Abstract
The cycloaddition reaction mechanism between interstellar molecules ketenimine and acetonitrile has been systematically investigated employing the second-order Møller-Plesset perturbation theory method in order to better understand the reactivity of heterocumulene ketenimine with acetonitrile. Geometry optimizations and vibrational analyses have been performed for the stationary points on the potential energy surfaces of the system. Calculations show that five-membered cyclic carbene intermediates could be afforded through pericyclic reaction processes between ketenimine and acetonitrile. Through the following intramolecular H-transfer processes, carbene intermediates can be isomerized to the corresponding 2-methylimidazole and 3-methylpyrazole derivatives, respectively. In addition, imidazole and pyrazole compounds can be produced through the intermolecular H-transfer processes on the basis of the formed cyclic carbene intermediates. The present study is helpful to understand the formation of prebiotic species in interstellar space.
Introduction
Ketenimine, CH2 = C = NH, has attracted much attention as a reactive intermediate in organic chemistry [1] and astrochemistry [2]. Jacox et al. predicted in 1963 that ketenimine is tentatively identified as a product of the reaction of the NH species with acetylene in solid argon [3]. In 1979, Jacox accomplished the first spectroscopic identification of ketenimine with the matrix isolation study of the products from the reaction of excited argon atoms with acetonitrile (CH3CN) [4]. A first study in gas phase, by microwave spectroscopy, was reported by Rodler et al. in 1984, in which 2-cyanoethanol was pyrolized at 800°C to form ketenimine [5]. It has been studied by computational methods that ketenimine can be formed through the insertion reaction between interstellar molecules azacyclopropenylidene and R-H (R = F, OH, NH2, CH3) [6].
Ketenimine has been detected in the star-forming region Sagittarius B2(N) by means of radio telescopes. It is a relatively abundant species in Sgr B2(N) and is likely formed directly from its isomer acetonitrile by tautomerization driven by shocks that pervade the star-forming region [2]. Therefore, several theoretical and experimental investigations have been performed to study the formation of ketenimine in the interstellar media. Nadia reported the combined crossed beam and theoretical studies of the N(2D)+C2H4 reaction and implications for the atmospheric models of Titan [7]. They verified the ketenimine is one of the mainly products for this reaction. Given that nitrile derivatives have been detected in Titan’s atmosphere, cometary comae, and the interstellar media, Hudson et al. reported laboratory investigations of the low-temperature chemistry of some of these compounds. In the absence of water, ketenimine is one of the photo- and radiochemical products of these derivatives [8]. Aminoacetonitrile, a species of astrochemical interest, has been detected in the interstellar media. The reaction of aminoacrylonitrile with Ni+ was investigated by means of mass spectrometry techniques and density functional theory calculations, where the ketenimine is one of the main products for this reaction [9]. Using Fourier transform infrared spectroscopy, ketenimine is trapped and identified by UV-irradiation of CH3COCN in an argon matrix at 10 K [10].
As an unstable nitrogenous cumulene, ketenimine can react with unsaturated compounds to form heterocyclic compounds. Song et al. [11] investigated the cycloaddition reaction between ketenimine and cyclopentadiene employing several theoretical methods, establishing that the activation energy for this reaction is 156.5 kJ/mol using the G3B3 method. Fang et al. studied the formation of four-membered compounds through the stepwise cycloaddition reaction between ketenimine and olefins using ab initio method [12]. In addition, Sun et al. [13] reported the reaction between ketenimine and water, where acetamide is finally formed by tautomerization. Sung [14] reported the amination reaction of ketenimine performing NMR and ab initio studies, which provided the formation pathway of vinylidenediamine.
The fact that organic chemistry started in space can be considered as a new challenge for the scientific community [15]. Among the extrasolar objects, the Galactic center molecular clouds, [16] hot cores, [17] and hot corinos [18] are particularly rich in organic molecules. Some more complex organic molecules have been observed in comets [19] and meteorites, where amino acids have also been detected [20]. Ketenimine and acetonitrile [21] are isomers and they are both interstellar molecules. Therefore, in the present study, we perform comprehensive theoretical investigations of the reaction mechanism between ketenimine and acetonitrile by employing the second-order Møler–Plesset perturbation theory (MP2) method, and confirm the formation of 2-methylimidazole and 3-methylpyrazole derivatives. Given that heterocyclic imidazole and pyrazole scaffolds are basic components for many biomolecules, we postulate that the reactions of ketenimine and unsaturated carbonitride compounds (acetonitrile) may be the formation pathways of nitrogenous heterocyclic compounds, which are the basic components of biomolecules. Expectedly, the present study could be helpful to understand the ketenimine reactivity and the formation of prebiotic species in interstellar space.
Calculational methods
The second-order Møller-Plesset perturbation theory (MP2) method [22] in combination with the 6–311+G* basis set [23] has been employed to locate all the stationary points along the reaction pathways without imposing any symmetry constraints. Frequency analyses have been carried out to confirm the nature of the minima and transition states. Moreover, intrinsic reaction coordinate (IRC) calculations have also been performed to further validate the calculated transition states connecting reactants and products. Additionally, the relevant energy quantities, such as the reaction energies and barrier height, have been corrected with the zero-point vibrational energy (ZPVE) corrections.
To further refine the calculated energy parameters, single point energy calculations for all the pathways have been performed using the coupled cluster with single, double, and triple excitations method (CCSD(T)) in combination with the 6–311+G* basis set based on the optimized geometries at the MP2/6–311+G* level of theory. As summarized in Tables 1 and 2, both levels can give consistent results for the calculated reaction profile. For the sake of simplicity, the geometric parameters at the MP2/6–311+G* level and the energetic results at the CCSD(T)//MP2/6–311+G* level have been mainly discussed below if not noted otherwise.
All the calculations have been performed using Gaussian 98 program [24].
Results and discussion
As displayed in Scheme 1, six possible pathways for the title reaction have been proposed. The geometric parameters for the reactants (R1-ketenimine and R2-acetonitrile), transition states (TS), intermediates (IM), and products (P) involved in pathways (A1), (A2), (B1), and (B2) are displayed in Fig. 1. Geometric parameters for the exclusive species involved in the pathways (A’) and (B’) are displayed in Fig. 2. The calculated relative energies for the available stationary points have been summarized in Tables 1 and 2. The reaction profiles are illustrated in Fig. 3.
Step (A): pericyclic reaction process to form a five-membered cyclic carbene intermediate IMa
Ketenimine can react with acetonitrile to form an intermediate in which to new σ bonds, namely C1-N2 and N1-C3 (see Fig. 1), are created. This intermediate, IMa, can be isomerized to Pa1 and Pa2, following the steps (A1) and (A2), respectively, through a hydrogen transfer process. The first intermediate IMa is formed via a pericyclic reaction process with an energy barrier of 153.9 kJ/mol. The unique imaginary frequency calculated for the corresponding transition state, TSa, was 497i cm–1 at the MP2/6–311+G* level of theory.
As shown in Fig. 1, in TSa, the distance of C1-N2 and N1-C3 are 1.853 and 2.106 Å, respectively. Thus, two new bonds, namely C1-N2 and N1-C3, are being formed in TSa. At the same time, the distance of N2-C3 in R2 fragment of TSa reached to 1.212 Å, which is 0.038 Å longer than that in acetonitrile. Therefore, based on the bond length data, the triple bond N2-C3 in acetonitrile is to be transformed into double bond in TSa. Moreover, the bond angle C1C2N1 decreased continuously. The formation of new σ bonds of C1-N2 and N1-C3 and the cleavage of one of π bonds of N2-C3 occurred simultaneously. Therefore, the formation of IMa is a concerted reaction process. As shown in Fig. S1 in the supporting information materials, these changes of bond lengths and angles can be further validated by an IRC calculation on the basis of TSa.
Qualitatively, the pericyclic reaction process can be understood from the frontier molecular orbital theory since the frontier orbitals of a chemical species are very important to define their reactivity and determine the way in that a molecule interacts with other species. As displayed in Fig. 4, the HOMO of ketenimine (upper left) and the LUMO+1 of acetonitrile (left below) are symmetry-allowed. Therefore, the HOMO of the electron-rich ketenimine and the LUMO+1 of the electron-poor acetonitrile can interact to form IMa (similarly, IMb, can be also formed through the interaction of the HOMO of ketenimine and the LUMO+1 of acetonitrile, see right of Fig. 4).
In IMa, there is a lone electron pair on C2, which means that is a carbene. Carbene species are usually unstable and, yet, the formation of the intermediate IMa is endothermic by 24.3 kJ/mol, compared with the energy of the separated reactants. Therefore, IMa undergoes an isomerization process, through an intramolecular proton transfer, to form the more stable products Pa1 and Pa2. It should be noted that IMa is treated as a singlet here. Corresponding triplet intermediate IMa t is also calculated as well, where it is higher in energy by 246.6 kJ/mol than that of IMa. Therefore, the present calculations are based on the singlet intermediate IMa.
Step (A1): Intramolecular H-transfer process to form Pa1 (2-methyl-1H-imidazole)
The reaction step (A1) involves an intramolecular H-transfer process, where the hydrogen H2 is transferred from C1 to the adjacent C2, resulting in the isomerization of IMa into Pa1 via TSa1. Here, the calculated barrier is 69.5 kJ/mol and the imaginary frequency of TSa1 is 929i cm–1. In detail, as shown in Fig. 1, the distance of C1-H2 in TSa1 has been elongated to 1.356 Å, and the distance of C2-H2 reached to 1.261 Å, which indicates that the H2 atom is being transferred from C1 to C2. At the same time, the bond length of C1-C2 in TSa1 decreased to 1.452 Å (the bond length of C1-C2 in IMa is 1.510 Å), suggesting that the single bond of C1-C2 in IMa is becoming a double bond in Pa1. As shown in Fig. S2 in the supporting information materials, those changes of bond lengths have been validated by means of an IRC calculation on TSa1.
Thus, after the cyclization process the stable aromatic compound Pa1, namely 2-methyl-1H-imidazole, is formed, the reaction being exothermic by 154.3 kJ/mol (compared to the energy of the isolated reactants).
Step (A2): Intramolecular H-transfer process to form Pa2 (2-methyl-4H-imidazole)
Similar to the reaction step (A1), the step (A2) is also the intramolecular hydrogen transfer process. Therefore, H3 atom in IMa migrates from N1 to C2, and IMa converts to Pa2 via TSa2, with a reaction barrier of 186.6 kJ/mol. As before, the formation of the aromatic compound Pa2, namely 2-methyl-4H-imidazole, is exothermic by 114.1 kJ/mol (compared to the energy of the isolated reactants).
Pathway (A’): Intermolecular H-transfer process to form Pa2 (2-methyl-4H-imidazole)
In addition to the intramolecular H-transfer process, we explored the possibility to have an intermolecular H-transfer process to form Pa2 on the basis of the formed IMa. The geometric parameters for the species involved in the pathways A’ and B’ are displayed in Fig. 2. Firstly, two IMa molecules react with each other to form a bimolecular complex IMa’, which is, as expected, a barrier-free exothermic reaction by 68.2 kJ/mol. In IMa’, the H3 atom of N1 can be further transferred to carbene C2’ atom of another IMa fragment. At the same time, the H atom of the other IMa fragment can also migrate similarly. The reaction barrier for this intermolecular H-transfer process is 35.5 kJ/mol, which is 151.1 kJ/mol lower than that of the intramolecular one. The product of intermolecular H-transfer process is, therefore, also Pa2, thus identical to the intramolecular one. Although the intermolecular H-transfer could also take place form the methylene group, all our attempts to find a proper transition state for this process were unsuccessful.
Pathways (B1), (B2), and (B’)
As mentioned above, ketenimine can react with acetonitrile to form the new C1-C3 and N1-N2 σ bonds as well (see Fig. 1). The formed intermediate IMb can be isomerized to Pb1 and Pb2, following the reaction pathways (B1) and (B2), respectively, through a hydrogen transfer process. The geometric parameters for the reactants, transition states, intermediates, and products involved in the reaction pathways (B1) and (B2) are displayed in Fig. 1. The calculated relative energies for the available stationary points have been summarized in Table 1.
The barrier of the first step of step (B), that is, the concerted reaction process to form a singlet carbene intermediate IMb, is 155.9 kJ/mol, which is 2.0 kJ/mol slightly higher than that of the pathways (A1) and (A2). Here, the corresponding triplet intermediate IMb t is also calculated, where it is higher in energy by 210.0 kJ/mol than IMb. Therefore, the following calculations are based on the singlet intermediate IMb. Because the electronegativity of the nitrogen, the distances for the new created bonds (1.952 and 2.403 Å for C1-C3 and N1-N2, respectively) are larger than those calculated for TSa (see Fig. 1). This can be also seen in the large difference between the MP2 and single point CCSD(T) calculations: 118.8 against 155.9 kJ/mol, respectively, probably due to an underestimation of the energy by MP2.
Subsequently, pathways (B1) and (B2) are similar to (A1) and (A2), respectively, taking place a hydrogen transfer process giving rise to Pb1 (step (B1)), 3-methyl-1H-pyrazole, and Pb2 (step (B2)), 3-methyl-4H-pyrazole.
Similarly to the pathway A’, two IMb molecules can also establish a barrier-free bimolecular complex lying 57.3 kJ/mol. The subsequent intermolecular H-transfer process, for which the reaction barrier is 43.7 kJ/mol, lead to the achievement of the final product Pb2. As in the case of pathway (A), this process is much more favorable than the corresponding intramolecular H-transfer reaction.
Comparisons of the six reaction pathways
Concerted reactions in pathways (A) and (B) give rise to carbene intermediates for which the energy barriers are 153.9 and 155.9 kJ/mol, respectively. Therefore, the carbene imidazole intermediate is slightly easier to form. Furthermore, the resultant intermediate, IMa, is thermodynamically more stable by 53.4 kJ/mol than that of IMb. Taking into account also the barrier to reach Pa1 (through TSa1), we need 69.5 kJ/mol, which is still lower than the 87.2 kJ/mol we need for TSb1. This means that the formation of Pa1 is quite straightforward.
If we also consider pathways (A’) and (B’), we can see that both mechanisms take place following a low-energy transition state, and thus the formation of both products Pa2 and Pb2 from the corresponding intermediates are very likely. Pathways (A’) and (B’) are kinetically favoured with respect to the (A) and (B) mechanisms, respectively, and IMa is 53.4 kJ/mol lower than IMb, then the formation of Pa2 should be faster.
Calculations and observations show that when several isomers of the same generic formula are identified, the most stable one is always the most abundant. Moreover, the abundance ratio of the most stable isomer to the other isomers is directly related to their energy difference. This can be seen as the minimum energy principle, which has been verified in molecular clouds, hot cores/corinos, photodissociation regions, and asymptotic giant branch stars [15]. Therefore, for the products of reaction between ketenimine and acetonitrile, Pa2 (2-methyl-4H-imidazole) should be more abundant than other isomers.
Conclusions
In this study, the cycloaddition reaction mechanism between ketenimine and acetonitrile have been investigated at the MP2/6-311+G* level of theory. It was found that five-membered cyclic carbene intermediates can be formed by means of a pericyclic reaction. Followed by intra- or intermolecular H-transfer processes, carbene intermediates can isomerize to give rise to imidazole and pyrazole derivatives. Moreover, the intermolecular H transfer is more favorable than that of the intramolecular one. From the kinetic point of view, the reaction pathway A’ is the most favorable channel. From the thermodynamical point of view, Pa2 is the major product. Most organic molecules with physiological activity have heterocyclic constituents. Therefore, the heterocyclic products (imidazole and pyrazole) of the reaction between ketenimine and acetonitrile in the interstellar space may play important roles in the origin of prebiotic species.
Footnotes
Acknowledgments
This work is supported by NSFC (21003082, 21303093, 31372356), and Shandong Province Outstanding Young and Middle-aged Backbone Teachers International Cooperation Program (J13LM06). The NSF of Shandong Province (ZR2014BM020) and the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2013-05) is also acknowledged.