 eV, respectively. The nitrogen atom of OCN anion has, therefore, a greater chance of attacking the electrophilic site C1 of cyanoacetylene leading to the formation of molecule 1. Molecule 1, which has global hardness g = 2.1428 eV and global philiciy x = 0.8226 eV, has the characteristics of a nucleophile. The addition of hydrogen atom to molecule 1 leads to the formation of molecule 2 with reaction energy of 24.41 kcal/mol (Table 4). It has a free energy DG of 25.04 kcal/mol indicating that it is an exothermic and spontaneous reaction. Molecule 2 has global hardness g = 1.1172 eV and the largest global philicity (x = 4.1819 eV) of all molecules (Table 5) and opens up sites C2 and C6 for nucleophilic attack. The reactive site C6 in

molecule 2 has the largest f k value 0.1584 and can accept nitrogen monohydride (NH) to form molecule 3. This reaction is also barrier-less and exothermic with reaction energy 105.85 kcal/mol (Fig. 3, Table 4). The site N4 of molecule 3 (f k = 0.1466) may accept a proton to form molecule 4. The addition of a hydrogen atom (H13) to molecule 4 leads to ring closer and formation of the six membered pyrimidine ring 5 (Fig. 4). Further addition of a proton at the N4 position (f k = 0.1399) changes the imino-group (@N–H) in 5 into the amino-group in 6. The abstraction of proton from molecule 6 at N10 position may result in the formation of cytosine with the release of a hydrogen molecule. Local softness and philicity values provide a similar information as given by Fukui function. All the reaction steps (Table 4 and Fig. 3) are found to be barrier-less and exothermic and have negative free energy (DG) values (Table 4) suggesting that cytosine formation by this reaction route has a high probability of occurrence.

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Fig. 3. Sketch map showing the steps of formation of cytosine from cyanoacetylene (HCCCN).

3.3. Effect of solvent on reaction mechanism It is known that ice mantles may coat interstellar or circumstellar dust grains under certain conditions, whereby they provide sites where the chemistry can be quite different from that which prevails in the gas phase (Ehrenfreund and Charnley, 2000; Herbst, 2001). Characterizing the range of chemical behavior that can occur on or within ice mantles is therefore essential for a full understanding of the nature and evolution of the gas and condensed phase composition in interstellar objects and elsewhere. The dust grains in molecular clouds consist of a core composed of refractory materials, and of a mantle of different frozen molecules covering the core, where ice water is generally the most abundant constituent (Greenberg and Li, 1998). At the extremely low pressures and temperatures at which icy grain mantles form, the dominant morphology is probably a high-density amorphous ice. Modeling an amorphous material at an ab initio level is rather problematic because of the presence of various surface sites and defects so that, one needs to adopt large unit cells, rendering the calculations very demanding. Two types of models have been proposed by Rimola et al. (2010) for considering the influence of icy-grains on reactions in interstellar space. In the first model the reactions may occur at the surface of the H2O–ice in which molecules from the gas-phase are adsorbed at the surface of icy grain particles. In the second model the reactants are confined within the cavities of the

icy mantles so that the reactive processes can suffer the effect of the dielectric response due to bulky ice. In the second model, the bulk is represented as a continuous medium which is characterized by a dielectric constant, e. In the present work, we have considered the bulk effect of the water icy-grains using the polarizable continuum model (PCM) for which the dielectric constant has been given a value 78.5, equal to that of liquid water. Real ices are mixtures of water and other species. Temperature, pressure, and phase also affect e. The actual value of the dielectric constant of the interstellar ices at cryogenic temperatures is vaguely known. The choice of the value e = 78.5 for water ice is based on the fact that the impact of solvent is fairly flat for large ranges of e. One point in favor of using 78.5 for both liquid water and cryogenic ices is that both are amorphous. Crystalline ices can have very different values of e. SCRF values should be viewed as approximate and upper limits of the impact of bulk ice. The results for the formation of cytosine starting from propynylidyne (CCCH) in the icy-grains are given in Tables 1 and Appendix A. It is found that the solvent under the PCM model reduces the potential barrier due to TS1 from 10.17 kcal/mol (B3LYP/631G**) in the gas phase to 0.25 kcal/mol in the icy-grain. This reduction in the barrier is mainly due to a greater stabilization of the transition state TS1 by 21.96 kcal/mol as against that of reactant 2 + HNCO by 12.048 kcal/mol (Appendix A). In contrast, the barrier of the transition stateTS2 increases from

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809

Fig. 4. Geometries of molecules involved in the formation of cytosine from cyanoacetylene (HCCCN).

5.71 kcal/mol in the gas phase to 11.04 kcal/mol in the solvent. The increase in this potential barrier may be attributed to greater stabilization of molecule 3 by 18.26 kcal/mol as against that of the transition state TS2 by 12.93 kcal/mol. The increase in barrier height is mainly due to lesser activity of the isocyanic acid in the polar solvent medium. A similar

observation regarding increase in barrier height in a polar solvent has been reported by Wang and Bowie (2012) in the reaction of CCCNH with mono-solvated urea against its reaction with urea. A similar situation is noted in the reaction between cyanoacetylene and cyanate anion in the water icy-grains. The

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reaction energies and enthalpies are significantly affected by the solvent (Table 4 and Appendix B) but the formation of cytosine remains to be exothermic and barrier-less.

icy-grains. It, therefore, has a greater probability of occurrence in the interstellar space. Acknowledgments

4. Conclusions Extensive search for small N-heterocycles and nucleobases suggest that they may be present in the interstellar medium though none of them has been identified so far. Their presence in the carbonaceous chondrites, however, indicates that they may be formed in non-terrestrial environments via abiotic pathways. It has been suggested that the exothermic reactions between :CCCNH and :CCCO with monosolvated urea may constitute viable interstellar syntheses of cytosine and uracil although the involved potential barriers are of the order of 32 kcal/mol. A justification for the occurrence of such high barrier reactions on interstellar ice has been given on the grounds of H tunneling at low temperatures and reduction in the barrier to the transition state due to solvation. The presence of several simple molecules and radicals such as isocyanic acid (HNCO), cyanoacetylene(HCCCN), propynylidyne (CCCH), nitrogen monohydride (NH), amino (NH2) radical and cyanate ion (OCN) in the interstellar medium, however, open up several new channels for the formation of cytosine. The possibility of formation of cytosine from these simpler molecules has been explored on the basis of quantum chemical methods that use radical–radical and radical-molecule interaction schemes in the gas phase and in the grains. PCM model was adopted to study the reaction in the grains, which were simulated as a continuum of dielectric constant e (=78.5). We calculated the electronic energies corrected for the zero point vibrational energy and the thermodynamic parameters of all molecules/radicals formed during the reaction path. The molecular geometries of the reactants, and their products were optimized and the harmonic frequencies were calculated to identify the stable states. Global and local descriptors such as chemical potential, molecular hardness and softness, electrophilicity and condensed Fukui functions and local philicity have been helpful in predicting the chemical reactivity and regio-selectivity of the radical additions. The quantum theory of atoms in molecules has additionally been used to locate the changes in structure along a reaction path. These have been useful in ascertaining the presence and nature of weak bonds in the reaction complexes and transition states formed during the reaction process. The study of the reaction energies and the structures of the reactants and products shows that cytosine formation is possible in the gas phase as well as in the water icy grains. At least two exothermic or very low barrier (12 kcal/mol) pathways are possible. The first pathway involves propynylidyne (C3H) and isocyanic acid (HNCO) which is exothermic but involves a reaction barrier of about 12 kcal/ mol in the gas phase. The other reaction pathway involves cyanoacetylene and cyanate (OCN) ion and is exothermic as well as barrier-less both in the gas phase and in water

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