Adsorption of nitrogen dioxide on C30B15N15 heterofullerene: AIM and NBO study via DFT

Ehsan Zahedi

doi:10.1016/j.crci.2012.09.011

Adsorption of nitrogen dioxide on C₃₀B₁₅N₁₅ heterofullerene: AIM and NBO study via DFT

Ehsan Zahedi ¹

¹ Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran

Comptes Rendus. Chimie, Volume 16 (2013) no. 2, pp. 189-194.

Résumé

Adsorption of nitrogen dioxide on the exterior surface of C₃₀B₁₅N₁₅ through B atom of adsorbent is studied using the DFT-B3LYP/6-31G^* level. Independent to the orientation, the NO₂ molecule presents strong chemisorption with large charge transfer from C₃₀B₁₅N₁₅ to adsorbed gas. NBO (natural electron configuration of adsorbing B atom), AIM analysis and spin density distributions indicate that the chemisorption of NO₂ on the C₃₀B₁₅N₁₅ in all studied complexes are covalent in nature.

Métadonnées

Reçu le : 2012-06-10
Accepté le : 2012-09-25
Publié le : 2012-11-01

DOI : 10.1016/j.crci.2012.09.011

Mots clés : DFT, C₃₀B₁₅N₁₅, QTAIM, NBO, Spin density, Chemisorption, Covalent interaction

Affiliations des auteurs :

Ehsan Zahedi ¹

¹ Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran

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     author = {Ehsan Zahedi},
     title = {Adsorption of nitrogen dioxide on {C\protect\textsubscript{30}B\protect\textsubscript{15}N\protect\textsubscript{15}} heterofullerene: {AIM} and {NBO} study via {DFT}},
     journal = {Comptes Rendus. Chimie},
     pages = {189--194},
     publisher = {Elsevier},
     volume = {16},
     number = {2},
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     doi = {10.1016/j.crci.2012.09.011},
     language = {en},
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Ehsan Zahedi. Adsorption of nitrogen dioxide on C₃₀B₁₅N₁₅ heterofullerene: AIM and NBO study via DFT. Comptes Rendus. Chimie, Volume 16 (2013) no. 2, pp. 189-194. doi : 10.1016/j.crci.2012.09.011. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2012.09.011/

Version originale du texte intégral

1 Introduction

Since its discovery in 1985 [1], the hollow spherical form of carbon allotropes has attracted increasing attention due to its potential applications. This allotrope was named fullerene. C₆₀ is the most plentiful and smallest stable fullerene with I_h symmetry. The construction principle of the fullerenes is an outcome of the isolated pentagon rule (IPR) [2] and Euler theorem. Based on these principles, for the closure of each spherical network with n hexagons, 12 pentagons are required (n ≠ 1) and none of the pentagons are in contact with each other. Compared to graphene sheets and carbon nanotubes (CNTs), the structures of these three-dimensional systems are wonderful and attractive. Spherical shape of fullerenes is due to the pentagons which are absent in graphene and CNT. Due to the spherical shape of C₆₀, the conjugate π bond should be less effective on the wall of a C₆₀ and there is hybridization between the π and σ orbitals. Therefore, conjugate π bonding among the carbon atoms is limited and this phenomenon causes high reactivity of C₆₀. On-ball doped (dopant is included in the fullerene shell) fullerenes in which one or more of carbon atoms that form the cage structure are replaced by heteroatoms are called heterofullerenes [2–4]. BN is a binary compound made of Group III and Group V elements in the periodic table that is much closer to the carbon system. BN materials are the best candidates to replace carbon because they are isoelectronic with their all-carbon analogues. Therefore, the inclusion of nitrogen and boron dopants for the formation of heterofullerenes with the general formula of C_60-2nB_nN_n is a promising way to modify the physical and chemical properties of fullerenes [3–5]. Existence of atoms with different electronegativities indicates the high ionicity of the heterofullerenes. The chemical functionalization of nano structures is an effective way to enhance their solubility and change their electronic properties [6]. Moreover, the results of these studies are very important on account of application of these materials for sensing and/or filtering of many hazardous molecules such as CO, HCOH, NH₃, NO_x, etc. Over the past decade, functionalization of various nanotubes has been extensively studied, while the functionalization of heterofullerenes has been less considered. As one of the early pioneers of theoretical study of functionalized heterofullerenes, the author has studied the chemical functionalization of heterofullerenes with NH₃ and NO₂ [7–10].

Nitrogen dioxide (NO₂) is one of a group of highly reactive oxidant and corrosive gasses known as nitrogen oxides (NO_x). This toxic gas is mainly emitted from the motor vehicle exhaust, burning of fossil fuels, petrol and metal refining, power plants, gas stoves, etc. [11–13]. Nitrogen dioxide is an important air pollutant because it contributes to the formation of ground-level ozone, a major component of photochemical smog, which can have significant impacts on human health. NO₂ is linked with a number of adverse effects on the respiratory system and is a precursor to nitrates, which contribute to increased respirable particle levels in the atmosphere [14,15]. As a radical, the reaction with NO₂ [16] is an interesting test of the reactivity of heterofullerenes. According to my knowledge, no prior theoretical investigations have been reported on the covalent/electrostatic nature of addend-heterofullerene interactions. The present work is a DFT study of functionalization of C₃₀B₁₅N₁₅ with NO₂ on the different adsorption sites, with a careful examination of a number of configurations. To get more information about the nature of the interaction, energy densities at the bond critical points (BCP) from Bader's [17] quantum theory of atoms in molecules (QTAIM) [18,19], and natural electron configuration of adsorbing atom using natural bond orbital analysis (NBO) [20] were calculated.

2 Model system and computational details

The initial geometry of C₆₀ was adopted for construction of C₃₀B₁₅N₁₅ as model adsorbent by replacing 15 carbon atoms by nitrogen atoms and another 15 carbon atoms by boron atoms (Fig. 1). Geometry optimizations and calculation of their electronic wave functions were performed at the 6-31G^* basis set with B3LYP functional [21,22], using Gaussian 03 software package [23]. This model chemistry for adsorption study in pi-extended systems is economical and accurate with respect to other model chemistries and used in many articles [7–10,24–26]. The AIM computations were carried out using the AIM 2000 program [18,19]. An NBO calculation was performed with the use of the NBO program version 5.0 [20] by the B3LYP method and 6-31G^* standard basis set in the optimized structures in order to evaluate the natural electron configuration of adsorbing atom and electron charge transfer between bound gas molecule and C₃₀B₁₅N₁₅.

Fig. 1
Schlegel diagram showing the C₆₀ core C-atoms, and the dopant atoms of nitrogen and boron.

3 Results and discussion

For a careful investigation of NO₂ adsorption on the exterior surface of a C₃₀B₁₅N₁₅ heterofullerene, different adsorption positions and orientation of the triangular shaped NO₂ relative to the C₃₀B₁₅N₁₅ surface must be taken into consideration. Three different adsorption sites for adsorption of NO₂ are available, adsorbed gas nearest to a C atom, N atom and B atom. Full geometrical optimization of NO₂-C₃₀B₁₅N₁₅ complexes with different orientation, including individual N or O atoms of adsorbed gas or both of them close to the surface, at several adsorption sites show that in the most stable configurations, the adsorbed gas is close to a B atom of C₃₀B₁₅N₁₅. Hence, the same labelling and numbering of C₆₀ as the Schlegel diagram was used to show the three-adsorption sites (Fig. 1). In this figure, adsorption sites pointed out in bold. Other boron sites could be considered but they are equivalent to those three, in such a manner that boron atoms labelled 34, 35, 36, 38 and 39 are in equivalent positions as B.37 (site I) and those at 53, 55, 56, 59 and 60 are equivalent to B.54 (site II). At last, boron atoms of sites 52 and 57 are equivalent to B.58 (site III). Equivalent sites indicate similarity in position symmetry and binding to neighbouring atoms with those three considered sites. On the other hand, there are three possible orientations for adsorption of NO₂ on the exterior of a C₃₀B₁₅N₁₅, as schematically illustrated in Figs. 2–4. In the nitro configuration (A), the NO₂ molecule is bonded to the surface with the nitrogen end. Bonding with one oxygen end produces the trans-nitrite configuration (B), while bonding with both oxygen ends produces the cis-nitrite configuration (C). For assurance that optimized NO₂-C₃₀B₁₅N₁₅ complexes are in global minimum, rotation of adsorbed molecule around the C₂ axis was performed, then complexes in minimum state reoptimized at the spin-unrestricted B3LYP/6-31G^* level of theory. To evaluate the interaction between a NO₂ molecule with different orientations and those three sites of C₃₀B₁₅N₁₅ heterofullerene, binding energies (E_b) computed, according to following:

E_{b} = E_{(C_{30} B_{15} N_{15} - N O_{2})} - E_{(C_{30} B_{15} N_{15})} - E_{(N O_{2})}

where

E_{(C_{30} B_{15} N_{15} - N O_{2})}

is the total energy of a NO₂ molecule adsorbed on the C₃₀B₁₅N₁₅ surface and

E_{(C_{30} B_{15} N_{15})}

and

E_{(N O_{2})}

are the total energy of the free surface and gas phase molecule, respectively. By the definition, negative values of E_b correspond to exothermic adsorption. The bond length and bond angle of optimized free NO₂ in the gas phase are 1.20 Å and 133.8°, respectively. Geometrical parameters of pristine model and 9 configurations of NO₂-C₃₀B₁₅N₁₅ complexes are gathered in Table 1 and Figs. 2–4. More detailed information including total electronic energies, binding energies and charge transfer based on NPA analysis are listed in Table 2. In the all 9 investigated NO₂-C₃₀B₁₅N₁₅ complexes, the adsorption energies are more negative than –10 kcal/mol, indicating that the adsorption process is chemical in nature. The geometric structure of C₃₀B₁₅N₁₅ at the adsorption site significantly deformed after the sorption of NO₂, where the adsorbing boron atom is slightly pulled out from the spherical surface and the corresponding bonds of adsorbing boron with three nearby atoms are elongated. Local structural deformation at the adsorption site is due to change in the local hybridization of the adsorbing boron atom with a charge transfer from C₃₀B₁₅N₁₅ to NO₂. Natural electron configuration of adsorbing B atom affords a proper picture about the nature of interaction between C₃₀B₁₅N₁₅ and NO₂. Natural electron configuration of adsorbing B atom for bare model and nine NO₂-C₃₀B₁₅N₁₅ complexes are tabulated in Table 3. Rehybridization of adsorbing B atom with increase in p character was predictable from flexible long bonds around the adsorbing atom. It is known that in covalent functionalization, the hybridization type of adsorbing atom is changed but in noncovalent (electrostatic) functionalization, the hybridization type of adsorbing atom is unchanged. Therefore these interactions have mainly covalent character. To further examine this issue, AIM analysis performed on the nine studied NO₂-C₃₀B₁₅N₁₅ complexes. Quantum theory of atom in molecules is a powerful method to study the nature of interactions [17]. Nature of chemical bonds and other valuable information about chemical bonding can be described by analysis of total electronic density, ρ(r), and its corresponding Laplacian, ▿²ρ(r), at corresponding bond critical point (BCP). Thus, at the BCP the first derivative of the charge density, ρ(r), is zero [17,27]. The Laplacian of total electronic density is related to the bond interaction energy by a local expression of the virial theorem at critical points [17]:

\frac{1}{4} \nabla^{2} ρ (r) = 2 G (r) + V (r)

where G(r) and V(r) are electronic kinetic and electronic potential energy densities, respectively. Negative and positive ▿²ρ(r) values are associated with shared-electron (covalent) interactions and closed-shell (electrostatic) interactions, respectively. For 2G(r) > |V(r)| > G(r), above description is not valid. Therefore, in this case total energy density, H(r), is a good indicator for bonding interactions:

H (r) = G (r) + V (r)

Fig. 2
Adsorption of NO₂ on the exterior surface of C₃₀B₁₅N₁₅ heterofullerene through B₃₇. The distances shown are in angstroms.

Fig. 3
Adsorption of NO₂ on the exterior surface of C₃₀B₁₅N₁₅ heterofullerene through B₅₄. The distances shown are in angstroms.

Fig. 4
Adsorption of NO₂ on the exterior surface of C₃₀B₁₅N₁₅ heterofullerene through B₅₈. The distances shown are in angstroms.

Model (Configuration)	B.37–C.3	B.37–N.40	B.37–N.42	B.54–C.4	B.54–N.44	B.54–N.45	B.58–C.14	B.58–N.46	B.58–N.47
Bare model	1.54	1.43	1.47	1.55	1.44	1.47	1.55	1.45	1.46
B₃₇-A	1.60	1.53	1.57
B₃₇-B	1.60	1.55	1.59
B₃₇-C	1.60	1.54	1.59
B₅₄-A				1.63	1.53	1.55
B₅₄-B				1.64	1.55	1.57
B₅₄-C				1.64	1.55	1.57
B₅₈-A							1.61	1.54	1.56
B₅₈-B							1.63	1.56	1.59
B₅₈-C							1.63	1.56	1.59

Model (Configuration)	Total Electronic Energy (a.u.)	E_b (kcal/mol)^c	Q_T (el)^b
Bare Model	–2338.1525321
B₃₇-A	–2543.2828574	–36.47	–0.449
B₃₇-B	–2543.2947684	–43.94	–0.508
B₃₇-C	–2543.2954393	–44.36	–0.485
B₅₄-A	–2543. 2879756	–39.68	–0.445
B₅₄-B	–2543. 3000198	–47.24	–0.499
B₅₄-C	–2543. 2998987	–47.16	–0.489
B₅₈-A	–2543. 2891551	–40.43	–0.439
B₅₈-B	–2543. 3003500	–47.45	–0.505
B₅₈-C	–2543. 3003660	–47.46	–0.486

^a E_ele (NO₂) = –205.0722062 Hartree.

^b Charge transfer from C₃₀B₁₅N₁₅ to NO₂ molecule.

^c The binding energy between the NO₂ molecule and C₃₀B₁₅N₁₅.

Model (Configuration)	Natural Electron Configuration
B₃₇	B₅₄	B₅₈
Bare Model	[core] 2S^0.52 2p^1.44 ≈ sp^2.77	[core] 2S^0.52 2p^1.45 ≈ sp^2.79	[core] 2S^0.52 2p^1.42 ≈ sp^2.73
A	[core] 2S^0.51 2p^1.61 ≈ sp^3.15	[core] 2S^0.51 2p^1.61 ≈ sp^3.15	[core] 2S^0.52 2p^1.60 ≈ sp^3.07
B	[core] 2S^0.49 2p^1.50 ≈ sp^3.06	[core] 2S^0.49 2p^1.48 ≈ sp^3.02	[core] 2S^0.50 2p^1.46 ≈ sp^2.92
C	[core] 2S^0.49 2p^1.52 ≈ sp^3.10	[core] 2S^0.50 2p^1.50 ≈ sp^3.00	[core] 2S^0.51 2p^1.49 ≈ sp^2.92

For negative H(r), the local potential energy density will dominate and an accumulation of electronic charge in the internuclear region will be stabilizing. This case is a covalent bond. For zero or positive H(r), there will be closed-shell or electrostatic interactions [28]. The values of ▿²ρ(r), G(r), V(r) and H(r) are presented in Table 4. For nine studied NO₂-C₃₀B₁₅N₁₅ complexes, the Laplacian of total electronic densities at BCPs are positive and reveals that electronic charges are depleted in the interatomic path, which is characteristic of closed-shell interactions, but according to above theoretical background, this discussion is not valid because in the all studied cases 2G(r) > |V(r)| > G(r). Negative values of H(r) emphasizing that interaction of NO₂ and C₃₀B₁₅N₁₅ in all studied gas-heterofullerene complexes are covalent in nature.

Model (Configuration)	▿²ρ(r) $e / a_{°}^{5}$	G(r) eV	V(r) eV	H(r) eV
B₃₇-A	0.243418	4.881	–8.107	–3.226
B₃₇-B	0.591966	6.883	–9.739	–2.856
B₃₇-C	0.528393	6.271	–8.947	–2.676
B₅₄-A	0.214331	4.710	–7.962	–3.252
B₅₄-B	0.612923	7.043	–9.917	–2.874
B₅₄-C	0.51732	6.266	–9.013	–2.747
B₅₈-A	0.207357	4.674	–7.938	–3.264
B₅₈-B	0.582856	6.880	–9.795	–2.915
B₅₈-C	0.511881	6.242	–9.001	–2.759

The results of natural electron configuration and AIM analysis were confirmed by spin density distribution analysis. Spin density is one of the diagnostic tools to determine the nature of the interaction between NO₂ and C₃₀B₁₅N₁₅. Spin density distributions of nine studied NO₂-C₃₀B₁₅N₁₅ complexes are visualized using GaussView software [29]. The visualized spin density distributions presented in Fig. 5. For all 9 studied NO₂-C₃₀B₁₅N₁₅ complexes, the spin densities are located somewhere other than the B-NO₂ units. In the NO₂-C₃₀B₁₅N₁₅ complexes, where the gas molecule interacted with B₃₇ and B₅₄ of C₃₀B₁₅N₁₅ heterofullerene, spin densities on the B-NO₂ units not observed. For adsorption of NO₂ through B₅₈ of C₃₀B₁₅N₁₅ heterofullerene, spin density distributions on the B-NO₂ units are very small. Spin density distributions of 9 studied NO₂-C₃₀B₁₅N₁₅ complexes show that the interactions between NO₂ and C₃₀B₁₅N₁₅ are covalent in nature.

Fig. 5
Visualized spin density distribution of NO₂-C₃₀B₁₅N₁₅ complexes.

4 Conclusions

The interaction between NO₂ and C₃₀B₁₅N₁₅ with different orientation of adsorbed gas investigated on the basis of density functional theory calculations. The results imply that NO₂ can be chemically adsorbed on the exterior surface of C₃₀B₁₅N₁₅ heterofullerene with a charge transfer from adsorbent to adsorbed gas. The results of NBO (natural electron configuration of adsorbing B atom), AIM analysis and spin density distributions revealed that the interaction of NO₂ and C₃₀B₁₅N₁₅ in all studied complexes are covalent in nature. From a qualitative aspect, all consequences are not dependent on the orientation of adsorbed gas.

Acknowledgements

The financial support of this work by the Iranian Nanotechnology Initiative is gratefully acknowledged. E. Zahedi expresses his gratitude to the Islamic Azad University of Shahrood.

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