Article pubs.acs.org/JPCA
The π···π Stacking Interactions between Homogeneous Dimers of C6FxI(6−x) (x = 0, 1, 2, 3, 4, and 5): A Comparative Study with the Halogen Bond Weizhou Wang,† Yu Zhang,† and Yi-Bo Wang*,‡ †
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, China Department of Chemistry, and Key Laboratory of Guizhou High Performance Computational Chemistry, Guizhou University, Guiyang 550025, China
‡
S Supporting Information *
ABSTRACT: The π···π stacking interactions between homogeneous dimers of C6FxI(6−x) (x = 0, 1, 2, 3, 4, and 5) have been investigated in detail using the state-of-the-art quantum chemistry methods. Computations clearly show that the π···π stacking interaction between the homogeneous dimer of C6FxI(6−x) is of the dispersion type interaction. At the same time, it is interesting to find that, for the π···π stacking interactions between homogeneous dimers of C6FxI(6−x), the M05-2X/def2-TZVPP computations give almost the same results as the CCSD(T)/SDD** computations. In the crystal growth and design, the formation of the π···π stacking interactions between homogeneous dimers of C6FxI(6−x) is always accompanied by the formation of the halogen bonds. Hence, competition and cooperation of the π···π stacking interaction and the halogen bond have also been studied theoretically by using C6FxI(6−x) and pyridine as coformers. At the M05-2X/def2-TZVPP theory level, it is found that the π···π stacking interactions in C6F5I···C6F5I and C6F4I2···C6F4I2 are weaker than the corresponding halogen bonds in C6F5I···NC5H5 and C6F4I2···NC5H5, and the π···π stacking interactions in C6FI5···C6FI5 and C6I6···C6I6 are stronger than the corresponding halogen bonds in C6FI5···NC5H5 and C6I6···NC5H5, while the strengths of the π···π stacking interactions in C6F3I3···C6F3I3 and C6F2I4···C6F2I4 are similar to the corresponding halogen bonds in C6F3I3···NC5H5 and C6F2I4···NC5H5. However, when the π···π stacking interaction and the halogen bond coexist, we find that the formation of the halogen bond will lead to the π···π stacking interaction much stronger, and vice versa.
1. INTRODUCTION Halogen bond, the noncovalent interaction involving a halogen atom as an acceptor of electron density,1 has been intensively studied during the past decade and can now be considered as a new item in the supramolecular toolbox for the design and synthesis of new supramolecular systems with desired architectures and functions.2−6 In supramolecular chemistry and especially in crystal engineering involving the halogen bond, aryl or heteroaryl iodide is often used as the halogen atom donor.2−8 Along with the formation of the halogen bond between the aryl or heteroaryl iodide and the electron density donor, the π···π stacking interaction is routinely observed between the homogeneous dimer of the aryl or heteroaryl iodide. To further confirm the above observation, we carried out a simple statistical analysis of the halogen bond in which the iodine atom of 1,3,5-trifluoro-2,4,6-triiodobenzene (C6F3I3) behaves as the acceptor of electron density with the Cambridge Structure Database (CSD version 5.22 + 35 updates).9,10 The program ConQuest,11 part of the CSD, was used to construct each interaction search since it offers a user-friendly interface. The following general search filters were also chosen from the ConQuest search menu: 3D coordinates determined, not disordered, no errors, not polymeric, no ions, only organics, © 2012 American Chemical Society
and R < 5%. The CSD search yields six individual crystal structures (CSD entry code: RUYJAX, UCEPEY, UCEPEY01, WEXVUR, WEXWAY, and WEXWEC) containing the halogen bond in which the iodine atom of C6F3I3 behaves as the acceptor of electron density. Inspection of the structures shows that five structures (UCEPEY, UCEPEY01, WEXVUR, WEXWAY, and WEXWEC) contain the π···π stacking interaction formed between the homogeneous dimer of C6F3I3. Although always coexisting with the halogen bond, the role of the π···π stacking interaction between the homogeneous dimer of the aryl or heteroaryl iodide in crystal engineering is seldom discussed and little understood. Pyridine is a strong Lewis base and often used as the halogen atom acceptor in the design of supramolecular structures and solid-state materials. In the present study, we selected C6FxI(6−x) (x = 0, 1, 2, 3, 4, and 5) and pyridine as coformers to model theoretically the competition and cooperation of the π···π stacking interaction and the halogen bond in the crystal growth and design (Figure 1). The purpose of this work is Received: August 12, 2012 Revised: November 28, 2012 Published: December 6, 2012 12486
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number of points used in the numerical integration.17−20 In the present study, an ultrafine integration grid (99 radial; 590 angular points) was used for all the DFT calculations to avoid the possible integration grid errors. CCSD(T)/SDD** computations were performed with MOLPRO quantum chemistry package.21 Other electronic structure calculations were carried out using the Gaussian09 suite of electronic structure programs.22 The newest version of the dispersion correction (DFT-D3) was calculated using the stand-alone code of Grimme et al.14 The natural bond orbital (NBO) theory analysis of Weinhold and co-workers was employed to quantitatively evaluate the electron density transfer involving the formation of the π···π stacking interaction and the halogen bond.23 NBO analysis used the M05-2X/def2TZVPP optimized structures, the M05-2X/def2-TZVPP densities, and the built-in subroutines of the Gaussian09 program.
Figure 1. Competition and cooperation of the π···π stacking interaction and the halogen bond. The blue mesh represents the positive part of the electrostatic potential, and the red and yellow meshes represent the negative part of the electrostatic potential.
3. RESULTS AND DISCUSSION 3.1. Stacking Interaction between Homogeneous Dimer of C6FxI(6−x). The π···π stacking interaction is another one of the most important noncovalent driving forces for supramolecular assembly. Benzene dimer is of key importance as a prototype of the π···π stacking interaction, and it has been extensively studied both experimentally and theoretically.24−35 One of the most important theoretical findings for the π···π interaction is that the π···π interaction is largely dependent on the long-range dispersion force. Even in the π-stacked complexes such as C6H6···C6F6 and C6H6···C6(CN)6 the electrostatic contribution to the interaction energy is still smaller than that of dispersion.15 It is well-known that the London dispersion interaction can not be correctly described by many theoretical methods. Although the M05-2X density functional is always recommended for studying equilibrium configurations of dispersion bound complexes,13,36 in order to avoid the possible computational artifacts, we still carried out a series of computations with different theoretical methods to check the reliability of the M05-2X/def2-TZVPP computations for the study of the π···π stacking interactions. All the computational results are listed in Table 1. The M05-2X/ def2-TZVPP optimized structures of the stacked dimers in Table 1 are shown in Figure 2. First, we selected the smallest dimer C6F5I···C6F5I as a model to examine the effect of the basis set size on the interaction energy. It can be clearly seen from Table 1 that the interaction energy values calculated using different basis sets are very similar, which indicates that the effect of the basis set size on the interaction energy is less pronounced. It is unrealistic to compute these stacked dimers at the CCSD(T)/CBS theory level, which is often referred to as
three-fold: (i) to explore the strength and nature of the π···π stacking interaction between the homogeneous dimer of C6FxI(6−x); (ii) to compare the strengths of π···π stacking interaction and the halogen bond; (iii) to study the cooperation of the π···π stacking interaction and the halogen bond. Evidently, the interaction between the highly polarized iodine and the nitrogen atom is strong. Now, it is generally accepted that the strong halogen bond is an electrostatically driven highly directional noncovalent interaction.12 Hence, we will not discuss the nature of the halogen bond any further in this article.
2. COMPUTATIONAL DETAILS All the structures were fully optimized with tight convergence criteria and characterized by frequency computations and wave function stability checks at the M05-2X/def2-TZVPP13 level of theory. In order to check the reliability of the M05-2X/def2TZVPP computations for the study of the π···π stacking interactions, additional single-point computations were carried out at the M05-2X/aug-cc-pVDZ, M05-2X/aug-cc-pVTZ, M05-2X-D3/def2-TZVPP,14 and CCSD(T)/SDD** levels of theory, respectively. SDD** is the core basis sets (D95 V for C and F, and [2s3p] for I) augmented by two sets of polarization functions at carbons and halogens [(C, F, I) = 0.8, 0.25; 0.8, 0.25; 0.4, 0.07].15 The binding energies of the complexes were calculated using the supermolecule method. All binding energies reported are corrected for basis set superposition error using the counterpoise method of Boys and Bernardi.16 Her, we will note that a negative interaction energy refers to a bound complex. It is well-known that the accuracy of density functional theory (DFT) calculations also depends on the
Table 1. Interaction Energies (in kcal/mol) for the Homogeneous Dimers of C6FxI(6−x), Where x = 0, 1, 2, 3, 4, and 5, at Different Levels of Theory dimer/method M05-2X/def2-TZVPP M05-2X/aug-ccpVDZ M05-2X/aug-ccpVTZ M05-2X-D3/def2TZVPP CCSD(T)/SDD** HF/SDD**
C6F5I···C6F5I C6F4I2···C6F4I2 C6F3I3···C6F3I3 C6F2I4···C6F2I4−1 C6F2I4···C6F2I4−2 C6FI5···C6FI5−1 C6FI5···C6FI5−2 C6I6···C6I6 −4.10 −4.26
−5.07
−5.97
−6.28
−5.85
−7.47
−6.79
−7.94
−4.43
−5.44
−6.37
−6.68
−6.24
−7.90
−7.20
−8.40
−3.91 7.63
−4.83 8.50
−5.98 9.10
−6.02 9.20
−5.87 8.98
−6.65 10.60
−6.69 9.37
−7.32 11.08
−4.10
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Figure 2. M05-2X/def2-TZVPP optimized structures (left, side view; right, bird view) of the stacked dimers C6F5I···C6F5I, C6F4I2···C6F4I2, C6F3I3···C6F3I3, C6F2I4···C6F2I4−1, C6F2I4···C6F2I4−2, C6FI5···C6FI5−1, C6FI5···C6FI5−2, and C6I6···C6I6.
energy differences between different configurations are generally small (less than 1 kcal/mol). Dispersion forces are due to electron correlation; the correlation energy can be approximately estimated by the difference between the CCSD(T) energy and the Hartree− Fock energy. Hence, the results from HF calculations can give us some hints on the role of each energy component. Table 1 lists the interaction energies calculated at the HF/SDD** level of theory. The positive values of the interaction energies indicate that these dimers are unbound at the HF theory level and the strong π···π stacking interactions between homogeneous dimers of C6FxI(6−x) are also of the dispersion type interactions. We also noticed that, with the increasing of the number of iodine atoms, the interaction energy at the HF theory level becomes more and more positive. This means that contribution of the dispersion to the total intermolecular interaction energy becomes larger and larger with the increasing of the number of iodine atoms. 3.2. Competition of the π···π Stacking Interaction and the Halogen Bond. Unlike the π···π stacking interactions between homogeneous dimers of C6FxI(6−x), the strong halogen bonds are of the electrostatic energy-dominated interactions. A large number of studies have proven that both density functional theory and MP2 methods are capable of proper description of the electrostatics-dominated complexes.36,38 Further, in a previous study,39 we found that M05-2X functional performs slightly better than the M05 and M06 functionals and much better than the M06-2X, M06-HF, M06L, and B3LYP functionals for the study of the halogen bond. So, in the present study, we still used the M05-2X functional to calculate the strength of the halogen bond. The M05-2X/def2-TZVPP optimized structures and interaction energies of the halogen-bonded dimers C6F5I···NC5H5,
the golden standard of quantum chemistry. Instead, we computed the interaction energies of these stacked dimers at the CCSD(T)/SDD** level of theory. As clearly shown in Table 1, the M05-2X functional gives almost the same results as the CCSD(T) ones. In recent papers,14,37 Grimme and coworkes reported that the accuracy of the calculations can be significantly improved by using the most recent version of modern dispersion corrected density functional theory (DFTD3). Here, the new DFT-D3 correction is also applied to the M05-2X functional in order to test its performance for the π···π stacking interaction between the homogeneous dimer of C6FxI(6−x). The results in Table 1 show that the differences with and without D3 dispersion energy correction are small. All these results indicate that the M05-2X/def2-TZVPP computations can give reliable results for the study of the π···π stacking interactions between homogeneous dimers of C6FxI(6−x). It can be clearly seen from Table 1 that the interaction energies calculated at the M05-2X/def2-TZVPP or CCSD(T)/ SDD** level of theory range from about −4.00 to 8.0 kcal/mol, which indicates that the π···π stacking interactions between homogeneous dimers of C6FxI(6−x) are all stronger than the π···π stacking interactions between benzene dimer. Let us add here that the best estimate of the interaction energy for the parallel-displaced configuration of the benzene dimer is −2.78 kcal/mol.34 In addition, with the increasing of the number of iodine atoms, the π···π stacking interaction between the homogeneous dimer of C6FxI(6−x) becomes much stronger. The configurations we selected in Figure 2 are mainly from the crystal structures. Without any doubt, there are many other stacked configurations for each homogeneous dimer of C6FxI(6−x). Comparisons of the interaction energies of C6F2I4···C6F2I4−1 and C6F2I4···C6F2I4−2 and the interaction energies of C6FI5···C6FI5−1 and C6FI5···C6FI5−2 show that the 12488
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Figure 3. M05-2X/def2-TZVPP optimized structures and interaction energies (red number; kcal/mol) of the halogen-bonded dimers C6F5I···NC5H5, C6F4I2···NC5H5, C6F3I3···NC5H5, C6F2I4···NC5H5, C6FI5···NC5H5, and C6I6···NC5H5.
C 6 F 4 I 2 ···NC 5 H 5 , C 6 F 3 I 3 ···NC 5 H 5 , C 6 F 2 I 4 ···NC 5 H 5 , C6FI5···NC5H5, and C6I6···NC5H5 are shown in Figure 3. The binding energies of these dimers are all larger than 5.40 kcal/ mol. As expected, the halogen bonds considered here are of the strong type. The fluorine atom acts as a good electron withdrawing agent, due to its high electronegativity. Hence, with the increasing of the number of iodine atoms, the halogen bond in the dimer C6FxI(6−x)···NC5H5 becomes more and more weak, as shown in Figure 3. The contrary behavior of the halogen bond and the π···π stacking interaction with the increasing of the number of iodine atoms leads to that the π···π stacking interactions in C6F5I···C6F5I and C6F4I2···C6F4I2 are weaker than the corresponding halogen bonds in C6F5I···NC5H5 and C6F4I2···NC5H5 and that the π···π stacking interactions in C6FI5···C6FI5 and C6I6···C6I6 are stronger than the corresponding halogen bonds in C6FI5···NC5H5 and C6I6···NC5H5, while the strengths of the π···π stacking interactions in C6F3I3···C6F3I3 and C6F2I4···C6F2I4 are similar to the corresponding halogen bonds in C6F3I3···NC5H5 and C6F2I4···NC5H5. This computational study, albeit performed on a relatively small number of dimers, indicates that the π···π stacking interaction is likely to be very competitive for the strong halogen bond in the crystal growth and design. 3.3. Cooperation of the π···π Stacking Interaction and the Halogen Bond. In the crystal structure, the π···π stacking interaction and the halogen bond always coexist, so it is significant to study the cooperativity between the π···π stacking interaction and the halogen bond. Figure 4 shows the M05-2X/def2-TZVPP optimized structures and interaction energies of the complexes C6F5I···C6F5I, C6F5I···NC5H5, C6F5I···C6F5I···NC5H5, and C5H5N···C6F5I···C6F5I···NC5H5. From the dimer to the trimer, both the π···π stacking interaction and the halogen bond become stronger, which indicates that a positive cooperativity is present in the trimer. From the dimer to the tetramer, the
Figure 4. M05-2X/def2-TZVPP optimized structures and interaction energies (red number; kcal/mol) of the complexes C6F5I···C6F5I, C6F5I···NC5H5, C6F5I···C6F5I···NC5H5, and C5H5N···C6F5I···C6F5I···NC5H5.
strength of the π···π stacking interaction increases about 0.55 kcal/mol and the strength of the halogen bond increases slightly, which indicates that a positive cooperativity is also present in the tetramer. Further, we also calculated the threeand four-body interaction terms between the monomers for the trimer C6F5I···C6F5I···NC5H5 and the tetramer C5H5N···C6F5I···C6F5I···NC5H5 with the method reported by Sherrill and Tauer.40 The three-body interaction term (Δ3E; −0.33 kcal/mol) for the trimer C6F5I···C6F5I···NC5H5 and the four-body interaction term (Δ4E; −0.14 kcal/mol) for the tetramer C5H5N···C6F5I···C6F5I···NC5H5 are both small but stabilizing contributions to the overall interaction, which is consistent with above results. The positive cooperativity between the π···π stacking interaction and the halogen bond can be explained by the 12489
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of China (2010GGJS-166) and the Natural Science Foundation of Henan Educational Committee (2010A150017 and 2011B150024).
NBO analysis. When the trimer or tetramer is formed, NBO analysis shows that the electron density transfer occurs from pyridine to C6F5I. We all know that the London dispersion forces become stronger as the atom or molecule in question becomes larger. So, the positive cooperativity of the π···π stacking interaction can be easily understood. The strength of the halogen bond mainly depends on the charge on I because of its electrostatical nature. In the isolated C6F5I, the I atom has a positive NPA charge of 0.230 e. When the dimer C6F5I···C6F5I is formed, the positive charge on I increases (0.232 e). Thus, the ability of I in the C6F5I···C6F5I dimer to form a halogen bond is stronger. This explains the positive cooperativity of the halogen bond in the trimer. The symmetrical structure of the tetramer in Figure 4 causes the strength of the halogen bond to decrease from the trimer.
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4. CONCLUSIONS Competition and cooperation of the π···π stacking interaction and the halogen bond in crystal engineering have been modeled theoretically in the present study. Like that in the benzene dimer, the π···π stacking interaction between the homogeneous dimer of C6FxI(6−x) is also of the dispersion type interaction. Interestingly, it is found that, for the π···π stacking interactions between homogeneous dimers of C6FxI(6−x), the M05-2X/def2TZVPP computations give almost the same results as the CCSD(T)/SDD** computations. At the M05-2X/def2TZVPP theory level, our computational study, albeit performed on a relatively small number of dimers, indicates that the π···π stacking interaction is likely to be very competitive for the strong halogen bond in the crystal growth and design. The cooperativity between the π···π stacking interaction and the halogen bond is also studied at the M05-2X/def2-TZVPP level of theory. We find that the formation of the halogen bond will lead to the π···π stacking interaction much stronger and that the formation of the π···π stacking interaction also leads to the halogen bond much stronger. The results presented here provide a good theoretical basis for the competition and cooperation of the π···π stacking interaction and the halogen bond in crystal engineering. At present, there are only small numbers of cocrystals that contain C6F5I, C6F3I3, C6F2I4, C6FI5, or C6I6. The samples are not enough to perform a statistical analysis. With the rapid development in this field,41 direct experimental evidence can be expected to be given in the near future.
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates and absolute energies for all the dimers in Table 1, and the full citation for ref 22. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (21173113). This work was also partly supported by the Aid Project for the Leading Young Teachers in Henan Provincial Institutions of Higher Education 12490
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