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The N•••I Halogen Bonding Interactions: The Influence of Lewis Bases on Their Strength and Characters Na Han, Yanli Zeng, Cuihong Sun, Xiaoyan Li, Zheng Sun, and Lingpeng Meng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp502558p • Publication Date (Web): 07 Aug 2014 Downloaded from http://pubs.acs.org on August 11, 2014

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The Journal of Physical Chemistry

The N···I Halogen Bonding Interactions: The Influence of Lewis Bases on Their Strength and Characters

Na Hana, Yanli Zenga,*, Cuihong Suna,b, Xiaoyan Li a, Zheng Suna, Lingpeng Menga,* a

Institute of Computational Quantum Chemistry, College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, PR China

b

College of Chemical Engineering, Shijiazhuang University, Shijiazhuang 050035, PR China

________________________

*To whom correspondence should be addressed. E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment


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The N···I Halogen Bonding Interactions: The Influence of Lewis Bases on Their Strength and Characters Na Hana, Yanli Zenga,*, Cuihong Suna,b, Xiaoyan Li a, Zheng Suna, Lingpeng Menga,* a

Institute of Computational Quantum Chemistry, College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, PR China

b

College of Chemical Engineering, Shijiazhuang University, Shijiazhuang 050035, PR China

Abstract: Halogen bonding (XB) as an emerging noncovalent interaction, due to its highly directional and devisable, has given rise to considerable interest for constructing supramolecular assemblies. In this work, the newly developed density functional M06-2X calculations and the quantum theory of “atoms in molecules” (QTAIM) studies were carried out on a series of N···I halogen bonding to investigate the influence of Lewis bases (XB acceptors) on the XB. For the Lewis base C6-nH6-nNn (n = 1, 2, 3), with the increasing number of nitrogen atom in the aromatic ring, the most negative electrostatic potentials (VS, min) outside the nitrogen atom becomes less negative and the XB becomes weaker. The positive cooperativity exists in the Y−−C6H5N···C6F5I, Y−−C4H4N2···C6F5I, and Y−−C3H3N3···C6F5I (Y− = Cl−, Br−, I−) termolecular complexes: the H-bond or anion-π interactions have the ability to enhance the N···I halogen bond, and vice versa. With the addition of halogen anions to the XB acceptor, the XB become more covalent, more electronic charge transfer from the XB acceptors to donors, the XB acceptors become more energetic stabilization and XB donors become more destabilization, the atomic volume attraction of both the nitrogen and iodine atoms become more obvious. From the view of the Laplacian of electron density function, for the XB acceptor, the reactivity zone is the region of valence shell charge concentration (VSCC), where have a (3, −3) critical point (CP) and referred to as lump, thus the XB interaction can be classified as lump-hole interaction. The more negative of VS,min outside the nitrogen atom, the stronger of the XB, resulting in the greater of the distance between the (3, −3) CP and the nitrogen nucleus. Keywords: halogen bonding, Lewis base, QTAIM, enhancing effect ________________________ E-mail: [email protected]; [email protected]


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1. Introduction Noncovalent interactions play a typical role in supramolecular chemistry1, 2, crystal engineering3, and materials self-assembly4-6. Recently, halogen bonding (XB) as an emerging noncovalent interaction, due to its highly directional and devisable, has given rise to considerable interest for constructing supramolecular assemblies. A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity7. For generally accepted the XB is a donor-acceptor interaction, the electron-deficient halogen atom acts as XB donor and the electron-rich Lewis base acts as XB acceptor8, 9. This XB interaction is often described as R-X···B-Y10, where halogen atoms (X) and Lewis base (B) are situated the centers of halogen bonding, while the remainder parts of the electron acceptor (R) and the electron donor (Y) are also the critical of influential factors for halogen bonding. Over the past decade, XB has been analyzed that multiple factors affecting for its structural and energetic properties from both experimentalists and theoreticians5, 8, 11-16

. Legon et al8 defined the early of halogen bond analogue to the hydrogen bond.

Politzer and his colleagues11 ascribe the formation of the highly directional XB interaction to the polarizabillity of halogen atom and the attractive electrostatic interaction in their theoretical research. Erdélyi et al5 summarized an overview of the methods hitherto applied for gaining an improved understanding of the behavior of XB in solutions. Sarwar et al12 studied that the thermodynamics of the halogen-bonding interaction in organic solution and predicting halogen-bonding affinities in solution. Beweries et al13 reported that the change of metal on metal-fluorine

bonding

is

an

important

contribution

to

XB

between

iodopentafluorobenzene and metal fluoride complexes in solution. Bauzá et al14 discussed that the Substituent effects of both halogen bond donor and acceptor aromatic moieties in halogen bonding complexes. Alkorta et al15 showed that the Hammett – Taft parameters provide reasonable correlations with the interaction



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energies. Auffinger et al16 found XB interactions offer halogenated proteins and nucleic acids the specific diversity. Most of the previous studies mainly concentrated on the impact of the XB donor, while neglected the XB acceptor part. For this reason, it would be of great significance to pay attention to the electron-rich Lewis base, which is a typical XB acceptor. Over the past decade, the researchers found that the interactions between nitrogen-containing heterocycles and iodo-perfluorocarbons (I-PFCs) are generally effective the building block of the halogen bonding in crystal engineering5,

17, 18

,

suramolecular architectures6, 19, and anion recognition20. In this work, the influence of the Lewis bases (XB acceptors) on the N···I halogen bonding interactions were investigated, the representative nitrogen-containing heterocycles17-19, C4H4N2,

and

C3H3N3)

were

chosen

as

the

XB

acceptor

21

(C5H5N, and

the

iodopentafluorobenzene was chosen as the XB donor. To take into account the influences of XB acceptor, the Y−−C6-nH6-nNn···C6F5I (n = 1, 2, 3; Y− = Cl−, Br−, I−) complexes were also investigated. The purpose of this work are: (1) to investigate the influencing effect of the number of the nitrogen atoms in the aromatic heterocycles on the XB; (2) to investigate the enhancing effect of halogen anion Y− (Y− = Cl−, Br−, I−) on the XB acceptor Y−−C6-nH6-nNn (n = 1, 2, 3) and furthermore on the strength of the XB; (3) to investigate how the halogen anions affect the bonding character of the XB, and the atomic integral properties of both XB acceptor and donor; (4) to investigate the relationship of the most negative electrostatic potentials and the reactivity region of valence shell charge concentration (VSCC) of the XB acceptor. 2. Computational details The newly developed density functional M06-2X is a high-non-local functional with twice the amount of nonlocal exchange22-24. Based on the recent study of Bauzá et al25, the M06-2X method could gives good results for both anionic and neutral halogen-bonding complexes, and M06-2X/aug-cc-pVTZ provides a good estimation of the XB energies when compared with CCSD(T)/aug-cc-pVTZ ones. Combined with the M06-2X or MP2 method, the aug-cc-pVDZ (aug-cc-pVDZ-PP) basis set is


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adequate to study the XB26-31. In this work, the M06-2X method was used to study all the monomers and complexes. The aug-cc-pVDZ-PP basis set32 applied to optimize the iodine atom, which includes the small-core energy-consistent relativistic pseudopotentials to account for relativistic effects. While for all other atoms the aug-cc-pVDZ33-34 basis set of Dunning and co-workers was used. Frequency calculations performed at the same theoretical level to indicate that all the geometries were local minima on their potential energy surfaces. Geometry optimizations and frequency computations were corrected for basis set superposition error (BSSE) which was obtained with the counterpoise procedure proposed by Boys and Bernardi35. For comparison, the geometries of the bimolecular complexes C5H5N···C6F5I, C4H4N2···C6F5I, C3H3N3···C6F5I, and the termolecular complexes Cl−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, I−−C5H5N···C6F5I were reoptimized at the M06-2X level combined with the convergent, partially augmented basis set levels36,37 corresponding to subsets of the augmented “aug-cc-pVTZ” basis sets of Dunning and co-workers. In this work, jul-cc-pVTZ, jun-cc-pVTZ, and may-cc-pVTZ basis sets were used to optimize the geometries and compute the interaction energies of the above complexes. The jul-cc-pVTZ basis set means that cc-pVTZ for the hydrogen atom and aug-cc-pVTZ for the carbon, nitrogen, florine, chlorine, and bromine atoms. According this rule, the jun-cc-pVTZ and may-cc-pVTZ basis sets could obtained36,37. For the iodine atom, the aug-cc-pVTZ-PP basis set32 was used when the jul-cc-pVTZ basis set was used for the other atoms; when the jun-cc-pVTZ basis set was used for the other atoms, the outermost f function was deleted from the aug-cc-pVTZ-PP basis set32 of iodine; and when the may-cc-pVTZ basis set were used for the other atoms, the outermost f function and the outermost d function were deleted from the aug-cc-pVTZ-PP basis set32 of iodine. All the above calculations were performed with the Gaussian 09 program package38. The electrostatic potentials were computed with the WFA surface analysis suite39, calculated on the 0.001 a.u. (electrons/bohr3) contour of the molecule’s electronic density40. As suggested by Bader et al, the molecular electrostatic potentials on the 0.001 a.u. isosurface encompasses roughly 97% of the molecule’s electronic charge



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and is very useful for predicting noncovalent interactions11, 41. The topological and energy properties based on electron density function and Laplacian of electron density function were computed by the AIM 2000 program42 and AIMALL program43. 3. Results and discussion 3.1 Comparison of the basis sets In this work, the bimolecular complexes C5H5N···C6F5I, C4H4N2···C6F5I, C3H3N3···C6F5I,

and

the

termolecular

complexes

Cl−−C5H5N···C6F5I,

Br−−C5H5N···C6F5I, I−−C5H5N···C6F5I were chosen as representative complexes, their geometries were optimized and the halogen-bonded interaction energies were obtained at the M06-2X method with the following basis sets: aug-cc-pVDZ, jul-cc-pVTZ, jun-cc-pVTZ, and may-cc-pVTZ basis sets, the corresponding basis set for the iodine atom was described in the Computational detail section. Table 1 gives the halogen-bonded interaction energies, ∆E(N···I), which have been corrected with BSSE and zero-point energies. From the aug-cc-pVDZ calculated ∆E(N···I) values, the halogen-bonded interactions become weaker with the increasing number of N atoms in the aromatic ring. On the other hand, the ∆E(N···I) values of Cl−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, and I−−C5H5N···C6F5I become more negative than that of C5H5N···C6F5I , and become greater along the sequence of Y−= I−, Br−, Cl−. These indicate that the introduction of Cl−, Br− and I− in the XB acceptor could lead to a prominent strengthening effect on the N···I halogen bonding interaction, especially Cl−. The above trends could be obtained from the jul-cc-pVTZ, jun-cc-pVTZ, and may-cc-pVTZ calculated ∆E(N···I) values. Table 1 (here) The halogen-bonded interaction distances, d(N···I), were collected in Table 2. From the aug-cc-pVDZ calculated d(N···I) values, the halogen-bonded interactions become shorter with the increasing number of N atoms in the aromatic ring. On the other hand, the d(N···I) values of Cl−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, and I−−C5H5N···C6F5I become more shorter than that of C5H5N···C6F5I, along the


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sequence of Y−= I−, Br−, Cl−. The above trends could also be obtained from the jul-cc-pVTZ, jun-cc-pVTZ, and may-cc-pVTZ calculated d(N···I) values. Table 2 (here) Comparing the halogen-bonded interaction energies and interaction distances, all the four kinds of basis sets could give consistent results. Therefore, the aug-cc-pVDZ (aug-cc-pVDZ-PP for iodine) basis set was used in the following discussions. 3.2 Molecular electrostatic potentials, equilibrium geometries and interaction energies 3.2.1 The XB in the C6-nH6-nNn···C6F5I (n = 1, 2, 3) bimolecular complexes The electronic properties on the surfaces of the molecular electrostatic potentials (MEPs) are useful as a tool for identifying the XB interactions. The MEPs of C5N5N, C4H4N2, and C3H3N3 on the 0.001 a.u. (electrons/bohr3) electron density isosurface are depicted in Figure 1. The most negative electrostatic potentials, VS, min, lies outside the nitrogen atom, associated with the lone pair electron region. Through which, the nitrogen atoms of C6-nH6-nNn (n = 1, 2, 3) interact with the iodine atom of C6F5I to form the N···I halogen-bonding complexes. Figure 1 (here) Table 3 (here) The VS, min magnitude of C5N5N, C4H4N2, and C3H3N3 are listed in Table 3. The VS, min of C5H5N with value of −36.16 kJ·mol−1 is remarkably more negative than that of C4H4N2 with value of −29.18 kJ·mol−1 and C3H3N3 with value of −24.89 kJ·mol−1. From the MEPs of XB acceptor alone, it should be expected that the C6-n H6-n Nn (n = 1, 2, 3) potential capacity for the interaction should be obviously different. Table 4 (here) Figure 2 (here) The main geometrical parameters that describe the halogen bonding in the bimolecular complexes C6-nH6-nNn···C6F5I (n = 1, 2, 3) are summarized in Table 4. The optimized geometries of C5N5N···C6F5I, C4H4N2···C6F5I, and C3H3N3···C6F5I are illustrated in Figure 2a-c. Table 4 and Figure 2 show that the angle N···I—C of the intermolecular halogen bond are approximately 180° in these bimolecular complexes, indicating that the halogen bonding interaction is highly directional. It also can be



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seen that the distances between the N and I atom, d(N···I), increases consistent with the increasing number of N atoms in the aromatic ring, and all N···I interaction distances are less than the sum of the van der Waals radii of N and I atoms (3.53 Å)44. The halogen-bonded interaction energies ∆E(N···I) of the bimolecular complexes are also listed in Table 4. It can be seen that the ∆E(N···I) values of C5H5N···C6F5I, C4H4N2···C6F5I, and C3H3N3···C6F5I are −27.37, −22.32, and −18.58 kJ·mol−1, respectively. The ∆E(N···I) values become less negative with the increasing number of N atoms in the aromatic ring, which suggest that the halogen-bonded interaction with C5H5N is the strongest one. Evidently, the strength of the halogen-bonded interaction is consistent with the VS, min values of XB acceptors. The more negative of the VS, min value, the stronger of the XB is. 3.2.2 The XB in the Y−−C6-nH6-nNn···C6F5I (Y− = Cl, Br, I; n = 1, 2, 3) termolecular complexes When the halogen anions are added to the XB accepter to form the termolecular complexes, there are two types of geometries, as shown in Figure 2d-f. For the Y−−C5H5N···C6F5I and Y−−C3H3N3···C6F5I (Y− = Cl−, Br−, I−) termolecular complexes, the interactions inside the XB acceptors Y−−C5H5N and Y−−C3H3N3 are H-bond interactions. While for the Y−−C4H4N2···C6F5I (Y− = Cl−, Br−, I−) termolecular complexes, the interactions inside the XB acceptors Y−−C4H4N2 are anion-π interactions. Actually, with the increasing number of N atom in electron donors, the anion-π complexes of Y−−C3H3N3 is stable than Y−−C4H2N2. However, in the Y−−C3H3N3···C6F5I termolecular complexes, the favorable electron donor site is Y−, not the nitrogen atom. For comparison study of the N···I halogen bond, for the Y−−C3H3N3 interaction, we choose the H-bond interaction, not the anion-π interaction. The VS, min values of the XB acceptors of the termolecular complexes, that is, H-bond type Y−−C5H5N, anion-π type Y−−C4H4N2 and H-bond type Y−−C3H3N3, are also listed in Table 3. In comparison with the XB acceptors (C5H5N, C4H4N2 and C3H3N3) of the bimolecular complexes, the VS, min values of Y−−C6-nH6-nNn (n = 1, 2, 3) become much more negative and in sequence of Y− = I−, Br−, Cl−. For example, the


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VS,min values of I−−C5H5N, Br−−C5H5N, and Cl−−C5H5N are −82.86, −84.81, and −86.55 kcal·mol−1, respectively. From the interaction energies ∆E(N···I) of Table 4, the N···I halogen-bonded intermolecular interactions in the termolecular complexes Y−−C6-nH6-nNn···C6F5I (n = 1, 2, 3; Y− = Cl−, Br−, I−) are much stronger than those of respective bimolecular complexes C6-nH6-nNn···C6F5I (n = 1, 2, 3; Y− = Cl−, Br−, I−). The ∆E(N···I) values of Y−−C6-nH6-nNn···C6F5I (n = 1, 2, 3; Y− = Cl−, Br−, I−) become greater along the sequence of Y−= I−, Br−, Cl−. The Cl− introduction in the XB acceptor leads to a prominent strengthening effect on the N···I halogen bonding interaction. From the most negative electrostatic potentials VS, min of Table 3 and the interaction energies ∆E of Table 4, the complexes C5H5N···C6F5I, Cl−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, and I−−C5H5N···C6F5I show that the more negative VS,min values of the XB acceptor, the stronger of the XB is. Similarly, this relationships also exist when n=2 and 3, respectively. The geometrical parameters of the termolecular complexes are also summarized in Table 4. Similarly to the bimolecular halogen-bonded complexes, the halogen-bond angles are close to 180º. It is evident that the N···I distances in the Y−−C6-nH6-nNn···C6F5I (n = 1, 2, 3) termolecular complexes are obviously shorter compared with those in respective C6-nH6-nNn···C6F5I (n = 1, 2, 3) bimolecular complexes, which are consistent with the halogen-bonded interaction energies. Table 5 (here) Table 5 shows the interaction energies of H-bond or anion-π interactions inside the XB acceptors in the bimolecular complexes and termolecular complexes, which are

denoted

as

∆EXB-acceptor(D)

and

∆EXB-acceptor(T),

respectively.

In

the

Y−−C6-nH6-nNn(n = 1, 2, 3; Y− = Cl−, Br−, I−) bimolecular complexes, ∆EXB-acceptor(D) ranges from −14.42 to −47.92 kJ·mol−1, and becomes more negative along the sequence of Y−= I−, Br−, Cl−. For example, the ∆EXB-acceptor(D) values of I−−C5H5N, Br−−C5H5N, and Cl−−C5H5N are −35.94, −42.03, and −47.92 kJ·mol−1, respectively. In the Y−−C6-nH6-nNn···C6F5I termolecular complexes, ∆EXB-acceptor(T) ranges from −39.13 to −71.52 kJ·mol−1, and also becomes more negative along the sequence of



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Y−= I−, Br−, Cl−. For example, the ∆EXB-acceptor(T) values of I−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, and Cl−−C5H5N···C6F5I are −55.96, −63.82, and −71.52 kJ·mol−1,

respectively.

Compared

∆EXB-acceptor(D)

values

with

respective

∆EXB-acceptor(T) values, the interaction energies of H-bond or anion-π interactions inside the XB acceptors become obviously more negative, indicating the H-bond or anion-π interactions are greatly enhanced with addition of N···I halogen bond of C6-nH6-nNn···C6F5I. In the Y−−C6H5N···C6F5I, Y−−C4H4N2···C6F5I, and Y−−C3H3N3···C6F5I (Y− = Cl−, Br−, I−) termolecular complexes, the H-bond or anion-π interactions have the ability to enhance the N···I halogen bond, meanwhile the H-bond or anion-π interactions are also strengthened by the N···I halogen bond. That is to say, the positive cooperativity exists in the termolecular complexes. 3.3 QTAIM analyses of XB 3.3.1 Topological and energy parameters of XB based on electron density function Topological analysis according to the quantum theory of “Atoms in Molecules” (QTAIM)45-47 can provide much useful information to analyze the weak interactions48-50. In this theory, critical points (CP) play an important role and are intuitive reflect the nature of interaction. The larger value of the electron density (ρb) at the bond critical points (BCP), the stronger the XB interaction is. From Table 5, for the bimolecular complexes, the XB of C5H5N···C6F5I is the strongest, C4H4N2···C6F5I is weaker, and C3H3N3···C6F5I is the weakest. In the termolecular complexes, ρb values at the BCPs of XB become greatly larger compared with respective bimolecular complexes, indicating that the halogen anions have the ability to strengthen the original N···I interactions, which are reflected in the changes in their respective interaction energies and interaction distances of Table 4. Table 6 (here) Table 6 also summarized the topological and energy parameters at the BCPs of XB based on electron density function (Laplacian of electron density ∇2ρb, the kinetic electron energy density Gb, the potential electron energy density Vb, the electron energy density Hb, and −Gb/Vb). QTAIM44-46 indicates that a positive value of ∇2ρb


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indicates that there are closed-shell systems: ionic interactions, van der Waals forces, or hydrogen bonding. It has also been claimed that if ∇2ρb is positive but Hb is negative, then the interaction is partly covalent in nature51, 52. The values of Gb and Vb indicate the type of interaction, −Gb/Vb as the balance between the positive value of Gb and the negative value of Vb may indicate the regions corresponding to covalent or noncovalent interactions. Generally, if the ratio of −Gb/Vb is greater than 1, the interaction is noncovalent; if it is between 0.5 and 1, the interaction is partly covalent in nature; it is less than 0.5, the interaction is a shared covalent. From Table 6, for the XB

in

the

bimolecular

complexes

C5H5N···C6F5I,

C4H4N2···C6F5I,

and

C3H3N3···C6F5I, ∇2ρb and Hb are all positive and the ratios of −Gb/Vb are greater than 1, indicating the halogen-bonded interactions in these bimolecular complexes display the characteristics of “closed-shell” noncovalent interactions. In the termolecular complexes Y−−C5H5N···C6F5I and Y−−C4H4N2···C6F5I (Y− = Cl−, Br−, I−) complexes, ∇2ρb values are positive, but Hb values become negative, and −Gb/Vb values become between 0.5 and 1, the halogen-bonded interactions in these complexes become partly covalent in nature. In the Y−−C3H3N3···C6F5I (Y− = Cl−, Br−, I−) complexes, ∇2ρb and Hb are all positive and the ratios of −Gb/Vb are greater than 1, the XB in Y−−C3H3N3···C6F5I still belong to “closed-shell” interactions. However, compared

with

the

XB

in

C3H3N3···C6F5I,

Hb

and

−Gb/Vb

values

in

Y−−C3H3N3···C6F5I become somewhat smaller. 3.3.2 Topological analysis of XB based on Laplacian of electron density function The electron-rich zones of XB acceptor is a special influential point for the XB interaction. The topology of the Laplacian of electron density function53-55 could characterize zones of concentration and depletion of electron density in valence and core electron shells; therefore it could explain the XB interaction from a more comprehensive vision. For the XB acceptor, the reactivity zone of the lone pair electron region where have (3, −3) critical point (CP), is called the valence shell charge concentration (VSCC) and is referred to as lump56. In the region of (3, −3) CPs, the ∇2ρ(r) is negative that signifies a local concentration of electron density. Correspondingly, the XB donor have the (3, +1) CPs corresponding to the local



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depletion of electron density where the ∇2ρ(r) is positive, this reactivity zone is often said the region of σ-hole in bonded halogen atom. It is using the definition of the Laplacian, the halogen-bonded interaction also can be classified as lump-hole interaction55, 57. Taking the complex of C5H5N···C6F5I as an example, the contour map of the Laplacian of electron density is shown in Figure 3. The negative domain of the Laplacian of the charge density ∇2ρ(r) is near the nitrogen atom, the lone pair electron region of the nitrogen atom corresponds to the (3, −3) CP with the sign of pink point. Distinctly, the formation of the complex results from the interaction between the lump domain of (3, −3) CP with the σ-hole region of the XB donor. Figure 3 (here) Table 7 summarized the main parameters of the Laplacian of electron density for the (3, −3) CPs in the XB acceptor upon complexation. The distances between the (3, −3) CPs and the nitrogen nucleus are denoted as R(CP-N). Compared R(CP-N) values with VS,min values of Table 3, and ∆E(N···I) values of Table 4, it can be seen that the more negative of VS,min values outside the nitrogen atom, the stronger of the XB, and the greater is the R(CP-N) distance, the relations of R(CP-N) with the halogen-bonded interaction energies ∆E(N···I) are displayed in Figure 4. From Table 7, the electron density in the VSCC critical point decreases with the increasing distance from the nitrogen nucleus. For example, The R(CP-N) values are 0.7401, 0.7463, 0.7472, and 0.7478 a.u, the ρ(r) values at the (3, −3) CPs are 0.5717, 0.5554, 0.5531, and 0.5515 for

the

C5H5N···C6F5I,

I−−C5H5N···C6F5I,

Br−−C5H5N···C6F5I,

and

Cl−−C5H5N···C6F5I complexes, respectively. The Laplacian of electron density at the (3,-3) CPs become less negative with the increasing R(CP-N) values. Table 7 (here) Figure 4 (here) 3.3.3 Integral characteristics of the XB The integration of atom charges was also performed here to deepen the nature of halogen bonds. In this part, we sum analysis of the integral characteristics for XB donors and acceptors in all complexes. The total net atom charges (∆q), the changes


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of atomic volumes (∆V), and the energetic stabilization (∆E) for both the N atom of the XB acceptors and I atom of the XB donors are listed in Table 8 and Table 9. Table 8 (here) Table 9 (here) From Table 8 and 9, it can be observed that with the formation of all complexes, the total net charges ∆q(N) are all to decrease and ∆q(I) are all to increase. In other words, charge transfer is aware from the XB acceptor (N atom) to the XB donor (I atom) in the formation of the halogen-bonded complexes. The formation of halogen bonding is a charge reallocating process. In the bimolecular halogen-bonded complexes, ∆q(N) becomes less negative and ∆q(I) becomes less positive in the series of C5H5N, C4H4N2, and C3H3N3, the trends of charge transfer are consistent with the interaction energies. With the introduction of halogen anions, the values of ∆q(N) become more negative and follows the reducing sequence of Y− = I−, Br−, Cl−. For example, ∆q(N) values of C5H5N···C6F5I, I−−C5H5N···C6F5I, Br−−C5H5N···C6F5I, and Cl−−C5H5N···C6F5I are −0.0452, −0.0520, −0.0528, and −0.0531a.u., respectively. Meanwhile, with the introduction of halogen anions, the values of ∆q(I) become more positive and follows the increasing sequence of Y− = I−, Br−, Cl−. For example, ∆q(I) values

of

C5H5N···C6F5I,

I−−C5H5N···C6F5I,

Br−−C5H5N···C6F5I,

and

Cl−−C5H5N···C6F5I are 0.0528, 0.0820, 0.0833, and 0.0843 a.u., respectively. For the halogen-bonded complexes, the energetic stabilization of the N atom in the XB acceptors, ∆E(N), are all negative. The ∆E(I) values of the XB donors are all positive. These indicate that the XB acceptors are energetic stabilization and XB donors are energetic destabilization upon the complexes formation. Both the energetic stabilization of XB acceptors and energetic destabilization XB donors follow the sequence of C6-nH6-nNn···C6F5I, I−−C6-nH6-nNn···C6F5I, Br−−C6-nH6-nNn···C6F5I, and Cl−−C6-nH6-nNn···C6F5I. The changes of atomic volumes, ∆V(N) and ∆V(I), are all negative, indicating that the atomic volume of XB acceptor atoms and XB donor atoms contract upon the formation of complexes. The general trend on the volume contraction agrees well with the halogen-bonded interaction energies of the complexes.



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Overall, with the introduction of halogen anions, more electronic charge transfer from the XB acceptors to donors, the XB acceptors become more energetic stabilization and XB donors become more destabilization, the atomic volume attraction become more obvious. 4. Conclusions In this work, M06-2X calculations and QTAIM studies were carried out on a series of N···I halogen bonding to investigate the influence of Lewis bases (XB acceptors) on the XB. (1) For the Lewis base C6-nH6-nNn (n = 1, 2, 3), with the increasing number of nitrogen atom in the aromatic ring, the VS,min value outside the nitrogen atom becomes less negative and the XB becomes weaker. (2)

The

positive

cooperativity

exists

in

the

Y−−C6H5N···C6F5I,

Y−−C4H4N2···C6F5I, and Y−−C3H3N3···C6F5I (Y− = Cl−, Br−, I−) termolecular complexes: the H-bond or anion-π interactions have the ability to enhance the N···I halogen bond, meanwhile the H-bond or anion-π interactions are also strengthened by the N···I halogen bond. (3) With the addition of halogen anions to the XB acceptor, the XB become more covalent in the termolecular complexes, more electronic charge transfer from the XB acceptors to donors, the XB acceptors become more energetic stabilization and XB donors become more destabilization, the atomic volume attraction of both the nitrogen and iodine atoms become more obvious. (4) From the view of the Laplacian of electron density function, the XB interaction can be classified as lump-hole interaction. The more negative of VS,min values outside the nitrogen atom, the stronger of the XB, resulting in greater of the R(CP-N) distance. The electron density in the VSCC critical point decreases with the increasing R(CP-N) distance.

Acknowledgements This project was supported by the National Natural Science Foundation of China


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Page 18 of 32

Analysis. J. Mol. Model. 2013, 19, 2035-2041.

Table 1: The N···I halogen-bonded interaction energies (in kJ·mol−1) obtained at the M06-2X level with different basis sets Complexes

aug-cc-pVDZ

jul-cc-pVTZ

jun-cc-pVTZ

may-cc-pVTZ

C5H5N···C6F5I

−27.37

-25.23

-25.06

-25.00

C4H4N2···C6F5I

−22.32

-20.22

-20.17

-20.09

C3H3N3···C6F5I

−18.58

-17.05

-16.88

-16.81

−

−50.97

-47.90

-47.48

-47.59

−

−49.17

-45.56

-45.14

-45.86

−47.40

-43.96

-42.89

-44.17

Cl −C5H5N···C6F5I Br −C5H5N···C6F5I −

I −C5H5N···C6F5I


Page 19 of 32

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Table 2: The N···I halogen-bonded interaction distances (in Å) obtained at the M06-2X level with different basis sets Complexes

aug-cc-pVDZ

jul-cc-pVTZ

jun-cc-pVTZ

may-cc-pVTZ

C5H5N···C6F5I

2.8993

2.8997

2.9089

2.9132

C4H4N2···C6F5I

2.9460

2.9473

2.9547

2.9575

C3H3N3···C6F5I

2.9871

3.0053

3.0121

3.0183

2.6908

2.6975

2.6994

2.7008

2.7048

2.7086

2.7144

2.7161

2.7210

2.7243

2.7402

2.7325

−

Cl −C5H5N···C6F5I −

Br −C5H5N···C6F5I −




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3: The most negative electrostatic potentials (VS, min, in kcal/mol) associated with the nitrogen atoms Species

VS, min

C5H5N

−36.16

−

Cl −C5H5N (H-bond)

−86.55

−

Br −C5H5N (H-bond)

−84.81

I−−C5H5N (H-bond)

−82.86

C4H4N2

−29.18

−

Cl −C4H4N2 (anion-π)

−137.01

−

−130.91

Br −C4H4N2 (anion-π) −

I −C4H4N2 (anion-π)

−122.77

C3H3N3

−24.89

−

Cl −C3H3N3 (H-bond)

−76.64

−

−74.64

Br −C3H3N3 (H-bond) −

I −C3H3N3 (H-bond)

−72.40


Page 20 of 32

Page 21 of 32

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Table 4: The halogen-bonded interaction energies (in kJ·mol−1), interaction distances (in Å), and interaction angles (in degree) Complexes

∆E(N···I)

C5H5N···C6F5I −

−

Cl −C5H5N···C6F5I Br −C5H5N···C6F5I

Geometrial parameters d(N···I)

∆d(N···I)

∠N···I−C

−27.37

2.8993

0.6307

180.00

−50.97

2.6908

0.8392

180.00

−49.17

2.7048

0.8252

180.00

−


−47.40

2.7210

0.8090

180.00

C4H4N2···C6F5I

−22.32

2.9460

0.5840

180.00

−

−51.74

2.7121

0.8180

177.10

−

−49.59

2.7332

0.7968

177.07

I −C4H4N2···C6F5I

−47.04

2.7525

0.7775

177.09

Cl −C4H4N2···C6F5I Br −C4H4N2···C6F5I −

C3H3N3···C6F5I

−18.58

2.9871

0.5429

178.47

−

−38.45

2.8284

0.7016

179.00

−

Br −C3H3N3···C6F5I

−36.60

2.8430

0.6870

178.91

I−−C3H3N3···C6F5I

−34.89

2.8579

0.6721

178.96

Cl −C3H3N3···C6F5I



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

Table 5: The interaction energies (in kJ·mol−1) of XB-acceptor in the halogen-bonded bimolecular complexes and termolecular complexes Complexes

∆EXB-acceptor(D)

∆EXB-acceptor(T)

Cl−−C5H5N···C6F5I

-47.92

-71.52

-42.03

-63.82


-35.94

-55.96

Cl−−C4H4N2···C6F5I

-18.94

-48.36

Br−−C4H4N2···C6F5I

-16.75

-44.02

-14.42

-39.13


-38.93

-58.80


-32.29

-50.31


-25.72

-42.03

−

Br −C5H5N···C6F5I −

−

I −C4H4N2···C6F5I −


Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 6: Topological properties and energy properties at the halogen bond critical points based on the electron density function (all values in a.u.) Complexes

ρb

λ1

λ2

λ3

∇2ρb

Gb

Vb

Hb

−Gb/Vb

C5H5N···C6F5I

0.0218

−0.0154

−0.0145

0.0929

0.0629

0.0151

−0.0145

0.0006

1.0420

−

0.0339

−0.0270

−0.0251

0.1368

0.0847

0.0228

−0.0244

−0.0016

0.9343

−

Cl −C5H5N···C6F5I Br −C5H5N···C6F5I

0.0329

−0.0260

−0.0242

0.1334

0.0832

0.0222

−0.0235

−0.0014

0.9425

−


0.0318

−0.0249

−0.0232

0.1295

0.0815

0.0215

−0.0226

−0.0011

0.9515

C4H4N2···C6F5I

0.0198

−0.0137

−0.0129

0.0849

0.0583

0.0138

−0.0130

0.0008

1.0627

−

0.0321

−0.0253

−0.0239

0.1317

0.0825

0.0218

−0.0229

−0.0011

0.9509

−

0.0306

−0.0239

−0.0226

0.1267

0.0802

0.0209

−0.0217

−0.0008

0.9628


0.0294

−0.0227

−0.0215

0.1223

0.0781

0.0201

−0.0206

−0.0006

0.9733

C3H3N3···C6F5I

0.0180

−0.0120

−0.0114

0.0780

0.0546

0.0126

−0.0116

0.0010

1.0880


0.0250

−0.0182

−0.0172

0.1054

0.0700

0.0173

−0.0170

0.0002

1.0136


0.0243

−0.0175

−0.0165

0.1025

0.0685

0.0168

−0.0164

0.0003

1.0208


0.0235

−0.0169

−0.0159

0.0997

0.0669

0.0163

−0.0158

0.0004

1.0282

Cl −C4H4N2···C6F5I Br −C4H4N2···C6F5I −

−



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Page 24 of 32

Table 7: Local properties at the (3, −3) critical points based on the Laplacian of electron density function in the vicinity of nitrogen atom (all values in a.u.) Complexes

R(CP-N) a

ρ(r)

G(r)

V(r)

∇2ρ(r )

C5H5N···C6F5I

0.7401

0.5717

0.7550

−2.1877

−0.6777


0.7478

0.5515

0.7292

−2.0887

−0.6304

Br−−C5H5N···C6F5I

0.7472

0.5531

0.7312

−2.0963

−0.6340

−


0.7463

0.5554

0.7340

−2.1075

−0.6395

C4H4N2···C6F5I

0.7374

0.5787

0.7611

−2.2197

−0.6975


0.7447

0.5600

0.7356

−2.1250

−0.6538

0.7439

0.5620

0.7383

−2.1349

−0.6583


0.7429

0.5647

0.7418

−2.1480

−0.6645

C3H3N3···C6F5I

0.7397

0.5712

0.7572

−2.1871

−0.6728

−


0.7458

0.5551

0.7370

−2.1080

−0.6341


0.7452

0.5566

0.7388

−2.1151

−0.6376


0.7444

0.5586

0.7412

−2.1249

−0.6425

−

Br −C4H4N2···C6F5I −

a

R(CP−N), the distance from the nitrogen nucleus to the (3, −3) critical points


Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 8: Atom integral properties for the nitrogen atom of XB acceptor a Complexes

∆q(N)

∆E(N)

∆V(N)

C5H5N···C6F5I

−0.0452

−0.0354

−11.6790


−0.0531

−0.0561

−19.7763

−

Br −C5H5N···C6F5I

−0.0528

−0.0543

−19.2505

−


−0.0520

−0.0514

−18.6111

C4H4N2···C6F5I

−0.0419

−0.0309

−10.5011

−

−0.0536

−0.0569

−18.8080

−


−0.0529

−0.0529

−17.6798


−0.0537

−0.0491

−16.3341

C3H3N3···C6F5I

−0.0395

−0.0263

−8.9089


−0.0484

−0.0389

−13.9204


−0.0475

−0.0372

−13.2962


−0.0470

−0.0363

−12.6206


a

∆q(N), ∆E(N), and ∆V(N) represent the difference of the properties between the termolecular complexes and XB acceptors; all values in a.u.

25



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Page 26 of 32

Table 9: Atom integral properties for the iodine atoms of XB donor a Complexes

∆q(I)

∆E(I)

∆V(I)

C5H5N···C6F5I

0.0528

0.0249

−12.2975


0.0843

0.0342

−16.7643

Br−−C5H5N···C6F5I

0.0833

0.0340

−16.5734

I−−C5H5N···C6F5I

0.0820

0.0337

−16.2721

C4H4N2···C6F5I

0.0449

0.0219

−11.2845


0.0997

0.0388

−17.0981

0.0979

0.0384

−16.5797


0.0946

0.0370

−16.0935

C3H3N3···C6F5I

0.0392

0.0195

−10.6623

−

0.0772

0.0316

−14.9363

−


0.0753

0.0310

−14.6660


0.0731

0.0302

−14.2765

−

Br −C4H4N2···C6F5I −


a

∆q(I), ∆E(I), and ∆V(I) represent the difference of the properties between the termolecular complexes and XB donors; all values in a.u.

26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1: Molecular surface electrostatic potential (MEP) of C5H5N, C4H4N2, and C3H3N3 on the 0.001 a u. (electrons/bohr3) contours of the molecule’s electronic density. Color ranges, in kcal/mol: Red, more positive than 18; yellow, 8-18; green, 0-8; blue, negative. Position of VS,min in blue. Figure 2: Optimized geometries of the complexes C6H5N···C6F5I (a), C4H4N2···C6F5I (b), C3H3N3···C6F5I (c), I−−C5H5N···C6F5I (d), I−−C4H4N2··C6F5I (e), I−−C3H3N3···C6F5I (f). Figure 3: Contour map of the Laplacian of the electron density in the C5H5N···C6F5I complex. The (3, −3) CP is denoted as the sign of pink point. Figure 4: The relations of R(CP-N) with the halogen-bonded interaction energies ∆E(N···I).

27



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Page 28 of 32

Figure 1: Molecular surface electrostatic potential (MEP) of C5H5N, C4H4N2, and C3H3N3 on the 0.001 a u. (electrons/bohr3) contours of the molecule’s electronic density. Color ranges, in kcal/mol: Red, more positive than 18; yellow, 8-18; green, 0-8; blue, negative. Position of VS,min in blue.

28


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Figure 2: Optimized geometries of the complexes C6H5N···C6F5I (a), C4H4N2···C6F5I (b), C3H3N3···C6F5I (c), I−−C5H5N···C6F5I (d), I−−C4H4N2··C6F5I (e), I−−C3H3N3···C6F5I (f).

29



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Page 30 of 32

Figure 3: Contour map of the Laplacian of the electron density in the C5H5N···C6F5I complex. The (3, −3) CP is denoted as the sign of pink point.

30


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Figure 4: The relations of R(CP-N) with VS, min outside the nitrogen atom and the halogen-bonded interaction energies ∆E(N···I).

31



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Table of Contents graphic

32


I Halogen Bonding Interactions - American Chemical Society

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