Adsorption and sensing properties of non-planar π surfaces towards high energy molecules: A density functional theory study

Journal Pre-proof Adsorption and sensing properties of non-planar π surfaces towards high energy molecules: A density functional theory study Senthilkumar Lakshmipathi, Agnes Lincy Arokiyanathan, Vidhyashree Ramasamy PII:

S0022-3697(19)30650-X

DOI:

https://doi.org/10.1016/j.jpcs.2019.109198

Reference:

PCS 109198

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 25 March 2019 Revised Date:

10 September 2019

Accepted Date: 11 September 2019

Please cite this article as: S. Lakshmipathi, A.L. Arokiyanathan, V. Ramasamy, Adsorption and sensing properties of non-planar π surfaces towards high energy molecules: A density functional theory study, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/j.jpcs.2019.109198. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical Abstract:

Adsorption and Sensing Properties of Non-Planar π Surfaces Towards High Energy Molecules: A Density Functional Theory Study Senthilkumar Lakshmipathi,* Agnes Lincy Arokiyanathan, Vidhyashree Ramasamy Department of Physics, Bharathiar University, Coimbatore-641046, Tamil Nadu, India Abstract: Using density functional theory (DFT), we have investigated the adsorption characteristics of several explosive compounds (nitromethane, nitrobenzene, nitroglycerin, pentaerythritol tetranitrate, hexogen, 2,4,6-trinitrotoluene and 1,1-diamino-2,2-dinitroethene) on the concave and convex surfaces of a variety of buckybowl materials (corannulene, sumanene, monoindenocorannulene, ortho-diindenocorannulene and 1,2,4-triindenocorannulene). Our results predict that the concave surface of the buckybowl materials shows enhanced adsorption properties for all of the explosive compounds studied. The curvature plays a vital role in the adsorption of explosives. The hydrogen bonding interactions in the complexes were studied using atoms in molecules (AIM) analysis, which confirmed the existence of closed-shell (noncovalent) interactions between the buckybowl materials and explosive compounds. Using natural bond orbital (NBO) analysis, the charge transfer within the complex was found to occur from the buckybowl materials, which act as electron acceptors, to the explosive compounds, which act as electron donors. Energy decomposition analysis showed that dispersion and electrostatic interactions stabilize the complexes. Thermochemical parameters, such as enthalpy, indicate that the adsorption process was exothermic. However, the Gibbs free energy values show that the adsorption process occurs at low temperature. The conductivity results show that concave buckybowl materials possess enhanced sensitivity towards the explosive compounds studied. Keywords: Buckybowl materials, explosives, concave, convex, adsorption, DFT, NBO. *

Corresponding author

Email: [email protected] 1

1. Introduction Explosive materials have been used worldwide in military battles and terrorist attacks. The detection of these explosives has become essential to save living beings and the environment. In this regard, several effective electronic, chemical and optical spectroscopic techniques [1-6] are available for the low cost and facile detection of explosives. However, in order to detect explosives compounds, a long exposure time is required due to their high vapor pressure [7]. Furthermore, due to the advancement in the range of explosive materials utilized for attacks, it has become necessary to develop highly efficient sensing materials with higher selectivity and sensitivity [8-11]. A recent review of the spectroscopic techniques used to detect explosives has summarized the advantages and limitations of various spectroscopy-based detection techniques [12]. This review concluded that improving the sensitivity and specificity of explosive detection technology is important toward the development of efficient detection techniques [12]. It has been well established that carbonaceous nanomaterials provide a natural platform for sensing and energy storage applications because of their structural and electronic properties [13-19]. Tang et al. have used a reduced graphene oxide film for the electrochemical detection of nitroaromatic explosives and attributed its excellent sensitivity to the effective adsorption ability and outstanding electrocatalytic activity of the film towards this class of explosives [20]. Vovusha and Sanyal [21] studied the adsorption of explosive compounds on boron nitride and graphene nanoflakes using density functional theory (DFT) and concluded that 2D flakes are useful for the design of nanomaterials used for the detection of aromatic and non-aromatic

2

explosives. Similarly, reports on the detection of explosive compounds using carbon nanotubes [22-24], quantum dots [25-27] and metal organic frameworks [28] are also available. Conversely, in this study, we have investigated a variety of buckybowl materials for their sensing properties toward explosive compounds. Buckybowls (π-bowls) [29], the curved surface unit of fullerene, which also acts as the capping structures of carbon nanotubes [30, 31], can exist with concave and convex shapes. Corannulene [32, 33] and sumanene [34] are the smallest subunits that maintain the curvaceous shape of fullerene. Among these two subunits, functionalizing corannulene with one or more indeno groups can create potentially useful derivatives. Using corannulene (C20H10) as the basic fullerene fragment, we have expanded the range of buckybowl materials up to pentaindenocorannulene (C50H20) with a stepwise increase in bowl depth, as suggested by Steinberg et al. and Filatov et al. [35, 36]. Previous studies have shown that these non-planar π systems exhibit larger binding energies towards gas compounds when compared to graphene [37, 38]. Therefore, it is imperative to study the interactions between explosives and non-planar π systems in order to understand their binding mechanism toward the curved π surface, which will subsequently lead to the development of explosive sensors. In the present work, we have explored the adsorption of several explosive compounds including nitromethane, nitrobenzene, nitroglycerin, pentaerythritol tetranitrate, hexogen, 2,4,6-trinitrotoluene and 1,1-diamino-2,2dinitroethene on the concave and convex surfaces of various buckybowl materials including corannulene

(C20H10),

sumanene

(C21H12),

monoindenocorannulene

(C26H12),

ortho-

diindenocorannulene (C32H14) and 1,2,4-triindenocorannulene (C38H16). Detailed analysis of the energetics, nature of the interactions and charge transfer process in the complexes may provide

3

fundamental insights into the adsorption properties of explosive compounds on buckybowl materials.

2. Computational details The buckybowl materials were functionalized along their edges with hydrogen atoms to avoid any dangling effects. The explosive compounds are adsorbed on both the concave and convex surfaces of the buckybowl materials. For convenience, we labelled the structures as CORA

(corannulene),

SUMA

(sumanene),

monoindenocorannulene

(M-IC),

orthodiindenocorannulene (O-IC) and 1,2,4- triindenocorannulene (T-IC); nitromethane (NM), nitrobenzene (NB), nitroglycerin (NG), hexogen (RDX) and 2,4,6-trinitrotoluene (TNT). Optimization of the geometry of all the investigated structures was carried out using density functional theory (DFT) with the CAM-B3LYP functional, which can be used to study longrange interactions [39, 40], and the 6-31G* basis set using the Gaussian 09 package [41]. The adsorption energy was calculated including the basis set superposition error (BSSE) correction using formula (1):

Eads = Ecomplex − ( Ebowl + Eexp. ) + EBSSE

(1)

where Ecomplex , Ebowl and Eexp . are the total energy of the explosive-buckybowl complex, buckybowl material and explosive molecule, respectively. Topological analysis was performed using the Morphy package [42] and NBO analysis was carried out using the Gaussian 09 package. Furthermore, the calculated HOMO-LUMO energy gap was used to characterize the electronic structure. To understand the nature of this interaction, energy decomposition analysis (EDA) [43, 44] of all the complexes was carried out with the B3LYP/DZP (double-zeta 4

polarized) level of theory using the Amsterdam density functional (ADF) package [45]. In addition, thermodynamic parameters, such as enthalpy (H), Gibbs free energy (G) and entropy (S), for the adsorption of the explosive compounds on the buckybowl materials at 298.15 K were calculated using formula (2): ∆X = ∆X complex − (∆X bowl + ∆X exp . )

(2)

where X can be H, G and S.

3. Results and discussion 3.1 Structural properties Figure 1 shows the optimized geometries of the hydrogen passivated buckybowl materials from the top-view and side-view. Among the non-planar π surfaces, CORA is composed of a central pentagonal ring along with five peripheral hexagonal rings (Fig. 1), whereas SUMA is composed of a central six-membered ring surrounded by alternate five- and six-membered rings (Fig. 1). The presence of two additional pentagonal rings in SUMA imparts a greater curvature to the molecule when compared to CORA. The remaining structures given in Fig. 1 (M-IC, O-IC and T-IC) were obtained by adding one or more indeno groups (benzene ring fused with cyclopentane) to CORA and are labeled according to the attachment of the indeno group. The curvature observed in the buckybowl materials induce two non-identical surfaces, one that is concave and the other, which is convex. Explosives interact independently on both the concave and convex surfaces. Figure 2 shows the optimized geometries of the seven different explosive compounds studied. NM is a primary nitroalkane in which one of the hydrogen atoms in methane is replaced with a nitro group (-NO2). NB is a nitroaromatic compound with a single NO2 group, whereas NG is the main component in dynamite. PETN is a nitrate ester explosive, which is structurally similar to NG, and consists of one central carbon atom bonded to four 5

CH2ONO2 groups. RDX has three axially positioned NO2 groups bonded to the nitrogen atoms in the triazine ring. Among the different conformers of RDX, the most stable conformer appears to be its triaxial chair form, which we have chosen for this work (AAA) [46]. TNT is also a nitroaromatic explosive with three -NO2 groups bonded to toluene. FOX-7 is an insensitive chemical compound, which has an ethene backbone bonded to two amino and two -NO2 groups. NM, PETN, NG and RDX are non-planar, non-aromatic compounds, whereas NB and TNT are planar aromatic compounds, while FOX-7 is a planar non-aromatic compound.

Corannulene (CORA)

Sumanene (SUMA)

Monoindenocorannulene (M-IC)

ortho-Diindenocorannulene (O-IC)

6

1,2,4-Triindenocorannulene (T-IC) Figure 1: Top- and side-views of the optimized geometries obtained for the buckybowl materials studied using the CAM-B3LYP/6-31G* level of theory.

Nitromethane (NM)

Nitrobenzene (NB)

Nitroglycerin (NG)

Pentaerythritol Tetranitrate (PETN)

Hexogen (RDX)

2,4,6-Trinitrotoluene (TNT)

7

1,1-Diamino-2,2-dinitroethene (FOX-7)

Figure 2: The optimized geometries obtained for the explosive compounds studied using the CAM-B3LYP/6-31G* level of theory.

3.2 Adsorption of the explosive compounds In order to find the most favourable adsorption configurations, the explosive compounds interact with both the concave and convex surfaces of the buckybowl materials. The calculated adsorption energies (kcal/mol) obtained for the explosive compounds interaction with the buckybowl materials studied are reported in Table 1 and shown in Figure 3.

Table 1: The BSSE corrected adsorption energies (kcal/mol) obtained for the interaction of the explosive compounds with the buckybowl materials studied. BSSE corrected adsorption energy, Eads (kcal/mol) Buckybowl/ Explosive

CORA

SUMA

M-IC

O-IC

T-IC

cnv

cvx

cnv

cvx

cnv

cvx

cnv

cvx

cnv

cvx

NM

–1.40

–1.59

–2.95

–1.31

–2.12

–1.74

–2.39

–1.87

–2.41

–2.05

NB

–0.50

–0.30

–0.50

–0.06

–0.04

–1.11

–0.98

–1.58

–1.56

–1.96

NG

–3.98

–2.98

–4.42

–3.47

–3.48

–2.86

–4.45

–1.75

–4.15

–2.73

PETN

–3.55

–0.41

–0.79

–0.68

–1.39

–1.67

–0.22

–0.38

–1.75

-0.53

8

RDX

–4.73

–3.86

–4.91

–4.10

–4.32

–3.63

–4.65

–3.55

–5.00

–3.40

TNT

–2.01

–0.89

–2.59

–1.28

–2.42

–1.47

-2.51

–0.23

–2.76

–0.55

FOX-7

–6.98

–2.81

–6.14

–1.33

–4.46

–2.48

–5.22

–2.15

–0.54

–5.08

Figure 3: The adsorption energies obtained for the interaction of the explosive compounds with the buckybowl materials studied using the CAM-B3LYP/6-31G* level of theory.

3.2.1 Adsorption on the concave surface As a starting point, we will discuss the adsorption of seven different explosive compounds on the concave surface of a variety of buckybowl materials. The optimized geometries of the explosive compounds interaction with the concave and convex surfaces of the buckybowl materials studied are shown in Figure S1–S5 (SI). The adsorption energy of the explosive compounds on the concave surface ranges from –0.04 to –6.98 kcal/mol, wherein FOX-7 is strongly adsorbed on all of the buckybowl materials studied except T-IC. Amidst the buckybowl surfaces studied, concave SUMA is the preferential surface for all the explosives considered. This scenario may be due to the higher number of hydrogen bonds (C-H•••O) that exist between the explosive compounds and concave surfaces of the buckybowl materials 9

studied. A previous study on the binding of various explosives (without BSSE correction) such as RDX and TNT on boron nitride (|BE| RDX = 13.73 kcal/mol, |BE| TNT = 19.59 kcal/mol) and graphene nanoflakes (|BE| RDX = 12.03 kcal/mol, |BE|TNT = 17.28 kcal/mol) have shown that the binding energy is large when compared to the present study (<13 kcal/mol; the adsorption energy without BSSE correction is shown in Table S1) [21]. Nevertheless, this lower adsorption energy on the buckybowl materials studied is advantageous from the recovery time perspective. The recovery time (τ) is related to the adsorption energy (Eads), as shown in formula (3):

τ = ν 0−1e[ − Eads / kT ]

(3)

where ν0 is the attempted frequency, k is Boltzmann’s constant and T is temperature. From the above equation, it can be perceived that a smaller adsorption energy will reduce the recovery time of the buckybowl materials exponentially, which is a key property of good sensor materials.

3.2.2 Adsorption on the convex surface Similarly, we have investigated the adsorption behavior of the different explosive compounds on the convex surface of the buckybowl materials studied. The adsorption energy on the convex surface lies in the range of –0.06 to –5.08 kcal/mol, which shows that the explosive molecules are weakly adsorbed on the convex surface when compared to the concave surface. Among the seven explosive compounds studied, FOX-7, RDX and NG are strongly adsorbed onto all of the buckybowl materials studied, whereas the adsorption of FOX-7 on the convex TIC surface is the strongest with an Eads value of –5.08 kcal/mol. On the other hand, NB, PETN and TNT are weakly adsorbed onto these buckybowl materials. Overall, the adsorption on the convex surface is weaker than the concave surface and this is primarily because of the lower number of hydrogen bonds formed. 10

3.3 Atoms in molecules (AIM) analysis AIM analysis [47] is widely used to characterize the strength and nature of hydrogen bonds using topological parameters such as the electron density ( ρ ) and its Laplacian (∇ 2 ρ ) at the bond critical point (BCP) [48]. A BCP (a point corresponding to ∇ρ = 0 ) found between

each pair of nuclei denotes a chemical bond with two negative curvatures (λ1 & λ 2 ) and one positive curvature (λ3 ) denoted as the (3,–1) critical point. The Laplacian of the electron density indicates whether the electron density is locally concentrated (∇ 2 ρ < 0) or depleted (∇ 2 ρ > 0) . In other words, ∇ 2 ρ < 0 (negative) at the BCP implies the covalent character of the bond, while

∇ 2 ρ > 0 (positive) indicates a non-covalent interaction such as ionic, van der Waals and hydrogen bonding. Generally, in a hydrogen-bonded complex, the values of ρ and ∇ 2 ρ are in the range of 0.002–0.034 a.u and 0.016–0.139 a.u, respectively, as proposed by Popelier [49, 50]. In this work, we found that most of the complexes possess a C-H•••O hydrogen bond, which can be studied considering the values of the electron density, Laplacian of electron density and ellipticity at the BCP’s. The topological parameters obtained for the C-H•••O bonds formed between the explosive compounds and the buckybowl materials for both the concave and convex surfaces are presented in Table S2 along with their corresponding bond lengths and bond angles. The value of the electron density and its Laplacian lie in a range of 0.0032–0.0208 a.u and 0.0027–0.0941 a.u, respectively, which satisfy the criteria proposed by Popelier. Among the concave surfaces studied, the M-IC[NM] complex has a higher electron density value of 0.0104 a.u, which suggests that the C5-H24•••O40 bond is stronger with a bond angle of 149.86°. Similarly, for the convex surfaces studied, the T-IC[NB] complex has the strongest bond, C48H51•••O63, which has an electron density value of 0.0099 a.u. The corresponding bond angle is 11

143.74°, which is almost linear. In general, the bond angles are almost linear for the stronger hydrogen bonds. The weakest bond is C20-H2•••O5 in the T-IC[NG] complex, which has an electron density value of 0.0042 a.u. The positive values of the Laplacian of the electron density indicate the closed-shell interactions in the complexes. The ellipticity value ranges from 0.0107– 3.2669 a.u. and its maximum value was observed for the C1-H3•••O5 bond in the O-IC[TNT] complex, indicating the higher possibility for structural change under external perturbations. The electron density and its Laplacian at the critical point of the C-H•••O bonds may be represented by an exponential function of the bond length. It shows that the electron density value and the corresponding Laplacian decreases exponentially with an increase in the bond length with a correlation coefficient value of 0.8830 and 0.8713, respectively.

(a)

(b)

Figure 4: The correlation between (a) Electron density and (b) Laplacian of the electron density versus the hydrogen bond distance for all the systems studied. 3.4 Natural bond orbital (NBO) analysis

NBO analysis provides information on the hydrogen bonds corresponding to a change in the charge density of the bonding and antibonding orbitals of the proton donor and acceptor 12

respectively, which is a measure of the intermolecular delocalization or hyperconjugation. Besides, hybridization or charge donation [51] can be used to precisely explain the formation mechanism of chemical bonds. Hydrogen bonds are formed due to charge transfer between the proton acceptor and proton donor. Therefore, the amount of charge transfer plays a significant role in defining the strength of the hydrogen bond formed. NBO parameters such as the occupation number of lone pairs in the proton acceptor (N(Y)) and occupation number of antibonding orbitals in the proton donor (N[σ*(X-H)]) along with the corresponding stabilization energy E(2) for the C-H•••O (X-H•••Y) bond formed between the explosive compounds and the buckybowl materials are listed in Table S3. For most of the complexes studied, the occupancy of the antibonding orbitals in the proton donor (buckybowls) increases upon complexation. In other words, the lone pair (O atom) in the explosive compounds act as an electron donor and the antibonding orbital (C-H) of buckybowls acts as an electron acceptor. A maximum amount of charge transfer (0.00751e) occurs for the C4-H25•••O32 bond present in the concave CORA[NB] complex. On the other hand, the occupancy of the lone pair (O atom) lies in the range of 1.9703 to 1.9998e, where concave O-IC[NB] and convex O-IC[PETN] possess the maximum value. The stabilization energy E(2) indicates the strength of the interaction between the monomers, which is higher for the C4-H25•••O32 bond in concave CORA[NB] complex with an energy of 1.98 kcal/mol and indicates the large charge transfer and subsequent strengthening of the hydrogen bonds formed in this complex structure. On the other hand, the weakest hydrogen bond (C21H28•••O37) in the concave SUMA[FOX-7] complex exhibits the minimum amount of stabilization energy (0.05 kcal/mol). Overall, the charge transfer during the interaction occurs from the explosive compounds interaction with the buckybowl materials. 3.5 HOMO-LUMO (H-L) energy gap 13

The H-L energy gap is the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and is an essential aspect of the electronic properties of the system. The HOMO represents the ability to donate an electron, while LUMO represents the ability to gain an electron. Furthermore, the H-L energy gap is a measure of the kinetic stability and reactivity. The H-L gaps for the bare structures and their complexes were calculated using formula (4): E L − H = E LUMO − E HOMO

(4)

where EL − H , ELUMO and EHOMO represent the H-L, HOMO and LUMO energy gaps, respectively. The calculated values are listed in Table 2a and 2b. Table 2a: The HOMO-LUMO energy gaps (eV) obtained for the bare buckybowl structures.

Buckybowl

EHOMO (eV)

ELUMO (eV)

EH-L (eV)

CORA SUMA M-IC O-IC T-IC

–7.33 –6.59 –6.87 –6.86 –6.75

–0.40 0.42 –0.90 –1.13 –1.32

6.93 7.00 5.98 5.72 5.44

The H-L energy gap for the bare structures lie in the range of 5.44 to 7.0 eV, where the energy gap decreases as the bowl size increases due to the increase in the number of πconjugated electrons. For the complexes, the H-L energy gap lies within the range of 4.78 to 6.93 eV. Correspondingly, there is a decrease in the H-L energy gap after the adsorption of the explosive compounds for both the concave and convex surfaces. Notably, the change in the H-L energy gap is significant in the buckybowl materials due to TNT adsorption, which is consistent with its adsorption behaviour. In other words, the H-L energy gap is substantially reduced after 14

TNT adsorption on both of the surfaces when compared to the other explosives studied. The difference in H-L energy gap is at its maximum (2.22 eV) for the concave SUMA[TNT] complex due to strong adsorption. Thus, the reduction in the H-L energy gap indicates the increased chemical reactivity of the TNT adsorbed buckybowl materials, as well as that found for the other complexes studied.

Table 2b: HOMO-LUMO energy gap (eV) for the interaction of the explosive compounds with the buckybowl materials studied. Buckybowl Parameter /Explosive (eV)

NM

NB

NG

PETN

RDX

TNT

FOX-7

EHOMO ELUMO EH-L EHOMO ELUMO EH-L EHOMO ELUMO EH-L EHOMO ELUMO EH-L EHOMO ELUMO EH-L EHOMO ELUMO EH-L EHOMO ELUMO EH-L

CORA cnv –7.03 –0.67 6.36 –7.42 –0.91 6.51 –7.88 –0.96 6.86 –7.71 –0.85 6.91 –7.89 –0.99 6.9 –7.56 –1.98 5.58 –7.75 –0.92 6.83

cvx –7.31 –0.46 6.84 –7.32 –0.91 6.41 –7.81 –0.91 6.74 –7.48 –0.75 6.9 –7.96 –1.03 6.93 –7.58 –1.83 5.76 –7.66 –0.79 6.87

SUMA cnv –6.78 –0.04 6.74 –6.64 –0.91 5.74 –7.1 –0.59 5.69 –6.63 –0.93 6.51 –7.25 –0.53 6.71 –6.81 –2.03 4.78 –7.02 –0.64 6.38

15

cvx –6.58 –0.2 6.38 –6.48 –0.95 5.52 –7.03 –0.59 6.12 –6.79 –0.67 6.43 –7.25 –0.47 6.78 –6.81 –1.91 4.9 –6.33 –1.06 5.27

M-IC cnv –6.83 –0.86 5.97 –6.97 –1.03 5.94 –7.26 –1.31 5.97 –7.05 –1.07 5.95 –7.35 –1.43 5.93 –7.08 –1.98 5.11 –7.15 –1.26 5.89

cvx –6.62 –0.74 5.88 –7.08 –1.11 5.97 –7.19 –1.25 5.96 –7.04 –1.08 5.94 –7.38 –1.45 5.94 –7.1 –1.83 5.27 –7.06 –1.11 5.95

O-IC cnv –6.94 –1.22 5.72 –6.97 –1.27 5.7 –7.27 –1.57 5.71 –6.89 –1.18 5.7 –7.28 –1.6 5.68 –7 –1.95 5.05 –6.37 –0.79 5.58

cvx –6.95 –1.23 5.71 -6.86 –1.15 5.71 –7.11 –1.42 5.72 –6.96 –1.25 5.69 –7.34 –1.64 5.7 –7.01 –1.83 5.18 –7.02 –1.31 5.71

T-IC cnv –6.69 –1.28 5.41 -6.82 –1.4 5.42 –7.05 –1.64 5.42 –6.94 –1.52 5.4 –7.13 –1.73 5.41 –6.86 –1.98 4.88 –6.58 –1.19 5.39

cvx –6.75 –1.31 5.43 –6.53 –1.46 5.07 –7 –1.58 5.43 -6.86 –1.44 5.42 –7.2 –1.78 5.41 –6.87 –1.86 5.01 –7.17 –1.8 5.37

3.6 Energy decomposition analysis (EDA)

In order to understand the nature of the interatomic interactions between the buckybowl materials and the explosive compounds, we performed EDA developed by Morokuma [43] and Ziegler and Rauk [44], where the total interaction energy (∆Eint) of the complex is decomposed into electrostatic interaction (∆Eelstat), Pauli repulsion (∆EPauli), orbital interaction (∆Eorbit) and dispersion interaction (∆Edis). The corresponding interaction energy values are listed in Table 3. Figure S9 shows the variation in the interaction energy corresponding to the adsorption energy. Table 3: EDA (kcal/mol) for the interaction of the explosive compounds with the buckybowl materials studied. Buckybowl/ Explosive

NM

NB

NG

PETN

RDX

TNT

Energy (kcal/mol) ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit

CORA cnv cvx 2.83 5.59 –3.15 –5.78 –1.66 –3.08 –1.88 –4.19 –3.87 –7.46 11.16 6.38 –11.54 –6.94 –6.7 –3.51 –12.43 –6.43 –19.51 –10.51 11.61 6.27 –12.61 –7.25 –7.18 –4.53 –12.97 –6.72 –21.15 –12.24 16.39 12.38 –16.11 –11.28 –8.39 –5.51 –17.3 –10.35 –25.42 –14.76 13.39 5.32 –13.92 –6.8 –7.98 –5.02 –14.27 –6.71 –22.78 –13.2 17.54 12.49 –18.13 –13.32 –8.78 –7.22

SUMA cnv cvx 9.51 6.82 –10.02 –6.57 –5.68 –3.65 –9.02 –5.41 –15.21 –8.82 10.15 6.39 –9.97 –5.82 –6.08 –3.75 –10.92 –6.64 –16.82 –9.83 14.61 6.54 –14.89 –7.45 –7.79 –4.34 –13.74 –6.78 –21.82 –12.01 8.98 12.87 –7.91 –11.86 –4.08 –5.87 –7.96 –11.68 –10.97 –16.54 9.82 6.61 –10.46 –7.56 –6.78 –4.6 –11.91 –6.73 –19.33 –12.27 18.83 12.19 –18.5 –13.15 –9.48 –7.02 16

M-IC cnv cvx 6.55 3.07 –6.71 –3.55 –3.75 –1.83 –4.89 –2.07 –8.8 –4.37 9.9 2.49 –10.21 –3.07 –7.02 –2.59 –13.26 –3.64 –20.59 –6.8 13.91 7.8 –13.69 –7.98 –8.3 –4.95 –16.09 –8.18 –24.17 –13.3 13.23 10.18 –12.33 –9.6 –6.6 –5.56 12.48 –10.2 –18.18 –15.18 12.69 5.36 –13.38 –6.42 -8.52 –4.65 -15.16 –6.4 –24.37 –12.11 16 11.43 –16.65 –12.14 –8.77 –7.01

O-IC cnv cvx 7.72 6.56 –8.31 –7.14 –3.92 –3.22 –5.98 –4.9 –10.49 –8.7 12.5 5.78 –13.56 –6.21 –8.21 –3.9 –15.12 –6.37 –24.38 –10.7 13.97 6.88 –15.07 –6.14 –9.43 –4.95 –16.94 –8.39 –27.46 –12.6 9.3 10.62 –7.51 –9.82 –4.31 –5.47 –8.05 –10.15 –10.56 –14.82 15.34 5.25 –15.75 –6.2 –10.73 –4.68 –21.16 –6.59 –32.29 –12.22 19.78 7.87 –19.88 –8.83 –11.06 –5.58

T-IC cnv cvx 6.77 6.18 –7.05 –6.64 –3.9 –3.46 –5.15 –4.76 –9.33 –8.68 12.33 4.22 –13.63 –4.49 –9.11 –2.34 –16.61 –3.07 –27.01 –5.67 13.38 7.64 –14.54 –7.8 –8.94 –4.67 –16.28 –7.94 –26.37 –12.77 16.97 11.37 –16.21 –10.26 –9.18 –5.52 –17.36 –10.4 –25.78 –14.8 13.91 5.09 –14.89 –5.84 –10.46 –4.63 –19.16 –6.19 –30.59 –11.57 19.29 9.39 –19.67 –9.67 –11.61 –5.68

FOX-7

∆Edis ∆Eint ∆EPauli ∆Eelstat ∆Eorbit ∆Edis ∆Eint

–17 –26.37 7.11 –9.03 –3.95 –6.06 –11.93

–13 –21.05 10.1 –12 –5.44 –8.07 –15.41

–17.41 –26.56 21.77 –21.26 –12.22 –16.8 –28.51

–12.37 –20.34 3.81 –4.1 –2.34 –3.33 –5.96

–17.14 –26.56 19.92 –19 –12.31 –19.09 –30.49

–12.84 –20.57 12.7 –14.34 –7.02 –10.76 –19.42

–20.85 –32.02 17.19 –18.56 –11.34 –17.43 –30.15

–10.99 –17.03 11.66 –12.31 –7.17 –11.02 –18.84

–21.83 –33.82 19.15 –15.66 –11.92 –18.72 –27.16

As for the explosive compounds considered in this study, the interaction energy was mainly due to the contributions from the electrostatic and dispersion interactions. In particular, the CORA and SUMA complexes are dominated by electrostatic interactions, while in the remaining complexes (M-IC, O-IC and T-IC) dispersion interactions play a major role. Table 3 clearly shows that the interaction energy for TNT with all the buckybowl materials studied is generally high and displays similar values for both the concave and convex surfaces. The maximum interaction energy (–33.82 kcal/mol) on the concave surface was observed for the T-IC[TNT] complex, whereas CORA[TNT] displays a maximum energy of –21.05 kcal/mol for the convex surface. In summary, the concave surface exhibits the highest interaction energies for all the complexes studied when compared to the convex surface due to the presence of a higher number of hydrogen bonds. 3.7 Thermochemical parameters of adsorption

Thermochemical parameters such as enthalpy (∆H ads ) , Gibbs free energy (∆Gads ) and entropy (T ∆S ads ) were calculated for the adsorption of the explosive compounds on the buckybowl materials studied at 298.15 K (Table 4) and are presented in Figure 5. These parameters were used to explore the energetic feasibility of the adsorption process. The enthalpy and entropy values for all the adsorbed complexes are negative and increases with an increase in the size of the buckybowl material studied for both surfaces. The negative enthalpy indicates the 17

–10.41 –16.37 7.31 –6.8 –6.13 –5.7 –11.33

adsorption process is exothermic. For any given explosive compound, the most favorable adsorption enthalpies were observed for the interaction with the concave surface of T-IC. This trend was in agreement with the EDA interaction energies and smaller H-L energy gap observed for the interaction of T-IC with the explosive compounds. The Gibbs free energy values for all the complexes were positive, indicating that the adsorption is a non-spontaneous and temperature dependent process. Furthermore, we propose that edge functionalization of these surfaces may reduce the Gibbs free energy values from the present values. A positive Gibbs free energy value has been previously reported for the interaction of an ionic liquid with graphene [52]. Figure S10 shows the temperature dependence for the spontaneity of the adsorption of the explosive compounds over the buckybowl materials studied. From the equation, ∆G = ∆H – T∆S, assuming

∆G = –0.01 kcal/mol for the adsorption to be spontaneous, we see that the adsorption becomes spontaneous below room temperature (<298.15 K). The temperature range studied was from 84 to 272 K for the concave surfaces, while it was 38 to 202 K for the convex surfaces. Interestingly, FOX-7 adsorbs at a higher temperature, while NB adsorbs at a lower temperature for both the concave and convex surfaces. Notably, FOX-7 has the lowest free energy for adsorption among all the explosives studied and therefore, has an enhanced spontaneity to adsorb on its curved surfaces. Overall, the thermochemical parameters show favourable results for the adsorption process toward the concave surface. Table 4: The thermochemical parameters (∆H and ∆G (kcal/mol) and T∆S (kcal/mol)) obtained for the interaction of the explosive compounds with the buckybowl materials studied. Buckybowl /Explosive

Parameters#

NM

∆H ∆G

CORA cnv –2.44 4.70

CORA cvx –2.76 6.19

SUMA cnv –4.81 4.60

SUMA cvx –2.98 6.26

18

M-IC cnv –3.81 5.81

M-IC cvx –2.90 3.63

O-IC cnv –4.30 5.69

O-IC cvx –2.55 6.04

T-IC cnv –4.18 5.32

T-IC cvx -3.28 5.62

T∆S ∆H ∆G T∆S ∆H ∆G T∆S ∆H ∆G T∆S ∆H ∆G T∆S ∆H ∆G T∆S ∆H ∆G T∆S

NB

NG

PETN

RDX

TNT

FOX-7 #

–7.14 –3.04 6.78 –9.82 –5.66 4.24 –9.90 –6.85 5.37 –12.22 –6.73 3.38 –10.11 –6.77 4.89 –11.66 –8.73 2.75 –11.48

–8.95 –1.47 6.55 –8.02 –3.39 4.51 –7.90 –3.73 7.22 –10.96 –3.81 3.14 –6.95 –3.32 5.49 –8.81 –4.26 4.60 –8.85

–9.41 –3.34 6.45 –9.78 –7.39 3.19 –10.58 –3.91 7.19 –11.10 –6.30 2.77 –9.07 –8.34 4.01 –12.35 –11.06 0.97 –12.03

–9.24 –1.77 5.99 –7.75 –4.18 2.99 –7.17 –4.70 6.78 –11.48 –4.70 3.66 –8.36 –4.52 5.31 –9.83 –2.95 4.12 –7.07

–9.62 –2.64 6.39 –9.04 –6.28 3.69 –9.96 –5.16 6.32 –11.48 –6.44 3.97 –10.41 –7.02 4.32 –11.34 –9.15 2.33 –11.48

Parameters: ∆H (kcal/mol), ∆G (kcal/mol) and T∆S (kcal/mol)

19

–6.53 –0.86 5.16 –6.02 –4.09 5.42 –9.51 –4.38 5.39 –9.77 –3.78 4.12 –7.90 –3.76 6.09 –9.85 –5.19 4.56 –9.75

–9.99 –4.33 6.36 –10.69 –7.15 3.51 –10.66 –3.41 6.87 –10.28 –7.75 2.97 –10.73 –8.25 4.60 –12.85 –9.62 1.26 –10.88

–8.59 –2.48 5.83 –8.31 –2.81 6.40 –9.21 –3.41 5.96 –9.37 –3.75 4.08 –7.83 –2.12 6.16 –8.27 –5.08 4.58 –9.66

–9.50 –5.18 5.01 –10.19 –7.51 3.53 –11.04 –6.88 5.91 –12.79 –7.77 2.43 –10.20 –8.88 3.49 –12.38 –6.07 5.07 –11.14

–8.90 –3.33 4.39 –7.73 –3.84 5.42 –9.25 –3.74 6.45 –10.19 –3.49 3.74 –7.24 –1.98 6.83 –8.81 –5.14 2.29 –7.43

Figure 5: The thermochemical parameters obtained for the adsorption process calculated at 298.15 K using the CAM-B3LYP/6-31G* level of theory. (Entropy corresponds to the T∆S value in kcal/mol)

20

3.8. Electrical conductivity

Using density functional theory, we have evaluated the sensing properties of the buckybowl materials in the vicinity of the explosive compounds studied via the expression for conductivity (σ). Accordingly, based on the studies reported by Ortiz et al. [53] and RamosBerdullas et al. [54], Zdetsis and Economou [55] put forth a theoretical framework for manipulating the conductivity of graphene related nanostructures using the dipole moment, edge length and energy difference of the materials with and without an electric field. Accordingly, the expression for conductivity (σ) along the x-axis can be expressed using formula (5): 2

8.098 σ x ≅ a × a × Dx × ∆Ex × eh y x

(5)

where Dx is the induced dipole moment (along the x-axis) for the external field along the x-axis in Debye, ∆Ex is the energy difference with and without an external field in eV, and ax and ay are the total length of the edge of the considered sheets along the x- and y-direction, respectively in Å. Formula (5) can be derived using two mathematical relationships: (i) The uncertainty principle: (∆E ) × (∆t ) ≥

h 2

21

(6)

where ∆E is the total energy difference of the molecular system with and without an electric field, and ∆t is a lifetime of the polarized state, and (ii) the relationship between the current (I), electron charge transferred (∆q) and ∆t, as shown in formula (7): I=

∆q 2∆q ≤ × (∆E ) ∆t h

(7)

I≅

2∆q × (∆E ) h

(8)

Applying formula (5) gives:

Using formula (8), we can measure the current density (J) as:

J=

I

a

(9) ⊥

where a⊥ is the perpendicular length of the system to the applied external electric field. Using the above formula in the conductivity relationship derived from Ohm’s law (equation (10)), one can obtain the conductivity relationship shown in formula (5).

σ=

J E

(10)

22

Figure 6 A schematic representation of the direction of the applied field observed during the interaction between an explosive compound and buckybowl material, where aǁ is the length of the molecule along a parallel axis in Å and a⊥ is the length of the molecule along a perpendicular axis in Å. The red arrow indicates the direction of the applied field (E∥ = 0.001 atomic units) However, in the present work, instead of planar structures, we have calculated the conductivity of non-planar π surfaces (buckybowls) with a variety of explosive compounds. This system inherently possesses a net dipole moment without applying an electric field. On applying an electric field along the parallel axis, as shown in Figure 6, the total dipole moment of the system changes. Therefore, the induced dipole moment due to the electric field is the difference in the dipole moment with and without an electric field along the parallel axis. Hence formula (5) can be transformed into formula (11) shown below: 2

8.098 σ || ≅ a × a × Dx × ∆E|| × eh || ⊥

23

(11)

Table 5 The conductivity (σ║) in units of e2/ħ calculated using equation (11) for the interaction of the explosive compounds with the buckybowl materials studied. σ║×10–2 (e2/ħ) Buckybowl

CORA

SUMA

M-IC

O-IC

T-IC

/Explosive

cnv

cvx

cnv

cvx

cnv

cvx

cnv

cvx

cnv

cvx

NM

0.0139

0.0170

0.0530

0.0510

0.2071

0.0309

0.2370

0.0014

0.2820

0.1741

NB

0.3080

0.2160

0.1870

0.0050

0.1130

0.4190

0.1860

0.1520

0.3160

0.0680

PETN

0.0156

0.0988

0.0557

0.1344

0.0463

0.0284

0.0657

0.0577

0.1411

0.0712

NG

0.2270

0.7100

0.2880

0.4820

0.0450

0.5300

0.2440

0.3920

0.0580

0.7300

RDX

0.2700

0.6550

0.5320

0.6990

0.0060

0.7200

0.2880

0.7400

0.1960

0.6200

TNT

0.1690

0.3360

0.2170

0.1340

0.0250

0.0130

0.2060

0.0970

0.2820

0.0028

FOX-7

1.2048

0.1194

0.0994

0.2967

0.0684

0.1298

0.0314

0.0144

0.1510

0.5342

24

Figure 7 The conductivity (σ║) in units of e2/ħ observed for the (a) concave (cnv) and (b) convex (cvx) surfaces of the buckybowl materials interactions with the explosive compounds studied. Using formula (11), the response of the buckybowl materials towards the explosive compounds as a function of the dipole moment, edge length of buckybowls and energy difference on applying the electric field was calculated and reported in Table 5. Graphs showing the variation in the conductivity for the interaction between the explosives and the concave and convex surfaces of the buckybowl materials are shown in Figure 7a and 7b, respectively. From Figure 7a, the sensitivity of concave SUMA for RDX was higher than the other systems studied. However, all the concave buckybowls were less conductive toward PETN. Nevertheless, the response toward the other explosive compounds by the concave buckybowl surfaces are very similar. This shows that the concave buckybowl materials are sensitive towards explosive compounds such as NM, NB, NG, RDX, TNT and FOX-7. While for the convex buckybowl materials (Figure 7b), the sensitivity toward explosives such as NG and RDX was higher than the other explosives, where RDX shows a greater response when compared to the other explosives in the case of the concave surfaces. In general, both concave and convex buckybowl materials are suitable for sensing explosive compounds. However, concave buckybowl materials can sense a wider range of explosives.

4. Conclusions The adsorption properties of explosive compounds such as NM, NB, NG, PETN, RDX, TNT and FOX-7 on the concave and convex surfaces of a variety of buckybowl materials have

25

been studied using density functional theory. Subsequently, the following conclusions were made: 1. The concave surface of the buckybowl materials has been found to be more accessible for the adsorption of explosive compounds. 2. The magnitude of the adsorption energy correlates with the number of hydrogen bonds. 3. Among the seven explosive compounds studied, TNT, RDX and FOX-7 are strongly adsorbed on the concave and convex surfaces of the buckybowl materials studied. 4. The interaction between the explosive compounds and buckybowl materials occurs via charge transfer with the buckybowl materials acting as an electron donor and the explosives acting as an electron acceptor. 5. EDA reveals that the complexes are stabilized by dispersion (concave and convex) and electrostatic (convex) interactions. 6. The positive Gibbs free energy and negative enthalpy value indicates that the adsorption is non-spontaneous and exothermic. 7. The electrical conductivity values reveal that the concave buckybowl materials are better explosive sensors.

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Highlights: •

Curvature plays a vital role in the adsorption of explosives

•

SUMA exhibits the enhanced adsorption of explosive compounds

•

Electrostatic interactions are predominant between SUMA and the explosive compounds studied

•

Concave buckybowl materials are sensitive towards all the explosive compounds studied