- Email: [email protected]
On the formation of naphthalene cation in space from small hydrocarbon molecules: A theoretical study
Chemical Physics Letters 564 (2013) 11–15
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
On the formation of naphthalene cation in space from small hydrocarbon molecules: A theoretical study Pierre Ghesquière, Dahbia Talbi ⇑ Laboratoire Univers et Particules Montpellier, UMR5299-CNRS-Université de Montpellier II, 34095 Montpellier Cedex 05, France
a r t i c l e
i n f o
Article history: Received 16 January 2013 In final form 7 February 2013 Available online 14 February 2013
a b s t r a c t We have investigated the formation of naphthalene cation from the successive addition of two ethynyl radical on benzene cation and from the addition of protonated diacetylene on benzene. Ab initio methods of quantum chemistry have been used for that purpose. Reactions paths have been optimized at the B3LYP/6-311G(d, p) theoretical level and the final energies recomputed at the CCSD(T)/6-311++G(d, p) one. This study shows that in cold space environments, there exists a very efficient route for the formation of naphthalene cation starting from benzene and protonated diacetylene. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Benzene has been detected both in the upper atmosphere of Jupiter, Saturn and Titan [1,2] and outside of our solar system toward the carbon-rich protoplanetary nebulae CRL 618 [3], a circumstellar envelope. The discovery of a band of blue luminescence in the Red Rectangle protoplanetary nebula has provided support for the presence of anthracene and pyrene by showing the close agreement of their emission spectra to the luminescence coming from this region [4,5]. Finally there is a general agreement that neutral and charged polycyclic aromatic hydrocarbons (PAHs) are widespread through the cold interstellar medium (ISM). The evidence for their presence is quite strong, arising from the 3.3, 6.2, 7.7, 8.6, 11.3 lm set of bands known as the unidentified infrared emission bands (IUR) [6] and coming from a wide variety of astronomical sources [7]. Under terrestrial conditions, benzene and more generally PAHs are known to form in hydrocarbon flames. By analogy and to explain the abundance of carbon grains, in evolved carbon rich stars, it has been suggested that their building blocks (benzene and PAHs) form in the inner envelops of these stars [8–10]. Indeed, the temperatures of these regions are of the order of 103 K and contain small carbon molecules such as C2H2 or C2H [11–12] and the similarities between these conditions and flames have led to the modeling of the formation of PAHs in stars on combustion chemistry [13]. In the much cooler environments (10–150 K), of the interstellar and circumstellar media or of the planetary atmospheres other mechanisms have been suggested for the existence of PAHs. Among them, those based on an ion–molecule chemistry [14–16] involving small hydrocarbon molecules.
⇑ Corresponding author. E-mail address: [email protected] (D. Talbi). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.010
Following this suggestion Bauschlicher et al. [17,17b] using ab initio technics of theoretical chemistry have studied the growth of benzene cation by addition of two acetylenes molecules following a Bittner–Howard-like mechanism. They have found a route that should allow for the formation of naphthalene cation through an almost barrier less path. However the mechanism they have considered involve the dehydrogenation of the benzene cation in the first step of the reaction, a dehydrogenation that regarding to the authors could be induced in space, either by UV photons or by reaction with atomic H. If such dehydrogenation mechanisms are possible in the diffuse part of the ISM they are unlikely in its denser regions. Indeed these regions are shielded from the UV radiations thanks to the dust particles they contain and Hydrogen there is mainly in its molecular form. We are therefore suggesting an alternative to this mechanism by considering the addition of two ethynyl radicals (C2H) on the benzene cation, the ethynyl radical having been observed in various space environments [12,18]. In the interstellar and circumstellar media, where benzene and acetylene have been detected diacetylene C4H2 has also been observed [3] and its protonated form C4 Hþ 3 is expected to exist [14–16]. In addition, C4 Hþ has been observed in the atmosphere 3 of Titan together with the benzene molecule [2]. Searching for more paths for the formation of naphthalene cation from benzene we have therefore investigated the addition of C4 Hþ 3 on C6H6. Choosing to study this reaction was also motivated by the CDSC (The cosmic Dust Simulation Chamber) experiment of Contreras et al. [19] where the growth of dust particles at low temperature is investigated starting from benzene and small hydrocarbon materials. In this experiment the reaction of C6H6 and C4 Hþ 3 is expected to occur and the question of the formation of naphthalene cation through this reaction posed [19]. In the present study we are presenting a theoretical investigation on the addition of two C2H radicals on benzene cation and on the addition of C4 Hþ 3 on benzene. Our goal is to search for paths
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P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15
Figure 1. The lowest energy path for the two successive addition of the ethynyl radical on the benzene cation. Structures at the minima (I), transitions states (T) and of products (F) can be viewed Figure 2. Reported relative energies calculated at the CCSD(T)/6-311++G(d, p) level and corrected for ZPE are given in kcal/mol.
that could lead to the formation of naphthalene cation (the smallest PAHs) in cold space environments where no excess of energy is available. i.e. paths that are exothermic and barrier less. 2. Theoretical approach and results The energy profile for the successive addition of C2H on C6 Hþ 6 and of C4 Hþ 3 on C6H6, has been investigated within the framework of density functional theory using the hybrid [20,21] B3LYP functional in conjunction with a triple zeta atomic basis set extended by polarization function: 6-311G(d, p). These are standard electronic structure calculations for such type of reactions [17,17b,22,23], with an atomic basis set of enough flexibility to ovoid artifacts from atomic basis of limited size [17,17b]. Structures at minima and maxima (transition states) on the reaction paths have been located by optimization using analytical gradients. The character of each structure has been confirmed by a vibrational analysis carried out at the same theoretical level making use of analytical second derivatives methods. For final and more accurate
electronic energies, single-point calculations have been performed at the CCSD(T) level using the previous basis set extend by diffuse functions (hereafter CCSD(T)/6-311++G(d, p). This approach corresponds to a coupled cluster singles and doubles method [24] with a perturbative treatment of the triple excitations. The CCSD(T) energies have been corrected for the unscaled B3LYP zero-point vibrational energies (ZPE) All calculations have been performed using the GAUSSIAN-09 package [25]. 2.1. The formation of naphthalene cation from C6 Hþ 6 and C2H The lowest energy path for the formation of naphthalene cation from C6 Hþ 6 and C2H is reported Figure 1. Related structures and corresponding energies are given respectively Figure 2 and Table 1. The formation of naphthalene cation from C6 Hþ 6 and C2H starts with the barrier less addition of the first ethynyl radical leading to structure I1. The addition of the second ethynyl radical is more favorable when occurring on the ortho position of the cycle. No
Figure 2. Optimized structures for the addition of two ethynyl radical on benzene cation. They are optimized at the B3LYP/6-311G(d, p) level.
P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15 Table 1 Energies of the structures reported Figure 2. Note that I0 corresponds to the sum of C6 H þ 6 and two C2H. Structure
C6 Hþ 6 C2H I0 I1 I2 I3 I4 I5 I6 F T1-2 T3-4 T4-5 T5-6 T6-F T5-F’ F’
Total energy (a.u.)
ZPE corrections (a.u.)
CCSD(T)/ 6-311++G(d, p)
B3LYP/ 6-311G(d, p)
231.326073 76.419330 384.164734 384.325299 384.371997 384.438511 384.472795 384.480248 384.547925 384.627991 384.285071 384.386286 384.413311 384.470162 384.526189 384.479071 384.591736
0.09745 0.016476 0.130402 0.119238 0.120271 0.139401 0.139193 0.13901 0.144564 0.146631 0.115014 0.133727 0.133540 0.139009 0.141348 0.138649 0.144761
Energies relative to the reactants I0 corrected for ZPE (kcal/mol)
0. 97.4 126. 166. 188. 193. 232. 281. 74.8 137. 154. 186. 220. 192. 259.
barrier is found for this second addition. This last can occur following two different paths. The first one corresponds to an addition on I1 leading to I3 that will rearrange to I4 (hydrogen migration). The second path involves a hydrogen transfer transforming I1 onto I2 on which C2H can add to give structure I4. Another hydrogen migration will transform I4 to I5. All hydrogen transfers involve transition states (T1–2, T3–4 and T4–5), which energies are far below the reactants energy (as can be seen from Table 1). The closure of the cycle from I5 to I6 is rather straightforward as well as the hydrogen transfer that leads to the formation of naphthalene cation (structure F). These last two steps involve barriers (T5–6, T6–F) that are also far below the energy of the reactants (see Table 1). I5 can also close by forming a five membered ring leading to structure F’, an isomer of naphthalene cation. 2.2. The formation of naphthalene cation from C6H6 and C4 Hþ 3 The lowest energy path for the formation of naphthalene cation from C6H6 and C4 Hþ 3 is presented Figure 3. The corresponding structures are given Figure 4 and the energies Table 2. This path corresponds to the addition of the acetylenic end of the carbon
13
chain, onto the cycle leading without any barrier to structure I7. Two successive hydrogen transfers will lead to structures I8 then I9. These migrations involve transition states (T7–8, T8–9) which energies are all below the reactants energy (see Table 2). The resulting product I9 can evolve toward two different isomers. It can close into a 5-member ring I13 which after losing a hydrogen will give F’ an isomer of the naphthalene cation. Or it can close into a six membered ring I10 that can rearrange to I12 (protonated naphthalene cation) to finally give naphthalene cation (F) after losing a hydrogen. There exist two paths connecting I9 to I12. Both involve transition states that are all below the reactants energy (see Table 2), with the most straightforward one being the one going through transition state T10–12. The other one requires one more step i.e. the formation of I11 and the passage over a transition state T10–11 higher in energy than T10–12 even though still below the reactants energy (see Table 2). For the structures that are common to the path studied by Bauschlicher et al. [17] and the present one, it is worth noting the good agreement between corresponding relative stabilities 3. Discussion and conclusion In this study we have searched for ion–molecule processes that could lead to the formation of naphthalene cation in the cold extra terrestrial environments where small hydrocarbons as the ethynyl radical, diacetylene or benzene are observed in a neutral or charged form. On the basis of accurate approaches of quantum chemistry we have found that: – the addition of two successive ethynyl radicals on benzene cation is a very exothermic process, with all intermediates involved in the path being far below the reactants energy. There is no barrier to prevent the formation of naphthalene cation from this reaction. However, because of the low density media we are investigating, the release of the excess of energy due to the formation of the final cation could lead to its rearrangement unless vibrational relaxation could efficiently dissipate this energy excess; a mechanism which could be envisaged regarding the size of this cation. The formation of an isomer of naphthalene cation with a five membered ring is also possible through this reaction, although its stabilization will also depend on how the energy excess resulting from its formation will be released.
Figure 3. The lowest energy path for the addition of protonated diacetylene on neutral benzene. Structures at the minima (I), transitions states (T) and of products (F) can be viewed Figure 4. Relative energies calculated at the CCSD(T)/6-311++G(d, p) level and corrected for ZPE are given in kcal/mol.
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P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15
Figure 4. Optimized structures for the addition of protonated diacetylene on neutral benzene. Structures are optimized at the B3LYP/6-311G(d, p) level.
Table 2 Energies of the structures reported Figure 4. Note that I’0 corresponds to the sum of þ C6H6 and C4 Hþ 3 , F and F’ to C10 H8 + H. Structure
Benzene C4 Hþ 3 H I’0 I7 I8 I9 I10 I11 I12 I13 T7-8 T8-9 T9-10 T10-11 T10-12 T11-12 T9-13 F’ F
Total energy (a.u.) CCSD(T)6-311++G(d, p)
ZPE correction (a.u.) B3LYP/6-311G(d, p)
231.659864 153.392245 0.499818 385.052109 385.108199 385.120771 385.170011 385.118942 385.239756 385.212094 385.199073 385.087214 385.107741 385.117608 385.108415 385.079833 385.201529 385.157585 384.627991 384.591735
0.100169 0.046567 0.0 0.146736 0.15313063 0.1512625 0.1542007 0.1547846 0.1578493 0.1568974 0.1561903 0.1486848 0.1493245 0.1539477 0.1526547 0.1512761 0.1545195 0.1533853 0.1447611 0.1466314
Corrected energy (kcal/mol)
0.0 31.2 40.2 69.3 36.9 110.8 94.0 86.3 20.8 33.3 36.6 31.6 14.5 88.9 62.0 26.0 47.6
interstellar and circumstellar media as well as in some planetary atmospheres as long as benzene and protonated diacetylene coexist. This reaction should be considered in the astrochemical networks related to PAHs growth as has to be considered the possible formation through this reaction of an isomer of naphthalene cation with a five membered ring. From an observational point of view, regarding the facility with which this reaction should occur we do encourage for the search of naphthalene in space even though the last attempts for its detection have remained unsuccessful. From an experimental point of view we encourage Contreras et al. [19] to consider the possible formation of both naphthalene cation and its isomer in their experiment, since this might have consequences on the growth of their carbon structures. The formation of naphthalene cation or its isomer from benzene cation and ethynyl radicals might be more questionable because of the energy excess that needs to be released by the final products in low density media. Acknowledgements We acknowledge the French national program PCMI for its financial support and the HPC resources from GENCI-[CCRT/ CINES/IDRIS] (grant 2012-[x2012085116]) for the computing time. References
– the addition on neutral benzene of protonated diacetylene is an exothermic process, and no barriers along the reaction path toward the formation of the naphthalene cation are high enough to prevent its formation. Moreover even though the reaction is exothermic the need to release the energy excess in the low density environment is not anymore an issue since it will be easily dissipated through kinetic energy with the departure of a hydrogen from the protonated naphthalene cation. It is important to outline here the possible formation through this addition of an isomer of the naphthalene cation with a five membered ring. To conclude we suggest that the formation of naphthalene cation from benzene and protonated diacethylene should occur in the
[1] B. Bézard, P. Drossart, T. Encrenaz, H. Feuchtgruber, Icarus 154 (2001) 492. [2] J.H. Waite Jr., D.T. Young, T.E. Cravens, A.J. Coates, F.J. Crary, B. Magee, J. Westlake, Science 316 (2007) 870. [3] J. Cernicharo, A.M. Heras, A.G.G.M. Tielens, J.R. Pardo, F. Herpin, M. Guelin, L.B.F.M. Waters, Astrophys. J. 546 (2001) L123. [4] U.P. Vijh, A.N. Witt, K.D. Gordon, Astrophys. J. 606 (2004) L65. [5] U.P. Vijh, A.N. Witt, K.D. Gordon, Astrophys. J. 619 (2005) 368. [6] A. Leger, J.L. Puget, Astron. Astrophys. 137 (1984) L5. [7] D.M. Hudgins, in: Polycyclic Aromatic Compounds, vol. 22, Taylor and Francis Ltd., Publisher, 2002, 469. [8] I. Cherchneff, J.B. Barker, Astrophys. J. 394 (1992) 703. [9] I. Cherchneff, J.R. Barker, A.G.G.M. Tielens, Astrophys. J. 401 (1992) 269. [10] I. Cherchneff, in: C. Joblin, A.G.G.M. Tielens (Eds.), PAHs and the Universe, EAS Publications Series, EDP Sciences, 2011, 46177. [11] S.T. Ridgway, D.N.B. Hall, S.G. Kleinmann, D.A. Weinberger, R.S. Wojslaw, Nature 264 (1976) 345. [12] J.K. Keady, K.H. Hinkle, Astrophys. J. 331 (1988) 539. [13] M. Frenklach, E.D. Feigelson, Astrophys. J. 341 (1989) 372. [14] T.J. Millar, E. Herbst, R.P.A. Bettens Mon, Not. R. Astron. Soc. 316 (2000) 195. [15] Y.H. Le Teuff, T.J. Millar, A.J. Markwick, Astron. Astrophys. 146 (2000) 157.
P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15 [16] P.M. Woods, in: C. Joblin, A.G.G.M. Tielens (Eds.), PAHs in the Universe, vol. 46, EAS publications series, 2011, 235. [17] C.W. Bauschlicher Jr., A. Ricca, Chem. Phys. Lett. 326 (2000) 283. [17b] C.W. Bauschlicher, A. Ricca, M. Rosi, Chem. Phys. Lett. 355 (2002) 159. [18] K.D. Tucker, M.L. Kutner, P. Thaddeus, Astrophys. J. 193 (1974) L115. [19] C.S. Contreras, C.L. Ricketts, F. Salama, in: C. Joblin, A.G.G.M. Tielens (Eds.), PAHs in the Universe, EAS Publications Series, 46, 2011, 201; C.S. Contreras, F. Salama (private communication). [20] A.D. Becke, J. Chem. Phys 98 (1993) 5648.
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[21] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994) 11623. [22] D.E. Woon Chem, Physics 331 (2006) 67. [23] N.W. Moriarty, M. Frenklach, Proc. Combust. Inst. 28 (2000) 2563. [24] K. Raghavachari, G.W. Trucks, J.A. Pople, M. Head-Gordon, Chem. Phys. Lett. 157 (1989) 479. [25] M.J. Frisch, et al., Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford, CT, 2009.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
On the formation of naphthalene cation in space from small hydrocarbon molecules: A theoretical study Pierre Ghesquière, Dahbia Talbi ⇑ Laboratoire Univers et Particules Montpellier, UMR5299-CNRS-Université de Montpellier II, 34095 Montpellier Cedex 05, France
a r t i c l e
i n f o
Article history: Received 16 January 2013 In final form 7 February 2013 Available online 14 February 2013
a b s t r a c t We have investigated the formation of naphthalene cation from the successive addition of two ethynyl radical on benzene cation and from the addition of protonated diacetylene on benzene. Ab initio methods of quantum chemistry have been used for that purpose. Reactions paths have been optimized at the B3LYP/6-311G(d, p) theoretical level and the final energies recomputed at the CCSD(T)/6-311++G(d, p) one. This study shows that in cold space environments, there exists a very efficient route for the formation of naphthalene cation starting from benzene and protonated diacetylene. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Benzene has been detected both in the upper atmosphere of Jupiter, Saturn and Titan [1,2] and outside of our solar system toward the carbon-rich protoplanetary nebulae CRL 618 [3], a circumstellar envelope. The discovery of a band of blue luminescence in the Red Rectangle protoplanetary nebula has provided support for the presence of anthracene and pyrene by showing the close agreement of their emission spectra to the luminescence coming from this region [4,5]. Finally there is a general agreement that neutral and charged polycyclic aromatic hydrocarbons (PAHs) are widespread through the cold interstellar medium (ISM). The evidence for their presence is quite strong, arising from the 3.3, 6.2, 7.7, 8.6, 11.3 lm set of bands known as the unidentified infrared emission bands (IUR) [6] and coming from a wide variety of astronomical sources [7]. Under terrestrial conditions, benzene and more generally PAHs are known to form in hydrocarbon flames. By analogy and to explain the abundance of carbon grains, in evolved carbon rich stars, it has been suggested that their building blocks (benzene and PAHs) form in the inner envelops of these stars [8–10]. Indeed, the temperatures of these regions are of the order of 103 K and contain small carbon molecules such as C2H2 or C2H [11–12] and the similarities between these conditions and flames have led to the modeling of the formation of PAHs in stars on combustion chemistry [13]. In the much cooler environments (10–150 K), of the interstellar and circumstellar media or of the planetary atmospheres other mechanisms have been suggested for the existence of PAHs. Among them, those based on an ion–molecule chemistry [14–16] involving small hydrocarbon molecules.
⇑ Corresponding author. E-mail address: [email protected] (D. Talbi). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.010
Following this suggestion Bauschlicher et al. [17,17b] using ab initio technics of theoretical chemistry have studied the growth of benzene cation by addition of two acetylenes molecules following a Bittner–Howard-like mechanism. They have found a route that should allow for the formation of naphthalene cation through an almost barrier less path. However the mechanism they have considered involve the dehydrogenation of the benzene cation in the first step of the reaction, a dehydrogenation that regarding to the authors could be induced in space, either by UV photons or by reaction with atomic H. If such dehydrogenation mechanisms are possible in the diffuse part of the ISM they are unlikely in its denser regions. Indeed these regions are shielded from the UV radiations thanks to the dust particles they contain and Hydrogen there is mainly in its molecular form. We are therefore suggesting an alternative to this mechanism by considering the addition of two ethynyl radicals (C2H) on the benzene cation, the ethynyl radical having been observed in various space environments [12,18]. In the interstellar and circumstellar media, where benzene and acetylene have been detected diacetylene C4H2 has also been observed [3] and its protonated form C4 Hþ 3 is expected to exist [14–16]. In addition, C4 Hþ has been observed in the atmosphere 3 of Titan together with the benzene molecule [2]. Searching for more paths for the formation of naphthalene cation from benzene we have therefore investigated the addition of C4 Hþ 3 on C6H6. Choosing to study this reaction was also motivated by the CDSC (The cosmic Dust Simulation Chamber) experiment of Contreras et al. [19] where the growth of dust particles at low temperature is investigated starting from benzene and small hydrocarbon materials. In this experiment the reaction of C6H6 and C4 Hþ 3 is expected to occur and the question of the formation of naphthalene cation through this reaction posed [19]. In the present study we are presenting a theoretical investigation on the addition of two C2H radicals on benzene cation and on the addition of C4 Hþ 3 on benzene. Our goal is to search for paths
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P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15
Figure 1. The lowest energy path for the two successive addition of the ethynyl radical on the benzene cation. Structures at the minima (I), transitions states (T) and of products (F) can be viewed Figure 2. Reported relative energies calculated at the CCSD(T)/6-311++G(d, p) level and corrected for ZPE are given in kcal/mol.
that could lead to the formation of naphthalene cation (the smallest PAHs) in cold space environments where no excess of energy is available. i.e. paths that are exothermic and barrier less. 2. Theoretical approach and results The energy profile for the successive addition of C2H on C6 Hþ 6 and of C4 Hþ 3 on C6H6, has been investigated within the framework of density functional theory using the hybrid [20,21] B3LYP functional in conjunction with a triple zeta atomic basis set extended by polarization function: 6-311G(d, p). These are standard electronic structure calculations for such type of reactions [17,17b,22,23], with an atomic basis set of enough flexibility to ovoid artifacts from atomic basis of limited size [17,17b]. Structures at minima and maxima (transition states) on the reaction paths have been located by optimization using analytical gradients. The character of each structure has been confirmed by a vibrational analysis carried out at the same theoretical level making use of analytical second derivatives methods. For final and more accurate
electronic energies, single-point calculations have been performed at the CCSD(T) level using the previous basis set extend by diffuse functions (hereafter CCSD(T)/6-311++G(d, p). This approach corresponds to a coupled cluster singles and doubles method [24] with a perturbative treatment of the triple excitations. The CCSD(T) energies have been corrected for the unscaled B3LYP zero-point vibrational energies (ZPE) All calculations have been performed using the GAUSSIAN-09 package [25]. 2.1. The formation of naphthalene cation from C6 Hþ 6 and C2H The lowest energy path for the formation of naphthalene cation from C6 Hþ 6 and C2H is reported Figure 1. Related structures and corresponding energies are given respectively Figure 2 and Table 1. The formation of naphthalene cation from C6 Hþ 6 and C2H starts with the barrier less addition of the first ethynyl radical leading to structure I1. The addition of the second ethynyl radical is more favorable when occurring on the ortho position of the cycle. No
Figure 2. Optimized structures for the addition of two ethynyl radical on benzene cation. They are optimized at the B3LYP/6-311G(d, p) level.
P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15 Table 1 Energies of the structures reported Figure 2. Note that I0 corresponds to the sum of C6 H þ 6 and two C2H. Structure
C6 Hþ 6 C2H I0 I1 I2 I3 I4 I5 I6 F T1-2 T3-4 T4-5 T5-6 T6-F T5-F’ F’
Total energy (a.u.)
ZPE corrections (a.u.)
CCSD(T)/ 6-311++G(d, p)
B3LYP/ 6-311G(d, p)
231.326073 76.419330 384.164734 384.325299 384.371997 384.438511 384.472795 384.480248 384.547925 384.627991 384.285071 384.386286 384.413311 384.470162 384.526189 384.479071 384.591736
0.09745 0.016476 0.130402 0.119238 0.120271 0.139401 0.139193 0.13901 0.144564 0.146631 0.115014 0.133727 0.133540 0.139009 0.141348 0.138649 0.144761
Energies relative to the reactants I0 corrected for ZPE (kcal/mol)
0. 97.4 126. 166. 188. 193. 232. 281. 74.8 137. 154. 186. 220. 192. 259.
barrier is found for this second addition. This last can occur following two different paths. The first one corresponds to an addition on I1 leading to I3 that will rearrange to I4 (hydrogen migration). The second path involves a hydrogen transfer transforming I1 onto I2 on which C2H can add to give structure I4. Another hydrogen migration will transform I4 to I5. All hydrogen transfers involve transition states (T1–2, T3–4 and T4–5), which energies are far below the reactants energy (as can be seen from Table 1). The closure of the cycle from I5 to I6 is rather straightforward as well as the hydrogen transfer that leads to the formation of naphthalene cation (structure F). These last two steps involve barriers (T5–6, T6–F) that are also far below the energy of the reactants (see Table 1). I5 can also close by forming a five membered ring leading to structure F’, an isomer of naphthalene cation. 2.2. The formation of naphthalene cation from C6H6 and C4 Hþ 3 The lowest energy path for the formation of naphthalene cation from C6H6 and C4 Hþ 3 is presented Figure 3. The corresponding structures are given Figure 4 and the energies Table 2. This path corresponds to the addition of the acetylenic end of the carbon
13
chain, onto the cycle leading without any barrier to structure I7. Two successive hydrogen transfers will lead to structures I8 then I9. These migrations involve transition states (T7–8, T8–9) which energies are all below the reactants energy (see Table 2). The resulting product I9 can evolve toward two different isomers. It can close into a 5-member ring I13 which after losing a hydrogen will give F’ an isomer of the naphthalene cation. Or it can close into a six membered ring I10 that can rearrange to I12 (protonated naphthalene cation) to finally give naphthalene cation (F) after losing a hydrogen. There exist two paths connecting I9 to I12. Both involve transition states that are all below the reactants energy (see Table 2), with the most straightforward one being the one going through transition state T10–12. The other one requires one more step i.e. the formation of I11 and the passage over a transition state T10–11 higher in energy than T10–12 even though still below the reactants energy (see Table 2). For the structures that are common to the path studied by Bauschlicher et al. [17] and the present one, it is worth noting the good agreement between corresponding relative stabilities 3. Discussion and conclusion In this study we have searched for ion–molecule processes that could lead to the formation of naphthalene cation in the cold extra terrestrial environments where small hydrocarbons as the ethynyl radical, diacetylene or benzene are observed in a neutral or charged form. On the basis of accurate approaches of quantum chemistry we have found that: – the addition of two successive ethynyl radicals on benzene cation is a very exothermic process, with all intermediates involved in the path being far below the reactants energy. There is no barrier to prevent the formation of naphthalene cation from this reaction. However, because of the low density media we are investigating, the release of the excess of energy due to the formation of the final cation could lead to its rearrangement unless vibrational relaxation could efficiently dissipate this energy excess; a mechanism which could be envisaged regarding the size of this cation. The formation of an isomer of naphthalene cation with a five membered ring is also possible through this reaction, although its stabilization will also depend on how the energy excess resulting from its formation will be released.
Figure 3. The lowest energy path for the addition of protonated diacetylene on neutral benzene. Structures at the minima (I), transitions states (T) and of products (F) can be viewed Figure 4. Relative energies calculated at the CCSD(T)/6-311++G(d, p) level and corrected for ZPE are given in kcal/mol.
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P. Ghesquière, D. Talbi / Chemical Physics Letters 564 (2013) 11–15
Figure 4. Optimized structures for the addition of protonated diacetylene on neutral benzene. Structures are optimized at the B3LYP/6-311G(d, p) level.
Table 2 Energies of the structures reported Figure 4. Note that I’0 corresponds to the sum of þ C6H6 and C4 Hþ 3 , F and F’ to C10 H8 + H. Structure
Benzene C4 Hþ 3 H I’0 I7 I8 I9 I10 I11 I12 I13 T7-8 T8-9 T9-10 T10-11 T10-12 T11-12 T9-13 F’ F
Total energy (a.u.) CCSD(T)6-311++G(d, p)
ZPE correction (a.u.) B3LYP/6-311G(d, p)
231.659864 153.392245 0.499818 385.052109 385.108199 385.120771 385.170011 385.118942 385.239756 385.212094 385.199073 385.087214 385.107741 385.117608 385.108415 385.079833 385.201529 385.157585 384.627991 384.591735
0.100169 0.046567 0.0 0.146736 0.15313063 0.1512625 0.1542007 0.1547846 0.1578493 0.1568974 0.1561903 0.1486848 0.1493245 0.1539477 0.1526547 0.1512761 0.1545195 0.1533853 0.1447611 0.1466314
Corrected energy (kcal/mol)
0.0 31.2 40.2 69.3 36.9 110.8 94.0 86.3 20.8 33.3 36.6 31.6 14.5 88.9 62.0 26.0 47.6
interstellar and circumstellar media as well as in some planetary atmospheres as long as benzene and protonated diacetylene coexist. This reaction should be considered in the astrochemical networks related to PAHs growth as has to be considered the possible formation through this reaction of an isomer of naphthalene cation with a five membered ring. From an observational point of view, regarding the facility with which this reaction should occur we do encourage for the search of naphthalene in space even though the last attempts for its detection have remained unsuccessful. From an experimental point of view we encourage Contreras et al. [19] to consider the possible formation of both naphthalene cation and its isomer in their experiment, since this might have consequences on the growth of their carbon structures. The formation of naphthalene cation or its isomer from benzene cation and ethynyl radicals might be more questionable because of the energy excess that needs to be released by the final products in low density media. Acknowledgements We acknowledge the French national program PCMI for its financial support and the HPC resources from GENCI-[CCRT/ CINES/IDRIS] (grant 2012-[x2012085116]) for the computing time. References
– the addition on neutral benzene of protonated diacetylene is an exothermic process, and no barriers along the reaction path toward the formation of the naphthalene cation are high enough to prevent its formation. Moreover even though the reaction is exothermic the need to release the energy excess in the low density environment is not anymore an issue since it will be easily dissipated through kinetic energy with the departure of a hydrogen from the protonated naphthalene cation. It is important to outline here the possible formation through this addition of an isomer of the naphthalene cation with a five membered ring. To conclude we suggest that the formation of naphthalene cation from benzene and protonated diacethylene should occur in the
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