A theoretical investigation of the competition between hydrogen bonding and lone pair⋯π interaction in complexes of TNT with NH3

Computational and Theoretical Chemistry 1064 (2015) 25–34

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A theoretical investigation of the competition between hydrogen bonding and lone pair  p interaction in complexes of TNT with NH3 Mei-Zhen Ao a, Zhi-qiang Tao a, Hai-Xia Liu a, De-Yin Wu b, Xin Wang a,⇑ a b

College of Chemistry, Sichuan University, Chengdu 610064, China State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Received 27 February 2015 Received in revised form 6 April 2015 Accepted 6 April 2015 Available online 1 May 2015 Keywords: Hydrogen bond Lone pair  p interaction 2,4,6-Trinitrotoluene Ammonia MP2 theory Density functional theory

a b s t r a c t A theoretical study of the interactions between 2,4,6-trinitrotoluene (TNT) and ammonia (NH3) has been carried out by means of MP2, DFT, and CCSD(T) methods. Eight stable structures of TNT  NH3 complexes were identified at the MP2/6-311++G(d,p) level. Various interactions such as lone pair  p interaction (lp  p interaction), conventional NAH  O, CAH  N, N–Hp hydrogen bonds, and novel noncovalent C  N carbon bonding, have been observed in the titled complexes. The competition between hydrogen bonding and lp  p interaction in the complexes was studied and discussed. The attractive lp  p interactions are observed when the lone pair of NH3 points toward TNT p ring. At the CCSD(T)/6-311++G(d, p)//MP2/6-311++G(d,p) level, the corresponding BSSE corrected interaction energy (DE) is 3.7 kcal mol1. The calculated results also show that the lp  p interaction competes successfully with the NAH  p and NAH  O hydrogen bonds. The competition between hydrogen bonding and lp  p interaction in TNT  NH3 complexes plays important role to affect the structures of complexes. However, the conventional CAH  N hydrogen bond is stronger than the lp  p interaction. The complex I merely with CAH  N hydrogen bond is the most stable structure among eight complexes. The theoretical studies on UV and IR spectra of the TNT  NH3 complexes show that the formations of hydrogen bonding and lp  p interaction have small effect on the UV spectra but new IR absorption peaks in range of 100–300 cm1 correlated to interactions between TNT and NH3 have been predicted, which may be helpful for the detection of TNT. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Noncovalent interactions have attracted considerable attentions due to their distinctive role in molecular biology and supramolecular chemistry [1–3]. Although previous researches for noncovalent interactions have mainly focused on the more common hydrogen-bonded interactions [4], more recently a growing number of experimental and theoretical evidences confirm the importance of lone pair  p (lp  p) interaction in stabilizing and constructing structures of biomolecules as well as materials [5– 13]. The noncovalent lp  p interaction refers to an interaction between a lone pair of electrons and the face of the p system [5,6]. The stabilizing effects of the lp  p interaction are somewhat counterintuitive and very interesting [5,11b,13c]. In 1995, Egli and co-workers demonstrated that the lp  p interaction between lone pair of deoxyribose oxygen and guanidine base provides stability in Z-DNA [7]. Later, the lp  p interaction between lone pair of the ⇑ Corresponding author. Fax: +86 28 85412800. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.comptc.2015.04.006 2210-271X/Ó 2015 Elsevier B.V. All rights reserved.

water oxygen and a nucleobase in an RNA pseudoknot was also reported by them using high-resolution X-ray crystallography [8]. Exhaustive investigations proved lpp interactions are comparable to hydrogen bonding in their role in supramolecular chemistry [5,12]. It is well recognized that the structural stability of proteins and cocrystallization reactions mainly result from a subtle balance of various classes of interactions [2,14]. Moreover, when two or more types of noncovalent interactions coexist during the formation of complexes, there is a competition among them. It is interesting and important to study and identify the competition in guiding the structure of the molecular complexes [15–18]. Many studies on the competition between halogen bond and other noncovalent interactions (hydrogen bond [15], lp  p [16] and p  p [17] interaction) have been reported. Li et al. [18] studied the competition between halogen bond and hydrogen bond in the complexes of formaldehyde with hypohalous acids. Their results indicated that hydrogen bonding interactions are always stronger than the halogen bonding interactions and the strength of the halogen bond is close to that of hydrogen bond as the size of the halogen atom increases [18].

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Compared with the fruitful studies on the competition between halogen bond and other noncovalent interactions [15–18], the investigations of the competition between lp  p interaction and hydrogen bond are relatively rare [12,19]. Quite recently, Das and coworkers have reported the first theoretical study on the weak lp  p interactions in the presence of conventional strong hydrogen bonding interaction in the molecular system [19]. 2,4,6-trinitrotoluene (TNT) is one of the most commonly used explosives for military and industrial applications. However, it is also a leading example of nitroaromatic explosives with significant pernicious effects on environment and health [20,21]. As a result, its detection is of great importance [21]. Several detection methods for TNT have been developed on the basis of TNT–amine complexes formed through charge transfer from electron-rich amines to the electron-deficient aromatic ring of TNT [21]. Recently, Qi et al. [21a] have developed electrochemiluminescence resonance energy transfer (ECRET) for the detection of TNT through the formation of TNT–amine complex. It follows that amines play an important role in the detection of TNT. In 1992, Constantinou and co-workers reported the sensitization of nitrocompounds by the addition of small amounts of amines with lower decomposition temperature and activation energies than pure nitrocompound using dropweight impact experiments [22]. Later, Oxley and co-workers studied the thermal decomposition of TNT in the presence of additive NH3, NO2, NTO and so on [23]. The experimental results indicated that additive of ammonia accelerated the decomposition of TNT. These investigations promoted us to characterize the interactions between TNT and ammonia. The three-dimensional (3D) molecular electrostatic potential (MEP) contour map of TNT was drawn in Fig. 1. The 3D MEP indicates that the positive charge mainly concentrates on the p ring and the area of methyl group in TNT. It is expected that the electrostatic interactions of NH3 with electron-deficient p ring be attractive. As a result, attractive lp  p interaction may be formed in the TNT  NH3 complex. In addition, the conventional NAH  O, CAH  N hydrogen bond, and the weak hydrogen bonds involving p system (NH/p interaction) can also coexist in TNT  NH3 complex. Consequently, the interactions between TNT and NH3 are complicated. It is very interesting and important to study the interactions in TNT  NH3 complex and how the competitions between hydrogen bonding and lp  p interaction affect the structures of the complexes. The previous investigations of TNT mainly focused on its detection [21] and mechanism of thermal decomposition [23,24]. However, to our best knowledge, no theoretical or

experimental studies on the interactions of TNT with NH3 have been done so far. In the present paper, we investigate interactions between TNT and NH3 and the structural competition between hydrogen bonding and lp  p interaction in the complexes. The present work provides a detailed evaluation of the competition of the various interactions in the TNT  NH3 complex. 2. Computational details All the structures of the isolated molecules and complexes were fully optimized using the second-order Møller–Plesset perturbation theory (MP2) [25] with 6-311++G(d,p) basis set. In order to confirm each structure corresponding to a minimum on the potential energy surfaces (PES), vibrational frequencies were also calculated at the same level of theory. The interaction energy (DE) was calculated with the changing vertical distance between TNT and ammonia. Basis set superposition error (BSSE) was corrected by the standard counterpoise (CP) correction method of Boys and Bernardi [26]. Five DFT methods (B3LYP [27], M06-2X [28], B97D [29], xB97XD [30], and B3LYP-D3 [31]) without or with dispersion correction were utilized to compare with MP2. In order to obtain more reliable DE, we performed single point calculations with the coupled cluster theory including single, double and perturbative triple excitations (CCSD(T)) [32]. Moreover, for a further understanding the lp  p interaction between TNT and NH3, the horizontal displacement’s effect on DE (X, Å) was also studied with a fixed distance between nitrogen lone pair and TNT p ring at the MP2/6-311++G(d,p) level. We also investigated the orientation dependence of DE for lp  p interaction. The substitution effects on the nitrogen lp  p interaction involving aromatic ring were probed by replacing the hydrogen atoms of NH3 with methyl groups and substituting the nitro groups of TNT with hydrogen atoms. We have also tried to search possible TNT  NH3 complexes without any restrictions. Eight TNT  NH3 complexes with lp  p interaction, or hydrogen bonding have been identified. The competition between hydrogen bonding and lp  p interaction in these complexes was studied and discussed. NBO analysis [33] was carried out at the MP2/6-311++G(d,p) level to understand the interaction model of the TNT  NH3 complexes. Additionally, based on the MP2 optimized structures, UV–vis spectra of the title complexes in gas phase and acetonitrile solvent were calculated by using time-dependent density functional theory (TD-DFT) [34] with the 6-311++G(d,p) basis set. Four DFT

Fig. 1. Calculated 3D molecular electrostatic potential contour maps of the isolated molecules at ±0.05 a.u. The color code of these maps is in the range between 0.05 a.u. (deepest red) to +0.05 a.u. (deepest blue) in molecules. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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methods, B3LYP, CAM-B3LYP (Coulomb-attenuated B3LYP) [35], PBE0 [36], and xB97XD, were used for the TD-DFT calculations. With the exception of the CCSD(T) calculations which were performed using the MOLPRO 2008.1 suite of programs [37] and the atoms in molecules (AIM) analyses [38a,38b] using the AIM2000 program [38c], all the other calculations were carried out with the Gaussian 09 program package [39].

3. Results and discussion 3.1. Lone pair  p interaction between TNT and ammonia For a better understanding of the TNT’s interaction with NH3, the 3D molecular electrostatic potential contour maps of TNT and NH3 were depicted in Fig. 1. To study the substitution effects on the interactions, the 3D MEP contour maps of toluene (C7H8) and trimethylamine (NMe3) have also been drawn. The 3D MEP clearly shows that the center of TNT p ring is positively charged due to the replacement of the hydrogen atoms of toluene (C7H8) p ring with electron-withdrawing nitro groups. For TNT has an electron-deficient p ring, it is expected that the lp  p interaction between TNT with NH3 or NMe3 would be attractive. However, the p ring of toluene is negatively charged. The electrostatic interaction between toluene and NH3 is supposed to be repulsive and the lp  p interaction between them may be repulsive. The intermolecular interaction potentials of the TNT  NH3 complex A (see Fig. 2) in which the nitrogen atom is pointed towards the TNT p ring were computed. Fig. 2 shows the effects of different methods (MP2 and five DFT functionals) with basis sets 6-311++G(d,p) on the DE. The results show that the depths of the intermolecular interaction potentials strongly depend on the selected methods. The calculated DEs and intermolecular distances (R) with different methods are summarized in Table 1. The results show that all the DEs with the six methods are negative, which indicates that the interaction between the lone pair of NH3 and TNT p ring is attractive. As shown in Table 1, the complex A has a minima at the intermolecular distance (R) of about 3.2 Å and the DE of about 3.7 kcal mol1 at the MP2/6-311++G(d,p) level. As we all known, CCSD(T) method can provide very accurate interaction energies but its computation costs are quite expensive. As a result, we have only performed the CCCSD(T) single point calculations. The DEs at the R of 3.1 and 3.2 Å were computed at the

Fig. 2. BSSE corrected MP2 and different DFT intermolecular interaction potentials of the TNT  NH3 complex A.

Table 1 Calculated interaction energies (DE, kcal mol1) and intermolecular distances (R, Å see Fig. 2) at MP2/6-311++G(d,p) level for TNT  NH3 complex A. Method

TNT  NH3

DE

R

MP2 xB97XD M06-2X B97D B3LYP-D3 B3LYP CCSD(T) CCSD(T)

3.7 5.0 5.3 4.1 4.7 2.1 3.5 3.7

3.2 3.1 3.1 3.2 3.2 3.5 3.1 3.2

CCSD(T)/6-311++G(d,p) level and their corresponding values were 3.5 and 3.7 kcal mol1, respectively. Thus, we used DEs of 3.7 kcal mol1 as the CCSD(T) result and compare it with other results with MP2 and DFT methods. Compared with the CCSD(T) result, the DEMP2 is same to that by CCSD(T) prediction and show that the MP2 method has better performance than other five density functionals. Thus, we have carried out the subsequent calculations at the MP2/6-311++G(d,p) level and used the MP2 result for discussion. According to Fig. 2 and Table 1, among the five density functionals, the B97D shows better performance and outperforms other four DFT functionals in the calculations of interaction energies of TNT  NH3 complex A. The B97D result (DE = 4.1 kcal mol1, R = 3.2 Å) is the closest to the CCSD(T) calculated values. The result indicates that the B97D may be a good choice to study structures involving lp  p interaction of TNT  NH3 complexes. Similarly, our previous work has also proved that the B97D is superior to other dispersion-corrected DFT functionals in investigating lp  p interaction in benzene–amines [13c] and tri-s-triazine–amines [13d] complexes. The B3LYP-D3, xB97X-D and M06-2X DEs are 4.7, 5.0, and 5.3 kcal mol1, respectively. Compared with the CCSD(T) results, they overestimate the interaction energies by about 1.0–1.6 kcal mol1. However, compared with the four DFT methods with dispersion correction, the B3LYP method without dispersion correction leads to considerable underestimation of the DE by about 1.6 kcal mol1 at a longer R of 3.5 Å. For the B3LYP-D3, the B3LYP functional with the Grimme’s D3 dispersion correction scheme [31], the corresponding DE and R are closer to the CCSD(T) values. The dispersion correction leads to the much larger DE and shorter R. The result illustrates that the dispersion force plays a significant role for lp  p interaction in the TNT  NH3 complex, which is also consistent with the previous conclusions on other lp  p systems [13c,13d,19]. The MP2 and the DFT methods with dispersion correction should be chosen to study these lp  p systems. Next we have studied the effect of horizontal displacement (X, Å) on the DE of complex A at the MP2/6-311++G(d,p) level. The results were drawn in Fig. S1 in the Electronic Supplementary Information (ESI). The vertical distance between the nitrogen atom of ammonia and the plane of TNT ring is fixed at 3.2 Å when translating NH3 in two directions along the C1AC4 axis (see Fig. S1). As shown in Fig. S1a, the DE decreased (more negative) by 0.2 kcal mol1 when the X is 2.0 Å along the C4 orientation. Likewise, the DE decreases by 0.1 kcal mol1 when the X is 1.5 Å along the C1 orientation. The changes of the interaction energies by translation along the C1 or C4 orientation are very small. The results show that moving NH3 horizontally has a little impact on interaction energies and NH3 may prefer to locate above the center of the TNT p ring. Burley and Petsko have also reported that the amino group prefers to locate above the center of the aromatic ring [40].

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In addition, we considered the orientation dependence of the MP2/6-311++G(d,p) interaction energy for TNT  NH3 complex with a fixed distance between lone pair and p ring of TNT. The intermolecular distance is fixed at 3.2 and 4.2 Å and NH3 was rotated along the C1 or C4 orientation to evaluate the orientation dependence of DE. As shown in Fig. S2 in ESI, the rotation of the ammonia above the TNT p system results in the obvious increment of the calculated DE, which indicates that the DE has strong orientation dependence. With a fixed distance of 3.2 Å, the DE changes from negative one to positive at the rotational angle (h) in the range of 110–130°. The calculated results indicate that the rotation of NH3 is energy unfavorable and the nitrogen lone pair prefers to point toward the TNT p ring vertically. Similar results have also been observed in attractive lp  p system such as the complexes of benzene and amines [13c]. But for the repulsive lp  p system such as benzene  ammonia complex, the rotation of NH3 lead to the more negative DE and a potential minimum with DE of about 2 kcal mol1 is observed at h = 120° [13a]. To better understand orientation dependence, we also calculated the interaction energies when the interaction molecular distance is 4.2 Å (Fig. S2). The calculated interaction energy of the complex has strong orientation dependence even in a long intermolecular separation (R = 4.2 Å). The orientation dependence trend is the same to that in a short separation (R = 3.2 Å). The same orientation dependence suggests that the directionality is controlled mainly by the electrostatic interaction, which is a long-range interaction. If short-range interactions such as charge transfer are the major source of the directionality, the directionality should disappear in a long intermolecular separation. We have also probed the substitution effects on the lp  p interaction between nitrogen lone pair and TNT p ring. Two types of substitution effects were taken into account, replacing the hydrogen atoms of ammonia with electron-donating methyl groups and nitro groups of TNT p ring with hydrogen atoms. The BSSE corrected MP2 potential energy curves of the TNT  NMe3 and C7H8  NH3 complexes with the basis set 6-311++G(d,p) and the geometries of these complexes are shown in Fig. 3. For comparison, the intermolecular interaction potential energy curve of TNT  NH3 complex A calculated at the MP2/6-311++G(d,p) level is also included in Fig. 3. The results show that the potentials of

C7H8  NH3 complex has no minima, and the calculated interaction energy of the complex is almost positive. As the distance increases, DE becomes less repulsive. The result indicates that there is no stabilization between the lone pair of NH3 and the C7H8 p ring. But the TNT  NH3 complex A has a minima at the intermolecular distance of about 3.2 Å with an DE of 3.7 kcal mol1. The result indicates that the introduction of the three nitro groups for the toluene ring leads to that a repulsive interaction observed between toluene and NH3 becomes an attractive interaction between TNT with NH3. As shown in Fig. 3, as expected, a stronger lp  p interaction is formed in the TNT  NMe3 complex. At the MP2/6-311++G(d,p) level, a distinct deep potential minimum is observed when R is 3.1 Å and DE is 6.3 kcal mol1, which is 2.6 kcal mol1 larger than that of TNT  NH3 complex. The stronger lp  p interaction between TNT and NMe3 is clearly due to the presence of electron-donating methyl groups of NMe3. This substitution effects on nitrogen lp  p interaction is similar to those of the previously reported lp  p interaction systems such as benzene  amine [13c] and tri-s-triazine  amine complexes [13d]. The calculation on the lp  p interaction between TNT and NH3 show that attractive lp  p interaction is observed when the lone pair of nitrogen points toward the TNT p ring. The exploration on the orientation and horizontal displacement dependence of DE show that both of the horizontal rotating and moving of ammonia lead to weaker interaction. Moving NH3 horizontally has a little impact on interaction energies but the DE has strong orientation dependence. The substitution effects on nitrogen lp  p interaction of present systems are similar to previous reported systems. 3.2. Competition of NAH  p hydrogen bonds and lone pair  p interactions of the TNT  NH3 complexes To study the competition of NAH  p hydrogen bonds and lp  p interaction of the TNT  NH3 complexes, we considered three complexes B, C, and D with the NAH  p interactions, which were shown in Fig. 4. The intermolecular interaction potentials of the four TNT  NH3 complexes A–D were calculated at the MP2/6-311++G(d,p) level and shown in Fig. 5. As shown in Fig. 5, the potential of complex A with lp  p interaction has the deepest minimum with the shortest intermolecular

H

H

H N R

H3C C7H8...NH3 H

H

H N R

H3C

O2N NO2 O2N

TNT...NH3

H3C H3C

CH3 N R

H3C

O2N NO2 O2N

TNT...NMe3

Fig. 3. BSSE corrected MP2 potentials curves of the TNT  NMe3, TNT  NH3, and C7H8  NH3 complexes, where the lone pair points to the center of the p ring.

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H

H

H

H N R

O2N

O2N

H3C

NO2 O2N

H

H N

N R

H3C

O2N

H

H

NO2 H3C O2N

A

H

R

C

N H R

O2N NO2

O2N

B

H H H3C

NO2 O2N

D

Fig. 4. The geometries of four TNT  NH3 complexes A–D considered in this work and the NH3 is above the center of the TNT p ring.

difference in orientation dependence between the two complexes is that negative electrostatic potential of benzene p ring leads to repulsive lp  p interaction. While the present TNT p ring has positive electrostatic potential due to the electron-withdrawing nitro groups and the TNT  NH3 complex has relatively strong attractive lp  p interaction. The result show that the NAH  p hydrogen bonds in the TNT  NH3 complexes are weak and the lp  p interaction competes successfully with the NAH  p hydrogen bonds when the NH3 is above the TNT p ring. 3.3. Competition effects on the structures of the TNT  NH3 complexes and NBO analysis

Fig. 5. The MP2/6-311++G(d,p) intermolecular interaction potentials of the four TNT  NH3 complexes A–D.

distance (R) of 3.2 Å. The calculated potentials of complexes B–D have their minima at intermolecular distances (R) of 3.6, 3.7 and 3.6 Å, respectively. The calculated interaction energies of the four complexes at the potential minima are summarized in Table 2. The DE of complex A is substantially larger than those of B, C, and D, which indicates that the lp  p interaction between TNT and NH3 is stronger than the NAH  p hydrogen bonds of the TNT  NH3 complexes. Ammonia above the center of the TNT p ring prefers to interact with TNT ring with the nitrogen lone pair but not the NAH bond. The bidentate TNT  NH3 complex C is slightly more stable than the monodentate and tridentate complexes B and D. The calculated potential of complex A (Fig. 4) shows that strong attraction still exists, even when two molecules are well separated. This indicates that the major source of the attraction is not short-range interactions (E  eaR), such as charge transfer, but long-range interactions (E  Rn), such as electrostatic and dispersion [41,42]. However, for the previously reported benzene–ammonia complex, it prefers a monodentate structure with one NAH bond directing towards the benzene p ring [13a]. The reason for the great

Table 2 Calculated interaction energies (DE, kcal mol1) and intermolecular distances (R, Å see Fig. 4) at the MP2/6-311++G(d,p) level for TNT  NH3 complexes A–D. Complexes

DE

R

A B C D

3.7 0.7 1.4 0.8

3.2 3.7 3.6 3.7

To further study how the competitions between the two molecular interactions affect the structures of the TNT  NH3 complexes, we have fully optimized the geometries of the TNT  NH3 complexes at the MP2/6-311++G(d,p) level without any restrictions and calculated their vibrational frequencies. Eight complexes have been identified and their optimized structures were shown in Fig. 6. All the complexes are found as local minima on the PES by vibrational frequency calculations. As shown in Fig. 6, complex I has only conventional CAH  N hydrogen bond. Structure II is a complex with novel C  N interaction, which is also called noncovalent ‘carbon bonding’ [43]. Complex III has a combination of NAH  O hydrogen bond and NAH  p interaction. For complexes IV, V, VI and VII, the NAH  O hydrogen bonds and lp  p interactions coexist. But complex VIII has only the lp  p interaction. It is expected that the structure II is a complex with CAH  N hydrogen bond. However, the three C9AH  N angles in complex II are 86.1, 93.7, 92.9 degree, respectively. These very small angles indicate a poor directionality of the CAH  N hydrogen bond. In addition, the C  N distance in II is quite short, which is 3.259 Å and shorter than that of other previous reported C  N carbon bonding complex H3N  CH3OH (3.337 Å at the MP2/6-311++G(3df,2p) level) [43a]. The distance is also close to 3.25 Å, the sum of van der Waals radii of C and N. Considering that the MP2 method may overestimate the bond distances of molecules, we have also optimized the structure II with M06-2X method. The C  N distance of complex II is 3.153 Å at the M06-2X/6-311++G(d,p) level, which is shorter than the van der Waals radii sum of C and N. Thus, structure II should be considered as an example of the novel noncovalent carbon bonding complex [43]. Row and co-worker have used the AIM analyses to show that the complexes with hydrogen bonding and carbon bonding have different bond paths [43b]. To further identify the noncovalent carbon bonding of complex II, we have also carried out the AIM analysis for the complex. The result was present in Fig. S4 of ESI. As shown in the Fig. S4, there is indeed a bond critical point (BCP) between N and C atoms in II and the complex exhibits a bond path originating from N atom and terminating at the carbon atom of methyl group. Thus, it is clear that the interaction in complex II is a noncovalent C  N carbon bonding but not a CAH  N hydrogen

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2.985 H11

H11

C9

3.259 C9

C2 C1 C6 C3

C4

C5

3.405

2.340

2.739

9 2.8 2.909

3.054

6

H7

I Δ E = -5.2 kcal mol -1

2.924

57 2. 7

3.160

II Δ E = -1.8 kcal mol-1

3.150

2.597

V Δ E = -4.2 kcal mol-1

III Δ E = -1.2 kcal mol-1

2.859 3.211

3.035

VI Δ E = -4.3 kcal mol -1

3.228

VII Δ E = -4.3 kcal mol-1

IV Δ E = -4.0 kcal mol-1

3.071

VIII ΔE = -3.6 kcal mol-1

Fig. 6. Optimized structures and interaction energies of eight fully optimized TNT  NH3 complexes at the MP2/6-311++G(d,p) level. The distances are in angstroms.

bond, which agree with the previous results on carbon binding complexes [43a,43b]. The C  N interaction is important in the stabilization of complex II. The BSSE-corrected DEs of the eight complexes have computed at the MP2/6-311++G(d,p) and the corresponding values are presented in Fig. 6. The DE of complex I merely with CAH  N hydrogen bond is 5.2 kcal mol1 while the complex VIII with only lp  p interaction is 3.6 kcal mol1. The result show that the lp  p interaction of the TNT  NH3 complexes is comparable to the conventional CAH  N hydrogen bond. But the latter is stronger than the former. The geometry of complex VIII is similar to that of TNT  NH3 complex A and its DEs is also very close to the minima of interaction potential curves of TNT  NH3 complex A (3.7 kcal mol1). At the CCSD(T)/6-311++G(d,p)//MP2/6-311++G(d,p) level, the complexes I and VIII have the DEs of 5.7 and 3.7 kcal mol1, respectively. The hydrogen-bonded complex I is most stable structure among the eight complexes. For the four complexes IV, V, VI and VII formed with NAH  O hydrogen bond and lp  p interaction, their DEs are 4.0, 4.2, 4.3, and 4.3 kcal mol1, respectively, which show that they are more stable than the complex VIII with only lp  p interaction. We have also considered the initial structures of the complexes V and VII formed with only the NAH  O hydrogen bonding in the aromatic plane, which were shown in

Fig. S3. However, the final optimized structures of the two complexes are obviously different to their initial structures. It is intriguing to notice that the two complexes form a similar stable structure with coexistence of the lp  p interaction and weak NAH  O hydrogen bond. The lp  p interaction brings the nitrogen

Table 4 Calculated maximum absorption wavelengths (kmax, nm) for TNT and complexes I and VIII in gas phase and acetonitrile solution (in parentheses). kmax

TNT I VIII

Exp.

xB97XD

CAM-B3LYP

216 [44] (229) [45] – –

221(238) 225(240) 227(240)

222(240) 227(244) 228(243)

Table 3 Second-order perturbation stabilization energies DE(2) (kcal mol1) of the eight TNT  NH3 complexes calculated at the MP2/6-311++G(d,p) level of theory. Complexes

En!r (NAH  O/CAH  N/ C  N)

En!p (lp  p)

ENH!p (NAH  p)

I II III IV V VI VII VIII

5.26 0.88 0.50 0.14 0.59 0.42 0.60 –

– – – – 1.25 1.16 0.97 0.49

– – 0.15 0.21 – 0.18 – –

ð2Þ

ð2Þ

ð2Þ

Fig. 7. Calculated UV-vis spectra with TD-CAM-B3LYP/6-311++G(d,p) of TNT, complexes I, and VIII in the solution phase (dotted lines) and the gas phase (solid lines).

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C-H of methyl 3185 cm-1

N-H 3682 cm-1

-1

C(3,5)-H 3255 cm

N-H 3647 cm-1

266 cm-1 140 km mol-1

N-H 3661 cm-1 C-H of methyl 3206 cm-1

154 cm-1 202 km mol-1

C(3,5)-H 3213 cm-1

N-H 3661 cm-1

N-H 3640 cm-1 -1

231 cm 148 km mol-1

N-H 3647 cm

-1

N-H 3647 cm-1

217 cm-1 134 km mol-1

203 cm-1 70 km mol-1

176 cm-1 185 km mol-1

N-H 3640 cm-1 C-H of methyl 3199 cm-1

N-H 3640 cm-1

182 cm-1 159 km mol-1

126 cm-1 69 km mol-1

Fig. 8. Theoretical infrared spectra of monomers and eight complexes at the MP2/6-311++G(d, p) level. The atomic numbers of the complexes can been found in Fig. 6.

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atom and the TNT p ring close to each other and adds the stability of the complex, which results to the final structure with two types of interaction. The results indicate that the competition between hydrogen bonding and lp  p interaction in TNT  NH3 complexes plays important role to affect the structures of complex. The lp  p interaction modulates the structures of the TNT  NH3 complexes with additional stability. AIM analysis has carried out in order to characterize the interaction of the eight complexes, which were shown in Fig. S4 of ESI. The results show the presence of intermolecular BCPs in all these complexes. One BCP between the H atom of TNT and the N atom of NH3 in complex I indicates the formation of CAH  N hydrogen bonds. For complex II, a BCP is found between the C9 atom of TNT and the N atom of NH3, which conforms the existence of novel C  N interaction. There is also one BCP between TNT p ring and NH3 in complex VIII. While more than one BCP exists in complex III–VII, demonstrating that there is a combination of various interactions. For the qualitative determination of the interaction model of the eight optimized complexes, NBO analysis has been carried out at the MP2/6-311++G(d,p) level. For brevity, the donor and acceptor orbitals are provided in Table S1. The second order perturbative energy E(2)s of the three main kinds of interactions (NAH  O/CAH  N/C  N, lp  p, and NAH  p interactions) are presented in Table 3. The larger E(2) value is, the greater contribution of a certain kind of interaction for the stability of complexes makes. For instance, the complex V has a the E(2) value for NAH  p interaction of 0.10 kcal mol1, which is so small that it can be neglected in comparison with the E(2) values for NAH  O and lp  p interactions (0.59 and 1.25 kcal mol1, respectively). The result suggests that there is no NAH  p interaction in complex V but the coexistence of the lp  p interaction and NAH  O hydrogen bond. For the complexes I, the NBO analysis shows that it is a simple hydrogen bonding complexes. But for the complex V, VI, and VII, lp  p interaction and NAH  O hydrogen bond coexist and the former competes successfully with the later. The E(2) values in Tables 3 and S1 can be further validation for the main interactions of the eight optimized complexes and help us to understand the complicated interactions in the TNT  NH3 complexes. 3.4. UV–vis and Infrared spectroscopy of the TNT  NH3 complexes The detection of TNT is very important [44–46]. The detection methods for TNT based on the TNT–amine complexes have been developed [23]. Here, we calculated the UV–vis and Infrared spectra of the TNT  NH3 complexes and compared them with those of the isolated TNT. TD-DFT calculations have been employed to predict UV–vis spectra for TNT and the selected TNT  NH3 complexes with only hydrogen bonding (I) and lp  p interaction (VIII). To obtain the UV absorption spectra of TNT, four TD-DFT methods were compared with basis set 6-311++G(d,p), including: B3LYP, CAM-B3LYP, PBE0, and xB97XD. The results are shown in Fig. S5 of ESI. Compared with the experimental values, CAM-B3LYP and xB97XD show better performance to predict the UV spectra of TNT. The two methods got similar results. So here we only show the theoretical UV–vis spectra of complexes I and VIII using CAM-B3LYP method. UV–vis spectra of TNT and complexes I and VIII in gas phase and PCM acetonitrile solution are shown in Fig. 7 from the CAM-B3LYP values and maximum absorption wavelengths are listed in Table 4. The UV absorption peak maximum (kmax) of TNT by CAM-B3LYP method is 222 nm in gas phase, which is consistent with the previous theoretical results [46]. To quantify solvent effects, the maximum absorption wavelengths in acetonitrile solution were also

calculated with polarizable continuum model (PCM) [47]. In acetonitrile solution, the absorption peak maximum is located at 240 nm in the UV–vis spectrum, showing a red-shift. The experimental spectrum for TNT is with absorption peak maximum in the vicinity of 216 nm in gas phase [44] and 229 nm in acetonitrile solution [45]. As shown in Fig. 7 and Table 4, maximum absorption wavelengths of complexes I and VIII are 227 and 228 nm respectively in gas phase. Compared to TNT, maximum absorption wavelengths of complexes I and VIII show a weak red-shift. Calculated UV–Vis spectra of complexes I and VIII in acetonitrile solution are similar to those in gas phase. The results indicate that hydrogen bonding and lp  p interaction may have only a little effect on the UV–vis spectrum and the maximum absorption wavelengths. Infrared (IR) spectroscopy is a useful spectroscopic tool to identify the existence of hydrogen bonds and can provide indirect evidence for the formation of hydrogen bonds. Here, theoretical IR spectra of the eight optimized TNT  NH3 complexes are computed to explore the characteristics of the hydrogen bonds. The theoretical IR spectra were drawn in Fig. 8. It can be seen that the IR spectra of eight complexes are different from that of isolated TNT in the area from 3000 to 4000 cm1. As shown in Fig. 8, one weak peak corresponding to the vibrational stretches of CAH bonds of TNT p ring and one very weak CAH stretches of methyl are observed in the IR spectrum of TNT. After forming complexes, other seven complexes (except for complex II) all have an elongated C/NAH bond with a red-shift stretching frequency of about 42 cm1. These shifts observed in the theoretical IR spectra of the eight TNT  NH3 complexes confirm the formation of hydrogen bonds. The CAH stretching frequency of complex II has an improper blue-shift of 21 cm1. The result is consistent with the previously studied CAH bond with a blue-shift vibrational frequency in NH3  CHnX4n [48]. In addition, new peaks evidently appear in the area from 100 to 300 cm1 in all the eight complexes and their relative infrared intensities are in the range of 69–202 km mol1, indicating that there is interaction between TNT and NH3. The vibrational modes of these new peaks in the area from 100 to 300 cm1 for the eight complexes are shown in Fig. S6 of ESI, which clearly show that these new peaks are correlated to the rotation of ammonia and come from the interaction between TNT and NH3. The theoretical studies on UV and IR spectroscopy of the TNT  NH3 complexes show that the formations of hydrogen bonding and lp  p interaction have small effect on the UV spectra but new IR absorption peaks in range of 100–300 cm1 have been predicted in the TNT  NH3 complexes, which may be helpful for the detection of TNT.

4. Conclusions Ab intio calculations were employed to study the interactions between TNT and NH3 by means of MP2 and five DFT methods. MP2 is the best overall performer for the titled complexes giving accuracy close to CCSD(T) method. The B3LYP method without dispersion correction underestimates the DE by about 1.6 kcal mol1, indicating the dispersion interaction is significantly important for the present lp  p interaction complexes. The MP2 and DFT methods with dispersion correction should be chosen to study these lp  p systems. The attractive lp  p interaction between TNT and NH3 is observed when the lone pair of nitrogen points toward the TNT p ring. The studies on the orientation and horizontal displacement dependence of DE show that both of the horizontal rotating and moving of ammonia lead to weaker interaction. Moving NH3 horizontally has a little impact on interaction energies but the DE has strong orientation dependence. The substitution effects on nitrogen lp  p interaction of present systems are similar

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to previous reported systems. The competition between hydrogen bonding and lp  p interaction in TNT  NH3 complexes has also been studied and discussed. The results show that the lp  p interaction competes successfully with the NAH  p and NAO  H hydrogen bonds but is weaker than the conventional CAH  N hydrogen bond. The results also indicate that the competition between hydrogen bonding and lp  p interaction in TNT  NH3 complexes plays important role to affect the structures of complexes. The stabilizing effects of the lp  p interaction result to that the TNT  NH3 complexes with only the NAH  O hydrogen bond cannot be located. The combination of hydrogen bonding and lp  p interaction controls the structures of complexes. Among the eight TNT  NH3 complexes, the complex I merely with CAH  N hydrogen bond has an DE of 5.2 kcal mol1 at the MP2/6-311++G(d,p) level and is the most stable one. Novel noncovalent C  N carbon bonding is observed in the complex II. The main interactions of the eight TNT  NH3 complexes are qualitatively determined by NBO analysis. TD-DFT calculations have been employed to predict UV–vis spectra for TNT and the selected TNT  NH3 complexes. The predicted UV kmax of TNT by CAM-B3LYP method in gas phase and solvent are in good agreements with the experiment values and the previous theoretical results. But the calculated UV–vis spectra of TNT  NH3 complexes show that the formation of hydrogen bonding and lp  p interaction has small effect on the UV spectra. The theoretical IR spectra show that the most complexes have classical hydrogen bonds with a stretching frequency red-shifted by about 42 cm1, except for complex II with an improper blue-shift. New IR absorption peaks in range of 100–300 cm1 have been predicted in the TNT  NH3 complexes. The new IR peaks provide further evidence for the formation of interaction between TNT and NH3 and may be helpful for the detection of TNT. Acknowledgments This project was supported by National Science Foundation of China under Grant No. 21472049 and Open Project Program of State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University), China (2010-14). We are grateful to the reviewer for her/his constructive comments and suggestions on the novel noncovalent carbon bonding in complex II. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2015.04. 006. References [1] A. Karshioff, Non-convalent Interactions in Proteins, Imperial College Press, Singapore, 2006. [2] J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995. [3] K. Müller-Dethlefs, P. Hobza, Noncovalent interactions: a challenge for experiment and theory, Chem. Rev. 100 (2000) 143–167. [4] (a) G.A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford University Press, New York, 1997; (b) G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond, Oxford University Press, Oxford, 1999; (c) S. Scheiner, Hydrogen Bonding, Oxford University Press, New York, 1997; (d) S.J. Grabowski, Hydrogen Bonding-New Insights, Springer, Dordrekht, Netherlands, 2006; (e) G. Gilli, P. Gilli, The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory, Oxford University Press, Oxford, UK, 2009; (f) S.J. Grabowski, What is the covalency of hydrogen bonding?, Chem Rev. 111 (2011) 2597–2625. [5] M. Egli, S. Sarkhel, Lone pair-aromatic interactions: to stabilize or not to stabilize, Acc. Chem. Res. 40 (2007) 197–205.

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