Theoretical Exploration of Halogen Bonding Interactions in the

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A Theoretical Exploration of Halogen Bonding Interactions in the Complexes of Novel Nitroxide Radical Probes and Comparison with Hydrogen Bonds Chengxi Zhao, Yunxiang Lu, Zhengdan Zhu, and Honglai Liu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03385 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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The Journal of Physical Chemistry

A Theoretical Exploration of Halogen Bonding Interactions in the Complexes of Novel Nitroxide Radical Probes and Comparison with Hydrogen Bonds

Chengxi Zhao1, Yunxiang Lu1*, Zhengdan Zhu2, Honglai Liu1

1

Key Laboratory for Advanced Materials and School of Chemistry & Molecular Engineering,

Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China 2

CAS Key Laboratory of Receptor Research, Drug Discovery and Design Center, Shanghai

Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT：：In this work, halogen bonding interactions in the complexes of two new nitroxide radicals, which contain both a halogen-bond-donor group and an EPR-active radical unit, were investigated using density functional theory calculations. For comparison, the corresponding hydrogen-bonded complexes were also examined. Halogen bonds in these systems are predicted to be linear and much stronger than hydrogen bonds. To further understand the nature of these interactions, many theoretical methods, such as atoms in molecules, noncovalent interaction index, localized orbital locator, energy decomposition analysis, electron density difference, and electron spin densities, were employed. Compared with hydrogen bonds, halogen bonds have more open shell and covalent interaction component. Particularly, the formation of halogen bonds changes the ratio of different conformations, leading to spin density shift on certain atoms. The results reported herein will assist in the design of new functional probes for the detection of halogen bonding.

2


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1. INTRODUCTION Halogen atoms, which exist widely in nature and artificial organic molecules, can form halogen bonding (XB) occasionally.1-4 XB shows a particularly directional ability,5,6 as a result of covalently-bonded halogen atoms exhibiting positive electrostatic potentials (the σ-hole)7 along the extension of the R−X bonds. Furthermore, halogen σ-hole can be readily tuned through changing halogen atoms and the moieties bound to these atoms.8,9 Owing to these specific features, XB has been successfully discovered and employed in such diverse fields as soft materials,10 crystal engineering,11 biomolecular systems,12 magnetic materials,13 and molecular recognition.14 Despite the broad range of applications employing XB, analytical techniques used for revealing the nature of this interaction were normally limited on microwave, infrared, UV-vis, NMR and X-rays.9 Recently, electron spin resonance (EPR) spectroscopy has been applied as a new analytical way for the detection of HB and XB-based complexes, and the combined use of nitroxide spin labels and EPR spectroscopy has proved to be effective in the characterization of these interactions.15 In 1972, Goto et al.16 firstly reported the formation of XB between CH3X and a free radical NO-(Me)2 in solution. Subsequently, the structural aspects of supramolecular self-assemblies of stable nitroxide radicals and 1,4-diiodo-tetrafluobenzene via directional XB were examined by the group of Schöllhron.17 Shortly after, a series of halogen/hydrogen bonded complexes of aliphatic and aromatic iodo-perfluocarbons with nitroxide radicals were detected by EPR in solution.18,19 However, all these nitroxide radicals indeed act as XB acceptors, which to some extent

limits

the

applications.

Very

recently, 3


a

new

nitroxide

radical,


2,3,5,6-tetrafluoro-4-iodobenzyl tert-butyl nitroxide (1I), has been used as spin probe for the detection of halogen-bonded complexes by EPR,20 and the formation of XB between this nitroxide and some nucleophilic molecules was evidenced by a significant change in the Hβ hyperfine splitting value upon complexation. This method constitutes the first direct EPR technique that enables a reliable determination of the strength of XB in solution. Unfortunately, the nature of such interaction and its working principles remains to be understood. In this work, XB interactions in the complexes of 1I and its hydroxyl derivative 2I, which contain both a halogen-bond-donor group and an EPR-active radical unit, with some nucleophilic molecules (quinulidine, Et3N, and halide anions) were systematically explored using density functional theory (DFT) calculations at the M06-2x level of theory. For comparison, the corresponding hydrogen-bonded complexes of 1H and 2H were also considered, as displayed in Scheme 1. To gain a deeper understanding of these interactions, the analyses of atoms in molecules (AIM), 21 noncolvalent interaction index (NCI), 22 localized orbital locator (LOL),23 electron density difference (EDD), energy decomposition analysis (EDA),24 and spin population analysis were performed. This article was organized as follows: computational methods are described in next section; the main results are given in section III; we show concluding remakes in section IV. Here it should be mentioned that the solvent can largely affect noncovalent interactions upon complexation, as revealed previously.25 However, the main purpose of this study was to explore the nature of radical XBs and HBs and the differences between them. Furthermore, as revealed by the EPR experiments,20 XBs in the corresponding complexes can be detected in 4


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several different solvents, such as C6F6, CH3CN, 2,2,4-trimethylpentane. Therefore, we believe that 1I and 2I act as XB donors irrespective of the solvent employed and the calculations in gas phase can provide reasonable results of these interactions. Scheme 1.

2. COMPUTATIONAL METHODS The geometries of all the complexes under study were fully optimized by means of the hybrid M06-2x functional,26 which has been widely used in the studies of noncovalent interactions and radical systems.27-29 This method was also proved efficient in calculating halogen-bonded systems.30 For the I atom, an adjusted effective core potential (ECP) basis set, aug-cc-pVDZ-PP,31 was employed (28 core electrons), while for the rest of the atoms Dunning’s basis set, aug-cc-pVDZ,32 was applied. No symmetry or geometry constraint was imposed during optimizations. The optimized geometries were verified as local minima on the potential energy surface via frequency computations at the same theoretical level. The interaction energies were corrected by the procedure of Boys and Bernardi33 to account for the basis set superposition error (BSSE). All these calculations were carried out with the Gaussian 09 suite of programs.34 Note that spin expection values of all radical systems were computed to be close to 0.75. The analyses of AIM, NCI, EDD, and spin density were undertaken by the Multiwfn program35, employing the wave functions generated with M06-2x/aug-cc-pVDZ(-PP), and visualized through the VMD package.36 EDA was done based on combining the extend transition state (ETS)37,38 method with natural orbitals for chemical valence (NOVC)39,40 5



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theory as embedded in the Amsterdam density functional (ADF)41 package. The Perdew– Burke–Ernzerhof (PBE) 42 exchange-correlation functional, including the semi-empirical DFT-D3 scheme proposed by Grimme,43 was used to take account of the dispersive effects in the EDA computations, which can provide reliable EDA results of radical systems.29,45 For the H atom, double-zeta STO with one set of polarization functions (DZP) was adopted, and a standard triple-zeta STO basis containing two set of polarization functions (TZ2P) was applied for the Br and I atoms. For the remaining atoms, a standard triple-zeta STO basis containing one set of polarization functions (TZP) was used. Zero order regular approximation (ZORA) were also applied for the Br and I atoms to account for the relativistic effects.

3. RESULTS AND DISCUSSION 3.1 The ESP Surface of the Molecules Under Study The electrostatic potential (ESP) describes the interaction energy between a unit positive charge and the system at a certain point. This is created by the nuclei and electrons of the molecule in the real space. ESP has a real physical property which means its value could be gotten not only by computation but also through experiments.44 The ESP is expressed as follows:

ρ′

r = r + r = ∑ | | − |′| dr′

(1)

where ZA is the charge on nucleus A at position RA. Figure 1. The ESP surfaces of four nitroxide radicals and two nucleophilic molecules under study, 6


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together with the most positive ESP (Vs.max) for the H and I atoms and the most negative ESP (Vs.min) for the O and N atoms, are shown in Figure 1. As can be seen, the I atom in 1I and 2I exhibits a region of positive ESP along the extension of the C−I bond, known as the σ-hole, and Vs.max for the I atom is calculated to be about 30.9 kcal/mol. The values of Vs.max for the H atom in 1H and 2H are computed slightly higher than those for the I atom. In contrast, the O atom in the nitroxide group (NO•) shows a region of negative ESP, attributed to the rich paired and unpaired electrons on this atom, and the N atom in the two nucleophilic molecules also has negative ESP values. Figure 2. Further analysis of the ESP on the donor I and H atoms in 1I and 1H was also employed herein and graphically depicted in Figure 2, which can interpret the orientation difference between XB and HB (vide infra). It is clear that the ESP around the H atom in 1H is spherically positive, so that all these sphere parts are available for nucleophilic molecules to form HB interactions. Furthermore, the ESP distribution on the H atom appears to be somewhat narrow (in the 10-30 kcal/mol range). However, the I atom in 1I exhibits a conical ship-like area (ω ≈ 67.1°) with positive ESP, i.e. the ESP distribution on I atom is hierarchically. Although the ESP values of the I atom are less than those of the H atom, the ESP for the I atom is specifically concentrated on the σ-hole. In addition, the positive ESP in the region parallel to the C−I bonds is nearly zero, hence indicating the anisotropic distribution of electron density for the I atom. 3.2 Geometries and Energetics of the Studied Complexes The optimized geometries for the complexes of 1I/1H and 2I/2H are displayed in Figure 3 7



and Figure S5, respectively. Key structural parameters and interaction energies for all the complexes under study are listed in Table 1. All XB angles in the complexes are predicted to be very close to 180°, thus implying the directionality of XB. However, HBs in the systems of quinulidine and Et3N deviate from linearity significantly, due to the formation of secondary F···H interactions between the F atoms in 1H/2H and the H atoms in quinulidine and Et3N. Upon complexation the C−I/C−H bonds in the nitroxide radicals undergo an elongation accompanied with a red shift of the C−I/C−H stretch vibrations. Figure 3. Table 1. All intermolecular I···N/X distances in the complexes are calculated to be significantly shorter than the sum of the van der Waals (vdW) radii of the atoms involved, and the computed interaction energies for the halogen-bonded complexes range from -9.3 kcal/mol to -25.3 kcal/mol. On the basis of these results, XBs in these systems are somewhat strong in strength, which can be ascribed to the powerful electron-withdrawing ability of the iodo-perfluorobenzene moiety in 1I and 2I. Particularly, the interaction energies for the halogen-bonded complexes are predicted much larger in absolute value than those for the hydrogen-bonded systems. Therefore, the I···N interactions become much stronger than the corresponding H···N interactions, even though secondary F···H interactions occur in the neutral hydrogen-bonded systems. In addition, the complexes of halide anions have considerably more negative interaction energies in comparison with the neutral systems. Generally, the intermolecular distances correlate with the interaction energies for the complexes. However, such correlation was not found for 1H-Quinuclidine and 1H-Et3N, 8


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which can be attributed to the steric energy (see below). To check the reliability of the base set used, single-point energy calculations were also performed at the M06-2x/aug-cc-pVTZ level using the M06-2x/aug-cc-pVDZ optimized geometries. From Table 1, it is seen that the interaction energies computed with aug-cc-pVDZ and aug-cc-pVTZ are very similar to each other. Here it is noteworthy that no obvious geometric and energetic difference was detected between the 1I and 2I systems. Therefore, we only concentrate on the complexes of 1I and 1H in the following analyses. 3.3 AIM, LOL, and NCI Analyses The analysis based on electron density, first proposed by Bader, is an important branch of atoms in molecules (AIM). Topological analysis of electron density based on real space functions has been used for searching the critical points (CPs) and the line segment of topological paths to link the CPs. At a CP, the gradient of electron density is zero and can be found by the Newton method. This theory has been widely applied to characterize covalent or noncovalent interactions through a BCP located between two neighboring atoms.45,46 For all XB and HB interactions in the studied complexes, a BCP is identified between the donor and acceptor atoms, as shown in Figure S1. Furthermore, additional BCPs are also found between the F and H atoms in the neutral hydrogen-bonded systems, which implies secondary H···F interactions. Here it is worth noting that in 1I-Et3N and 2I-Et3N, the I atom is also involved into HBs with the H atoms in Et3N, as revealed by the AIM analysis. This is not surprising, considering the anisotropic distribution of electron density of this atom (vide supra). Owing to the larger values of ρbcp for the halogen-bonded complexes, XBs become much stronger in strength than HBs, consistent with the energetic results demonstrated above. In addition, the 9



complexes of halide anions possess considerably greater ρbcp compared with the neutral systems. Table 2. Dieter et al.47 have pointed out that the bonding type can be judged through electron energy density H, the sum of G (local one electron kinetic energy density) and V (potential energy density). A positive H indicates electrostatic dominant and a negative H implies covalent dominant. Due to the negative values of H, all XBs in the complexes are very strong and show some covalent character, while all HBs in the complexes belong to weak electrostatic interactions because of positive H. Note that both electron energy density H and Laplacian of electron density ∇2ρbcp at BCPs are usually used to evaluate the type of interactions between the atoms. As can be seen from Table 2, ∇2ρbcp for all the studied complexes is predicted to be positive. To further illustrate this viewpoint, the LOL analysis, which can be used to analyze the spatial distribution of electrons with simple and clear function form, was adopted. Value of LOL was renormalized from 0 to 1. Electrons tend to delocalize in the low value parts and localize in high value parts. As shown in Figure 4, an area of high electron localization between the I atom and the acceptor atoms is observed, whereas the electron near HB is somewhat delocalized. On the basis of these findings, XBs have more open shell and covalent interaction component. In addition, compare with 1I-Et3N, electron localization for the interaction in 1I-I− and 1I-Cl− is markedly stronger, in line with the results of interaction energy and AIM (see above). Figure 4. 10


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The NCI theory, in which the real space interaction is separated by the relationship between electron density ρ and the reduced density gradient s, can provide a more global description of noncovalent interactions. The region far from nuclei has small ρ and large s while the region near nuclei has large ρ. Noncovalent interaction has a feature of low ρ and low s and shows a spike on the NCI scatter plot. The reduced density gradient s is expressed as follows: =

π

×

|∇#| #

$

(2)

The type of noncovalent interaction can be characterized by the sign of λ2: attractive interaction has a negative λ2 and repulsive interaction has a positive λ2. The NCI isosurface (RDG=0.5) for four representative systems are displayed in Figure 5. Repulsive interaction region is shown in red, weak interaction in green, and strong interaction in blue. The blue isosurface between the I atom and the N/Br atom indicates the strong I···N/Br interaction, while the green isosurface between the H atom and the N/Br atom corresponds to the weak H···N/Br interaction. Furthermore, the green isosurface between the F atom and the H atom corresponds to secondary H···F interaction in the neutral hydrogen-bonded systems. The scatter plots for the four chosen systems are also given in Figure 5, where the blue points represent the halogen-bonded complexes and the red points represent the hydrogen-bonded complexes. It is clear that the spike of the I···N/I interaction lies at a more negative λ2 value with respect to the H···N/I interaction, thus signifying stronger XBs in the complexes. Figure 5. 3.4 EDD and EDA Analyses The EDD plots, obtained by subtracting the electron density of the complexes from the separated monomers, for three selected systems are displayed in Figure 6. Evident electron 11



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density redistribution can be found along the XB and HB axes. The density around the I/H atom is increased and thus electron density is shifted from the N/Cl atom to the I/H atom. Particularly, the halogen-bonded complex exhibits a larger magnitude of electron density shift with respect to the hydrogen-bonded complex, in good agreement with the energetic, AIM and NCI results. In addition, compare with 1I-Quinuclidine, electron density shift in 1I-Cl− is much more intense, thus suggesting significantly stronger XBs in the complexes of halide anions. Figure 6. In the ETS-NOCV method, the total interaction energy between two fragments is divided into the following components: ∆Eint = ∆Eelstat + ∆Epauli + ∆Eorb + ∆Edisp

(3)

∆Eelstat refers to the classical electrostatic interaction, and ∆Epauli represents the repulsive Pauli interaction between the two fragments caused by their occupied orbitals. ∆Eorb represents the interactions between one occupied molecular orbital and one unoccupied molecular orbital with the same fragment as well as with different fragment. This energy team also linked to electronic bonding coming from formation of a bond48. The final term, ∆Edisp, is the dispersion contribution. In addition, ∆Esteric (the sum of ∆Eelstat and ∆Epauli), which is generally related to charge cloud mutual penetration of two fragments, was also calculated herein. Note that the interaction energies calculated by ETS-NOCV correlate well with those obtained with M06-2x/aug-cc-pVDZ (R2=0.99). Table 3. From Table 3, it is evident that for the HB complexes, ∆Eorb has a main contribution to the 12


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attraction, while ∆Eelestat is the second contribution. In the cases of the XB systems, the contribution of ∆Eorb to the attraction becomes smaller but still dominant, and ∆Eelestat tends to be more significant. To verify the finding of electrostatic interaction in XB and HB, the mutual penetration distance of the vdW surface of the fragments for the four neutral complexes was examined, as shown in Figure 7. Clearly, longer mutual penetration distances are predicted for the XB complexes, consistent with the larger contribution of ∆Eelestat. As expected, dispersion interaction plays a more important role in the HB systems due to the weaker HB interactions, and the contribution of the dispersion term to the attraction is much smaller in the neutral systems compared with that in the charged complexes. In addition, ∆Esteric is computed to be negative for the complexes of 1H with halide anions, which indicates that the attractive interaction is enhanced by steric energy. However, 1H-Quinuclidine and 1H-Et3N show a positive steric energy, because the approach of the H and N atoms is forced by the second H…F weak interactions leading to the increased ∆Esteric. Notably, ∆Esteric is computed to be larger for 1H-Et3N with respect to 1H-Quinuclidine, which may give rise to the longer intermolecular distance in 1H-Et3N. As a result of the larger charge cloud mutual penetration, positive values of ∆Esteric are obtained for all the XB complexes. Figure 7. 3.5 Analysis of Spin Density Distribution The spin density is defined by the following equation in real space: Spin density = Alpha electron density – Beta electron density

(4)

It is known that atomic spin populations mainly come from the nitroxide moiety (NO•) and 13



thus the distance between this group and other atoms plays a key role in the spin density distribution of a certain system. According to the experimental findings, the EPR feedback of the XB/HB formation rely primarily on the spin population change of the Hβ atoms20. To explore the influence of the XB/HB formation on spin density transfer, two stable conformational isomers (A and B) were considered for 1I and 1H, as displayed in Figure S1. These two stable conformational isomers were obtained with a full optimization on the basis of the energy scan of the C-C-N-O dihedral angle (cf. Figure S4). In conformation A the nitroxide moiety is nearly parallel to one Hβ atom and the t-Bu group resides at a staggered site, while in conformation B the t-Bu group is in an eclipsed position that makes the nitroxide moiety in a staggered site. Conformation A is only slightly more stable than conformation B, and these two conformational isomers can be interconverted just through rotating the C−N bond. The reason for choosing these two structures is due to the significant different distance between the nitroxide moiety and the Hβ atoms. Note that the two conformation isomers, which both exist in the system, should exhibit a competitive relationship. Unpaired spin density plots for the two conformation isomers of 1I are also depicted in Figure S2. In general, the positive spin density will be mainly localized near the N and O atoms, and the unpaired electron will reside on the π* orbital since the energy of the σ* orbital is much higher. However, the unpaired spin is, indeed, not completely concentrated on the nitroxide moiety, and the C, Hβ and F atoms also possess unpaired spin density. As shown in Figure S1, the positive spin density lies heavily on Hβ but slightly on the F atoms in conformation 1IB, whereas only moderate spin density lies on Hβ and the F atoms have 14


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almost no spin density in conformation 1IA. It is reasonable that negative spin density is located proportionally on the atoms adjacent to those having a large positive spin density. Based on the experimental EPR spectra, the nitroxide radical 1I can be used as a XB probe through the decrease of A-tensor, which is associated with the spin density change on specific atoms upon XB formation15. However, accurate description of molecular EPR properties, especially the electronic g-tensor and hyperfine coupling A-tensor, is still a great challenge for quantum-chemical calculations 49 . Due to the dependence of hyperfine coupling on spin density distribution near the nuclei, it is necessary to examine spin density distribution in more detail. Despite the difficulty of obtaining a perfect match between the experiment and calculation, the estimated spin density distribution is much less sensitive to the choice of basis sets and functionals used.49 Computed atomic spin densities via the Becke method for the two conformations of 1I/1H and their complexes with quinuclidine are summarized in Table 4. As can be seen, 1IB is predicted to be dominant in Boltzmann distribution, and the spin density values on the H, N and F atoms in 1IB are higher than those in 1IA. Particularly, for the XB complexes the contribution of 1IA-quinuclidine, which has a higher spin density value on the O atom, becomes more significant in Boltzmann distribution. Therefore, in the whole system the unpaired spin density population on the O atom would be increased at the expense of the decrease on the H, N and F atoms. This leads to a decrease of the benzylic hyperfine splitting (aH) upon XB formation, in good agreement with the experimental EPR results. Similarly, the contribution of 1HA-quinuclidine becomes more significant in Boltzmann distribution. However, the weaker HB interaction will simultaneously attenuate the variation of total spin 15



densities in the whole system, resulting in no obvious change of aH upon complexation. Table 4. 3.6 Discussion As shown in Figure 2, the high positive ESP of the I atom is concentrated along the extension of the C−I bond, while half sphere of the H atom shows high positive ESP. Therefore, HBs tend to deviate from the linearity more easily, especially when secondary weak interactions occur. In fact, although the H atom in 1H/2H possesses a slightly lager Vs.max compared with the I atom in 1I/2I, XB interactions in the complexes of 1I/2I are predicted more directional and much stronger than HBs in the systems of 1H/2H. Commonly, the nitroxide radicals used to evidence the XB formation work as XB acceptors. The probes studied in this work, nonetheless, contain a XB donor group and also maintain the benzyl tert-butyl nitroxide with favorable EPR features, which provides a reliable determination of the strength of XB interactions. As revealed previously, the EPR change is attributed to the direct redistribution of spin density on the N atom. However, according to our calculations, the spin density in present systems redistributes to a less degree upon complexation, because of the long real-space distance between the I atom and the nitroxide group. Actually, the formation of XB breaks the equilibrium conformation ratio of the whole system, leading to the decreased unpaired spin density population on the Hβ atom. Here it is worth mentioning that an equally split on both Hβ atoms was observed in the EPR experiments, while our calculations disclosed that the spin density distribution mainly localize on one of them. The experimental coupling analysis was based on a direct and averaged measure, which ignores that these two H atoms may not be equally polarizable at instant. 16


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4. CONCLUSIONS In the present work, XB interactions in the complexes of two novel nitroxide radicals, which contain both a XB donor group and an EPR-active radical unit, with some nucleophilic molecules were studied at the M06-2X level of theory. For comparison, the corresponding hydrogen-bonded complexes were also taken into account. Based on our calculations, the following conclusions can be drawn: 1. XB interactions appears to be much stronger than HBs, and HBs are easier to deviate from the linearity, especially when secondary weak interactions occur. 2. XB interactions exhibit more covalent nature than HBs, and electron density shift is somewhat more intense in halogen-bonded complexes. 3. The EPR characteristics was influenced by the ratio of the conformations with different spin density distribution in the whole system. 4. No obvious geometric and energetic difference was observed between the 1I and 2I systems. We hope that these results will assist in the design and application of halogen-bond-based materials containing radicals.

ASSOCIATED CONSTENT Support Information Figure S1 displaying the scan result of the C-C-N-O dihedral angle for 1I. AUTHOR INFORMATION Corresponding Author 17



* E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (21473054).

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Page 24 of 33

Table 1. Calculated Geometric and Energetic Data for the Studied Complexesa complexes

∆Eintb

distance

angle

∆ν C-I(H)

HB complexes 1H-Cl−

-15.2 (-15.2)

2.155

178.5

483.5

1H-Br−

-12.8 (-12.9)

2.411

173.5

284.4

1H-I−

-10.7 (-10.7)

2.706

171.8

187.1

1H-Quinuclidine

-5.0 (-5.2)

2.170

161.9

159.9

1H- Et3N

-5.2 (-5.5)

2.187

164.9

167.6

2H-Cl−

-15.1 (-15.2)

2.154

178.8

478.1

2H-Br−

-12.6 (-12.7)

2.394

177.9

317.2

2H-I−

-10.4 (-10.4)

2.704

175.9

175.3

2H-Quinuclidine

-4.9 (-5.3)

2.168

162.1

142.4

2H-Et3N

-5.2 (-5.6)

2.186

164.9

147.1

XB complexes

a

1I-Cl−

-25.3 (-25.1)

2.792

179.6

28.0

1I-Br−

-21.9 (-21.8)

2.985

179.6

25.9

1I-I−

-18.6 (-18.3)

3.230

179.6

25.4

1I - Quinuclidine

-9.7 (-9.1)

2.751

179.7

11.5

1I-Et3N

-9.3 (-8.8)

2.874

179.4

10.3

2I-Cl−

-25.2 (-25.0)

2.791

179.3

27.9

2I-Br−

-21.7 (-21.5)

2.986

179.8

25.6

2I-I−

-18.2 (-18.2)

3.230

179.8

29.3

2I-Quinuclidine

-9.6 (-9.2)

2.754

179.8

8.1

2I-Et3N

-9.3 (-8.9)

2.868

179.7

5.7

Energies are given in kcal/mol, distances in angstroms, and angles in degrees. bThe energies

in parentheses are obtained with M06-2X/aug-cc-pVTZ(-PP).

Table 2. The AIM results for the Complexes Under Studya 24


Page 25 of 33 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


interaction

102ρbcp 102∇2ρbcp 102G bcp

type

102V bcp 102H bcp

HB complexes 1H-Cl−

H-Cl

2.893

7.474

1.828

-1.788

0.040

1H-Br−

H-Br

2.073

4.892

1.188

-1.152

0.035

1H-I−

H-I

1.534

3.239

0.799

-0.788

0.011

1H-Quinuclidine

H-N

2.033

5.512

1.327

-1.276

0.051

F-H

0.665

2.615

0.569

-0.485

0.085

F-H

0.644

2.567

0.553

-0.465

0.088

H-N

2.046

5.292

1.286

-1.250

0.037

F-H

0.162

0.841

0.142

-0.073

0.068

F-H

0.430

2.006

0.386

-0.271

0.115

F-H

0.692

2.866

0.614

-0.512

0.102

F-H

0.288

1.306

0.243

-0.160

0.083

1H-Et3N

XB complexes

a

1I-Cl−

I-Cl

3.934

8.069

2.341

-2.664

-0.323

1I-Br−

I-Br

3.275

6.103

1.724

-1.923

-0.199

1I-I−

I-I

2.675

4.452

1.232

-1.351

-0.119

1I-Quinuclidine

I-N

3.153

7.713

2.061

-2.195

-0.133

1I-Et3N

I-N

2.515

6.269

1.601

-1.634

-0.034

I-H

0.660

1.971

0.404

-0.315

0.089

I-H

0.637

1.909

0.390

-0.302

0.087

I-H

0.649

1.930

0.395

-0.308

0.087

All parameters are given in a.u.

Table 3. The Results of the EDA Analysis for some selected systems a complex

∆Eint

∆EPauli

∆Eelestat HB complexes 25


∆Eorb

∆Edisp

∆Esteric b


Page 26 of 33

1H-Cl−

-27.82

14.96

-16.53(38.64%) -25.29(59.12%)

-0.96(2.24%)

-1.57

1H-Br−

-24.31

10.82

-13.03(37.09%) -21.08(60.01%)

-1.02(2.90%)

-2.21

1H-I−

-20.95

8.43

-10.43(35.50%) -17.81(60.62%)

-1.14(3.88%)

-2.00

1H-Quinuclidine -14.95

9.00

-8.10(33.82%)

-13.15(54.91%) -2.70(11.27%)

0.90

11.02

-8.72(33.17%)

-13.73(52.23%) -3.84(14.61%)

2.30

1H-Et3N

-15.27

XB complexes

a

1I-Cl−

-44.19

48.04

-40.63(44.05%) -50.64(54.91%)

-0.96(1.04%)

7.41

1I-Br−

-39.79

42.77

-36.27(43.93%) -45.23(54.78%)

-1.06(1.28%)

6.50

1I-I−

-35.04

37.70

-31.44(43.22%) -40.08(55.10%)

-1.22(1.68%)

6.26

1I-Quinuclidine

-21.00

29.80

-24.30(47.83%) -23.31(45.89%)

-3.19(6.28%)

5.50

1I-Et3N

-20.69

26.15

-20.90(44.62%) -21.74(46.41%)

-4.20(8.97%)

5.25

All energies are given in kcal/mol. b∆Esteric=∆Eelestat+∆EPauli.

Table 4. Relative Energies and Becke Atomic Spin Populationsa spin populations value Erelc

percentageb

1HA

1.40 (0.42)

8.6%

0.014808 0.411511 0.513284 0.003192

1HB

0

91.4%

0.019766 0.416628 0.505654 0.006576

1HA+Quinuclidine

0.75 (0.24)

22.0%

0.014946 0.413005 0.511511 0.003228

1HB+Quinuclidine

0

78.0%

0.019777 0.417274 0.504728 0.006627

H

N

O

F

spin populations value Erel

percentage

H

1IA

1.14 (0.37)

12.9%

0.014668 0.411056 0.513589 0.003138

1IB

0

87.1%

0.019876 0.416339 0.505924 0.006502

1IA+Quinuclidine

1.05 (0.04)

14.6%

0.015033 0.413244 0.511301 0.003325

1IB+Quinuclidine

0

85.4%

0.019811 0.416974 0.505151 0.006501

26


N

O

F

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a

Energies are given in kcal/mol. Simulated temperature is 298 ℃ .

b

Conformational

populations are calculated with Boltzmann distribution. cThe relative energies in parentheses are corrected by zero point energy.

Scheme 1. Chemical structures of the studied donors (1H, 1I, 2H, 2I) and the acceptors. 27



Figure 1. ESP surfaces of the radicals under study (1H, 1I, 2H, 2I) and two XB acceptors by mapping the electrostatic potential onto the surface of electron density (0.001electron per Bohr3).

28


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Figure 2. Top diagrams: ESP with Vs≥0 is displayed. Red point represents Vs,min and blue point represents Vs,max. Bottom diagrams: the ESP distribution of the whole molecule and the specific H/I atom.

29



Figure 3. Optimized structures for the complexes of 1H and 1I.

Figure 4. LOL maps for the selected complexes. Any value more than 0.8 was filled with white. The core electrons for I were filled with black since pseudo potential was admitted. Length unit are given in Bohr.

30


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Figure 5. The NCI isosurface and scatter plot of selected systems. In scatter plots, the blue points represent XB complexes and red points represent HB complexes. In real-space plots, repulsive interaction is shown in red, weak interaction in green, and strong interaction in blue.

Figure 6. The EDD maps for the selected complexes. The isodensity contours is 0.001electron bohr3. 31



Figure 7. Mutual penetration distance of fragments’ vdW surfaces for four selected complexes.

32


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TOC Graphic

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Theoretical Exploration of Halogen Bonding Interactions in the

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