Article pubs.acs.org/JPCA
Theoretical Study of Interactions between Halogen-Substituted s‑Triazine and Halide Anions Yishan Chen* School of Chemistry & Chemical Engineering, Qujing Normal University, Qujing 655011, Yunnan, China S Supporting Information *
ABSTRACT: The interactions between halogen-substituted s-trazine (C3H2N3X) and halide anions (Y−) have been investigated at the MP2/aug-cc-pVDZ (aug-cc-pVDZ-PP) level. C3H2N3X can interact with halide anions to form five types of complexes (C3H2N3X·Y−): a strong σ-type interaction complex, a weak σ-type interaction complex, an anion−π interaction complex, a hydrogen-bonding complex, and a halogen-bonding complex. The binding energies, structures, and bonding characteristics of these complexes have been discussed. The local details of potential energy surfaces around the binding sites for some selected complexes have been depicted. The results indicate that the binding behavior of F− is quite different from that of Cl−, Br−, and I−. The potential energy surface is separated into two parts, the HB-σ−π region and the XB region, by a relatively high energy barrier for complexes C3H2N3Cl·Cl−, C3H2N3Br·Cl−, and C3H2N3I·Cl−. The HB-σ−π region is characterized by the flat potential energy surface, indicating that the binding strength is retained when the anion is held over the HB-σ−π region. The XB region is characterized by the steeper potential energy surface, indicating that the binding strength is more sensitive to the anion position in this region. The binding strength of the HB-σ−π region is stronger than that of the XB region for C3H2N3Cl·Cl− and C3H2N3Br·Cl−, whereas the binding strength of the XB region is stronger than that of the HB-σ−π region for C3H2N3I·Cl−.
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INTRODUCTION
In this study, we select halogen-substituted s-trazine (Figure 1a, abbreviated as C3H2N3X) as a model receptor molecule to understand the interactions between electron-deficient arenes and anions. As shown in Figure 1b−f, C3H2N3X can interact with halide anions (Y−) to form all five types of complexes (abbreviated as C3H2N3X·Y−): (1) strong σ-type interaction complexes in which the fluoride anion attacks a partially positive aromatic carbon, changing the hybridization of arene-C to sp3; (2) weak σ-type interaction complexes, where the anion is located over the periphery of the aromatic ring, which can be formed with less nucleophilic halides (Y = Cl, Br, I); (3) anion−π interaction complexes, which may also be formed with less nucleophilic halides (Y = Cl, Br, I); (4) HB complexes, involving the interaction between the aryl C−H donor and the anion; and (5) XB complexes, which can be formed when X is a less nucleophilic halide (X = Cl, Br, I). In this work, we investigate the binding energies, structures, and bonding characteristics of these complexes and compare the relative strengths of various interactions. Furthermore, we examine the local details of potential energy surfaces around the binding sites for some selected complexes. All of the studies would be useful to provide new insight into the interactions between electron-deficient arenes and halide anions.
Anion complexation by synthetic host molecules is an important theme in supramolecular chemistry and is an area of intense interest that has relevance to biology, industry, and environment.1−10 In the past decade, experimental and theoretical studies have accumulated evidence for the existence of a variety of binding modes for complexes of anions with electron-deficient arenes.11−45 When the halide lies above the plane of the π system, the studies establish that three distinctly different types of interaction are possible: strong σ-type interactions, weak σ-type interactions, and anion−π interactions.17 Additionally, both experiment and theory reveal that electron-deficient arenes bearing C−H groups are potent hydrogen-bond donors, and aryl C−H···anion interactions are very common in supramolecular systems.18 Joining hydrogen bonding (HB) and anion−π interactions, halogen bonding (XB) between electron-deficient arenes and anions has also emerged as a useful method for selective anion recognition processes.40−42 The interactions between arenes and anions have been extensively investigated using quantum chemical methods. In a previous study, Berryman et al. refined the nature of the first four interactions mentioned above (strong σ-type interactions, weak σ-type interactions, anion−π interactions, and HB).17 The theoretical studies concerning XB between halogen-substituted arenes and anions have also been reported in recent years.43−45 However, to the best of our knowledge, a comprehensive and comparative study of all five interactions is still absent in the literature. © 2013 American Chemical Society
Received: July 12, 2013 Revised: August 1, 2013 Published: August 13, 2013 8081
dx.doi.org/10.1021/jp4069015 | J. Phys. Chem. A 2013, 117, 8081−8090
The Journal of Physical Chemistry A
Article
Figure 2. Electrostatic potential surfaces (ranging from −20 to 20 kcal/mol) of receptors.
Figure 1. Receptor and complexes investigated in this study.
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COMPUTATIONAL DETAILS The geometries of all of the complexes studied in this work were fully optimized at the MP2 level of theory using the Gaussian 03 programs.46 The aug-cc-pVDZPP basis set, which uses pseudopotentials to describe the inner core orbitals, was employed for iodine, whereas aug-cc-pVDZ was applied for other atoms. The vibrational frequencies were calculated for all of the optimized geometries to verify that these optimized structures are indeed minima on the potential energy surfaces. Single-point energy calculations were performed using the augcc-pVTZ basis set (aug-cc-pVTZPP for iodine) to obtain more accurate energies. Basis set superposition error (BSSE) correction was carried out following the counterpoise (CP) method.47 Moreover, the energies at the MP2/CBS level were obtained by means of extrapolation according to the Helgaker method48 based on the aug-cc-pVDZ and aug-cc-pVTZ energies. The relaxed potential energy surface scans were performed for some selected complexes at the MP2/aug-ccpVDZ (aug-cc-pVDZ-PP) level. Natural bond orbital (NBO) analysis49 was performed via the procedures contained within Gaussian 03, and atoms in molecules (AIM) analysis50 was carried out using the AIM 2000 program.51
listed. The energies with BSSE correction at the MP2/aug-ccpVTZ level will be used in the discussion. The optimized structures of five representative complexes (17, 18, 30, 46, and 58) are illustrated in Figure 3. The equilibrium distances of other complexes are also collected in the tables. The binding energies and equilibrium Y···C distances for σtype interaction complexes are listed in Table 1. As shown in Figure 3 (complex 17), F− can attack the aromatic carbon atom, and the ring carbon under attack adopts a tetrahedral geometry. The F−C distance is smaller than 1.5 Å, indicating that this interaction is a strongly covalent interaction. Different from F−, the less nucleophilic halides (Cl−, Br−, I−) interact with C3H2N3X by weak σ-type interactions. As displayed in the structure of complex 18, the ring carbon atom still keeps a planar geometry. The binding energies of weak σ-type interaction complexes (8−12 kcal/mol) are obviously smaller than those of strong σ-type interaction complexes (35−38 kcal/ mol). The binding strength of a weak σ-type interaction is in the sequence of Cl− > Br− > I−. On the other hand, the binding strength of different receptor molecules goes in the order 1 < 2 < 3 < 4, but this difference is rather small. For example, the binding energies of complexes 6, 10, 14, and 18 are −11.39, −11.72, −12.04, and −12.15 kcal/mol, respectively. The binding energies and equilibrium distances for anion−π complexes are listed in Table 2. Anion−π complexes can only be formed with less nucleophilic halides, and the anion−π structure is not a stable geometry for F−. Different from weak σtype interaction complexes in which the halide anion is located outside of the ring perimeter, the halide is located directly above the arene centroid in anion−π complexes, as shown in Figure 3. Although there is an obvious difference in geometry between weak σ-type interactions and anion−π complexes, their binding energies are very similar. For example, the binding energies of complexes 21 (−10.94 kcal/mol), 22 (−9.82 kcal/ mol), and 23 (−8.49 kcal/mol) are very close to those of 6 (−11.39 kcal/mol), 7 (−9.89 kcal/mol), and 8 (−8.28 kcal/ mol), respectively. In fact, the potential energy surfaces between weak σ-type interactions and anion−π binding sites are very flat, as discussed below.
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RESULTS AND DISCUSSION Binding Energies and Structures of Complexes. The molecular electrostatic potential (MEP) surfaces of receptor molecules are represented in Figure 2. The MEP maps clearly indicate that an area of positive charge concentrates on the center of the molecule and expands to the hydrogen atoms along the C−H bonds. As an example, the approximate binding sites of σ-type interactions, anion−π interactions, and HB are illustrated in the MEP map of molecule 2 (C3H2N3Cl). It can be observed that there is a progressive increase of the positive halogen surface region (σ-hole) from molecule 2 to 4. The absence of a σ-hole in molecule 1 (C3H2N3F) implies that 1 cannot interact with halide anions by XB. In Tables 1−4, we collect the binding energies with and without BSSE correction for complexes 5−60 at the MP2/augcc-pVTZ level, and the energies at the MP2/CBS level are also 8082
dx.doi.org/10.1021/jp4069015 | J. Phys. Chem. A 2013, 117, 8081−8090
The Journal of Physical Chemistry A
Article
Table 1. Binding Energy without (in parentheses) and with BSSE Correction at the MP2/aug-cc-pVTZ Level and MP2/CBS Level (ΔE, in kcal/mol), Equilibrium Distance (RY···C, in Å), Second-Order Perturbation Stabilization Energy (E(2)LP(Y−)→π*(CN), in kcal/mol), Charge Transfer from the Anion to the Arene Derived from Natural Population Analysis (qCT, in e), Wiberg Bond Index (WBIY···C), and Electron Density (ρY···C, in au) for σ-Type Interaction Complexes ΔE complex −
5 (1+F ) 6 (1+Cl−) 7 (1+Br−) 8 (1+I−) 9 (2+F−) 10 (2+Cl−) 11 (2+Br−) 12 (2+I−) 13 (3+F−) 14 (3+Cl−) 15 (3+Br−) 16 (3+I−) 17 (4+F−) 18 (4+Cl−) 19 (4+Br−) 20 (4+I−)
aug-cc-pVTZ −35.28 −11.39 −9.89 −8.28 −36.95 −11.72 −10.13 −8.46 −38.07 −12.04 −10.41 −8.69 −38.84 −12.15 −10.49 −8.78
(−37.70) (−12.31) (−11.87) (−11.08) (−39.42) (−12.71) (−12.26) (−11.47) (−40.61) (−13.11) (−12.64) (−11.82) (−41.44) (−13.27) (−12.80) (−12.00)
E(2)LP(Y−)→π*(CN)
CBS
RY···C
−36.87 −11.78 −10.27 −8.70 −38.61 −12.16 −10.57 −8.94 −39.79 −12.52 −10.88 −9.21 −40.51 −12.63 −10.97 −9.30
1.495 2.796 3.000 3.252 1.489 2.760 2.964 3.227 1.485 2.742 2.949 3.212 1.481 2.729 2.939 3.214
15.19 11.11 8.27 16.90 12.27 8.61 18.08 12.99 9.06 18.87 13.36 8.70
qCT
WBIY···C
ρY···C
0.4575 0.0636 0.0516 0.0431 0.4631 0.0705 0.0568 0.0448 0.4667 0.0750 0.0600 0.0472 0.4703 0.0778 0.0615 0.0458
0.6406 0.0802 0.0641 0.0528 0.6482 0.0885 0.0704 0.0550 0.6531 0.0941 0.0743 0.0578 0.6582 0.0976 0.0761 0.0561
0.1744 0.0194 0.0160 0.0131 0.1771 0.0209 0.0172 0.0139 0.1788 0.0217 0.0177 0.0143 0.1806 0.0223 0.0181 0.0143
strength of those formed by O−H and N−H groups. Multiple substitution with electron-withdrawing substituents can result in very strong HB interactions.18 The binding energies of HB complexes (33−48) are 8−23 kcal/mol, which range from moderate to strong hydrogen bonds. Similar to the σ-type and anion−π interactions, the binding strength of HB interactions is in the sequence of F− > Cl− > Br− > I−, and once again, the difference in binding strength of different receptor molecules is rather small. The binding energies and equilibrium Y···X distances for XB complexes are listed in Table 4. Similar to the HB complex, the halogen bond in complex 58 also possesses a strictly linear geometry, as displayed in Figure 3. The calculation indicates that receptor 1 cannot bind the anions by XB, and the binding strength of the other three receptor molecules goes in the order 2 < 3 < 4. This result is consistent with the postulation derived from the MEP results. The binding strength of the XB interaction is in the sequence of F− > Cl− > Br− > I−, which is identical to the results of σ-type, anion−π, and HB interactions. However, different from the other three interactions in which different receptor molecules are very similar in binding strength, the binding strength of different receptors shows a great difference in XB complexes. For example, the binding energies of complexes 50, 54, and 58, are −6.08, −10.13, and −17.20 kcal/mol, respectively. A comprehensive comparison of these interactions will be presented in the discussion of potential energy surfaces around the binding sites. Before that, we examine the bonding characteristics of various complexes by NBO and AIM analyses. NBO and AIM Analysis. NBO analysis can provide some bonding information on the formation of complexes. The orbital interaction between filled and empty natural bond orbitals can be estimated with the respective second-order perturbation stabilization energy (E(2)). Atomic charges derived from natural orbital analysis reflect the extent of charge transfer (CT) from the anion to the arene. The bond strength can also be estimated with the Wiberg bond index (WBI). The leading orbital interactions and E(2) values, CT, and the corresponding WBI for various complexes have been given in the tables. We
Figure 3. Optimized geometries of the selected complexes; distances are in Å.
The binding energies and equilibrium Y···H distances for HB complexes are listed in Table 3. As shown in the structure of complex 46, the hydrogen bond in the complex shows a strictly linear geometry. It is generally believed that C−H groups form much weaker hydrogen bonds than conventional donor groups such as O−H and N−H. However, the theoretical calculations and experimental binding energies suggest that even in the absence of electron-withdrawing substituents, simple arenes form hydrogen bonds with anions that can exceed 50% of the 8083
dx.doi.org/10.1021/jp4069015 | J. Phys. Chem. A 2013, 117, 8081−8090
The Journal of Physical Chemistry A
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Table 2. Binding Energy without (in parentheses) and with BSSE Correction at the MP2/aug-cc-pVTZ Level and MP2/CBS Level (ΔE, in kcal/mol), Equilibrium Distance (Re, in Å), Charge Transfer from the Anion to the Arene Derived from Natural Population Analysis (qCT, in e), Wiberg Bond Index (WBIsum), and Electron Density (ρcage, in au) for Anion−π Interaction Complexes ΔE complex 21 22 23 24 25 26 27 28 29 30 31 32
(1+Cl−) (1+Br−) (1+I−) (2+Cl−) (2+Br−) (2+I−) (3+Cl−) (3+Br−) (3+I−) (4+Cl−) (4+Br−) (4+I−)
aug-cc-pVTZ −10.94 −9.82 −8.49 −11.13 −10.02 −8.69 −11.39 −10.26 −8.91 −11.56 −10.44 −9.10
(−11.79) (−11.80) (−11.35) (−12.06) (−12.12) (−11.71) (−12.42) (−12.50) (−12.09) (−12.68) (−12.77) (−12.38)
CBS
Re
qCT
WBIsum
ρcage
−11.27 −10.20 −8.95 −11.50 −10.44 −9.19 −11.79 −10.72 −9.47 −11.98 −10.92 −9.68
3.080 3.248 3.488 3.068 3.232 3.467 3.061 3.225 3.460 3.050 3.215 3.452
0.0102 0.0095 0.0082 0.0110 0.0100 0.0084 0.0114 0.0104 0.0088 0.0116 0.0107 0.0092
0.0216 0.0202 0.0181 0.0236 0.0217 0.0191 0.0243 0.0224 0.0196 0.0244 0.0225 0.0201
0.00703 0.00653 0.00583 0.00713 0.00668 0.00603 0.00720 0.00674 0.00609 0.00730 0.00681 0.00613
Table 3. Binding Energy without (in parentheses) and with BSSE Correction at the MP2/aug-cc-pVTZ and MP2/CBS Levels (ΔE, in kcal/mol), Second-Order Perturbation Stabilization Energy (E(2)LP(Y−)→σ*(C−H), in kcal/mol), Charge Transfer from the Anion to the Arene Derived from Natural Population Analysis (qCT, in e), Wiberg Bond Index (WBIY··H), and Electron Density (ρY···H, in au) for HB Complexes ΔE complex 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
−
(1+F ) (1+Cl−) (1+Br−) (1+I−) (2+F−) (2+Cl−) (2+Br−) (2+I−) (3+F−) (3+Cl−) (3+Br−) (3+I−) (4+F−) (4+Cl−) (4+Br−) (4+I−)
aug-cc-pVTZ −22.83 −12.57 −10.85 −8.89 −23.09 −12.58 −10.82 −8.84 −23.46 −12.76 −10.98 −8.98 −23.51 −12.71 −10.92 −8.91
(−23.96) (−13.48) (−12.85) (−11.76) (−24.25) (−13.50) (−12.87) (−11.76) (−24.65) (−13.72) (−13.07) (−11.95) (−24.74) (−13.70) (−13.05) (−11.93)
CBS
RY···H
E(2)LP(Y−)→σ*(C−H)
qCT
WBIY···H
ρY···H
−22.93 −12.76 −11.04 −9.10 −23.23 −12.78 −11.02 −9.06 −23.62 −12.98 −11.20 −9.21 −23.65 −12.92 −11.14 −9.14
1.485 2.250 2.428 2.641 1.470 2.242 2.420 2.634 1.462 2.235 2.415 2.628 1.454 2.230 2.409 2.622
65.05 20.99 18.05 16.10 68.14 21.51 18.43 16.37 70.12 21.95 18.70 16.63 72.07 22.23 18.97 16.84
0.0862 0.0426 0.0390 0.0374 0.0898 0.0436 0.0398 0.0380 0.0920 0.0445 0.0405 0.0388 0.0942 0.0451 0.0411 0.0393
0.1110 0.0530 0.0484 0.0463 0.1158 0.0542 0.0493 0.0471 0.1188 0.0553 0.0501 0.0479 0.1217 0.0560 0.0507 0.0484
0.0680 0.0228 0.0194 0.0170 0.0705 0.0232 0.0197 0.0172 0.0720 0.0235 0.0199 0.0174 0.0735 0.0238 0.0202 0.0176
have performed the AIM analysis of all complexes included in this study. The distribution of critical points (CPs) in some selected complexes is illustrated in Figure 4. The electron densities at the corresponding bond and cage CPs are listed in the tables. As shown in Table 1, the leading orbital interaction between receptor and anion in weak σ-type interaction complexes is LP(Y−) → π*(CN). The NBO analysis indicates that the receptor and F− are within a unit for strong σ-type interaction complexes, and therefore, the E(2) values for strong σ-type interaction complexes are not listed in Table 1. The CT of strong σ-type interaction complexes is about 0.46, and this value is obviously larger than that (0.04−0.08) of weak σ-type interaction complexes. Similarly, the WBI of strong σ-type interaction complexes is about 0.65, and this value is obviously larger than that (0.05−0.1) of weak σ-type interaction complexes. The AIM analysis also indicates that the densities (>0.17) of strong σ-type interaction complexes are much larger than those (
