Oriented External Electric Fields - Tweezers and Catalysts for

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Oriented External Electric Fields - Tweezers and Catalysts for Reactivity in Halogen-Bond Complexes Chao Wang, David Danovich, Hui Chen, and Sason Shaik J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02174 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Journal of the American Chemical Society

Oriented External Electric Fields - Tweezers and Catalysts for Reactivity in Halogen-Bond Complexes Chao Wang,†,‡, David Danovich,† Hui Chen*,‡ and Sason Shaik*,† †

‡

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190407, Israel

Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of

Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

*Correspondence authors, [email protected]; [email protected]

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ABSTRACT This theoretical study establishes ways of controlling and enabling an uncommon chemical reaction; the displacement reaction, B:---(X—Y) ® (B—X)+ + :Y–, which is nascent from a B:---(X—Y) halogen bond (XB) by nucleophilic attack of the base, B: on the halogen X. In most of the 14 cases examined, these reactions possess high barriers in either the gas phase (where the X-Y bond dissociates to radicals) or in solvents such as CH2Cl2 and CH3CN (which lead to endothermic processes; e.g. Figure 3). Thus, generally, the XB species are trapped in deep minima and their reactions are not allowed without catalysis. However, when an oriented-external electric field (OEEF) is directed along the B---X---Y reaction axis, the field acts as electric tweezers that orient the XB along the field’s axis, and intensely catalyzes the process, by tens of kcal/mol, thus rendering the reaction allowed (Figures 9c and 10c). Flipping the OEEF along the reaction axis inhibits the reaction and weakens the interaction of the XB. Furthermore, at a critical OEEF, each XB undergoes spontaneous, and barrier-free reaction. As such, OEEF achieves quite tight control of structure and reactivity of XB species. A valence-bond (VB) modeling is used to elucidate the means whereby OEEFs exert their control. Keywords: Halogen bond, Charger transfer, Electric Field Effects, Anion displacement reactions, Electric Tweezers


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1. INTRODUCTION A typical halogen bond, so-called an X-bond (XB), involves a Lewis base B: interacting with a halogen X that is bonded to a fragment or an atom Y, as depicted in Chart 1a. The XB is analogous to the hydrogen bond (HB),1 in Chart 1b, both sharing B-X-Y and B-H-Y angles which are approximately 180°. The interest in the features and utility of XBs is currently enormous.2 Thus, due to its linear B-X-Y moiety, the XB plays a role as a ubiquitous architectural element in crystal engineering, liquid crystal materials and polymer chemistry materials in various areas.2,3 Furthermore, as shown by Riedel et al,3 XBs give rise to fascinating novel halogen- and inter-halogen chemistry in 3D.

Chart 1: (a) A Halogen Bond (XB) of a Base B: with a Halogen (X) in an X-Y Molecule. (b) An Analogous Hydrogen Bond (HB). The approximate BXY and BHY Angles are depicted by the species.

Alongside this structural role, there are fundamental issues, such as the origins of the XB bonding interaction, which has attracted considerable attention.2d,f,4,5 When halogen bonded complexes (e.g., amines and dihalide, etc.) had been originally discovered and characterized,6a the species were referred to as charge transfer complexes (CTC).6b,7 This classification reflected their spectral properties and intense colors, which fitted the theoretical description by Mulliken,7 who attributed the bonding and spectral features to the charge-transfer interaction, between the base B: donor and the X-Y acceptor. While the ACS Paragon Plus Environment

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CTC theory has been supported recently by various energy decomposition analysis (EDA) methods,4 as well as by experimental methods,2d,5 the more commonly used current explanation for the bonding in XB species is electrostatic, as manifested by the s-hole concept, which refers to the positively charged head of the halogen inside the negatively charged belt along the X-Y axis.8 Our goal here is not concerned, with the discussion of which bonding picture is more ‘correct’ or more ‘useful’. In our view, novel species may exhibit also unusual reactions, which may prove in time to be useful. As such, our interest focuses, herein, on a potential displacement reaction, in Chart 2, whereby the “base”, B:, attacks the halogen X, and displaces Y, as an anion, Y:–, which itself may be a halide, or a fluorinated alkyl group, etc. This displacement reaction, which is an analogue of the textbook SN2 reaction, is not common, and there might be considerable interest in defining a possible approach to stimulate and catalyze such ‘reluctant’ reactions, which form ion-pairs, be these in the gas phase, solids, in polymers, or in proteins.

Chart 2: A Displacement Reaction in a Generic XB.

An initial impulse for seeking these displacement reactions, is the pioneering study of Legon et al,2d,5a who investigated the microwave spectra of a variety of XB complexes. Legon et al. concluded that a satisfactory interpretation of the rotational spectral constants for e.g., (CH3)3N---ClF and (CH3)3N---F2 requires significant contributions from the ion-pair forms [(CH3)3NX]+---F– (X = F or Cl). This characterization was supported by a theoretical analysis of these XBs4a using valence bond (VB) and ACS Paragon Plus Environment

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Block-localized Wavefunction (BLW) methods.9 Furthermore, Legon et al.1,2d,5a compared the XB complexes with the corresponding HB complexes, e.g. (CH3)3N---HI, and found that the trimethylammonium iodide is nearly an ion pair, even in the gas phase. Related findings were reported by Rosokha et al.10 who investigated the CT spectra and kinetics of XB complexes. X: –----Br-R and amine---Br-R (X = I, R = CBr2Z; Z = H, Br, CN, NO2; (amine = TMPD)), and found an XB-assisted electron-transfer reaction, with a net displacement of R:– mostly for the cases with Z = CN and NO2. The previous treatments of classical SN2 nucleophilic displacements on alkyl halides, by one of us,11 has shown that this textbook process transpires due to the charge transfer state that crosses the reactants state X: –----R-X, and leads to the halide exchange. By analogy, we may envision that in the process depicted in Chart 2, the XB complexes (B:----XY), of bases/nucleophiles B: with dihalides or alkyl halides may undergo displacement reactions by a nucleophilic attack of B: on the halogen. However, since the thermodynamics of such processes are highly unfavorable for the neutral bases (see also later), these reactions are uncommon and “non-allowed”, requiring a catalyst to transpire. Indeed, as this study shows, with some exceptions, these displacements cannot take place neither in the gas phase, nor in solvents like CH2Cl2 or CH3CN. However, as shall be shown, an oriented external electric field (OEEF),12-14 along the reaction-axis, B-X-Y (in Chart 3), acts as electric tweezers that orient the XB species along B-X-Y axis, and is capable of efficiently catalyzing these nonreactive XBs and controlling their reactivity at will. Chart 3. Illustration of halogen bond complexes subsect to OEEF along the reaction axis Z.


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2. TARGET SYSTEMS The target 14 XBs we investigated here are depicted in Scheme 1. Scheme 1a lists the simplest XB complexes between ammonia and the four dihalogen molecules. Scheme 1b shows the analogous complexes where pyridine replaces ammonia, whereas Scheme 1c shows the XB complexes between pyridine and four trifluromethyl halides (halogen = F, Cl, Br, and I). Finally, Scheme 1d shows XB systems, which studied by Legon et al,2d,5a involving trimethylamine (Me3N:) as a base, and F2, and ClF.

Scheme 1. The σ halogen bond systems studied in this work. (a) ammonia dihalogens complexes, (b) pyridine dihalogen complexes, (c) pyridine trifluoromethyl halide complexes. (d) Trimethylamine with F2, ClF.

These XB complexes, will be studied by means of density function theory (DFT), Möller-Plesset second order perturbation theory (MP2), coupled-cluster CCSD(T), valence bond (VB) calculations, and multi reference configuration interaction (MRCI) calculations (see supporting information (SI) document). As shall be demonstrated, an OEEF along the reaction axes of the XB complexes, will make these


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“nonallowable” processes exothermic and enable the displacement reactions by lowering the respective barriers by tens of kcal/mol.

3. COMPUTATIONAL DETAILS Electronic and Geometries Calculations: All molecular geometries were optimized in “vacuum”, employing the M06-2X functional15 and using the Gaussian 09 quantum chemistry package.16 The M062X functional has been tested and found to be reliable for various kinds of non-covalent interactions.17 The correlation consistent polarized valence triple-ζ basis set cc-pVTZ18 was used for all atoms except for Bromine and Iodine, for which we used the correlation consistent polarized valence triple-ζ small-core relativistic pseudopotential basis set cc-pVTZ-PP.19 Bond-dissociation energy (BDE) calculations were calculated at the same level of geometry optimization (see Table S1 in the Supporting Information (SI) document). Group charges were determined using the NBO method,20 as implemented in Gaussian 09. The binding energies of the XB complexes, DEb, were determined with basis-set superposition error (BSSE) correction.4a,b The XBs with Me3N were also investigated in CH2Cl2 and CH3CN solvents, using the polarizable continuum solvation model.21 Energy Profiles and Transition State Calculations: All the potential energy profiles for the reactions in Scheme 1, were scanned along the reaction coordinate, using the M06-2X/cc-pVTZ-PP (for Br and I cases) and M06-2X/cc-pVTZ levels. Whenever appropriate, the highest point on the energy scan was subject to transition state (TS) search. All TSs were characterized by having a single imaginary frequency. The products of the displacement reactions of the XB complexes were taken along the reaction coordinate all the way to X-Y distances of 12-15Å. Using either solvents or OEEF, these dissociation limits where fully ionic, (B-X)+ + :Y–. However, in the gas phase, the charge distribution indicated the occurrence of self-interaction or delocalization errors, such that the displaced Y had a partial negative charge. Using the LC-wPBE functional and modifying the w value (using IOP(3/107 = 2000000000), see SI) for short- and long-range exchange22 corrected these SIE or delocalization errors. However, upon further inspection, we found that in the gas phase and in the absence of OEEF, there exist two solutions ACS Paragon Plus Environment

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at the product limit, for most of the cases in Scheme 1. One is an open-shell singlet diradical solution (B: •X) + •Y, and the other is the ionic solution (B-X)+ + :Y–. By calculating the open-shell singlet (OSS)

solution, we were able to verify that all the complexes gave the diradical state at the product limit. To check the reliability of the DFT calculations, we further tested the XB H3N---Cl-Cl’ and the energy barriers for the displacement reactions, using B2PLYP/cc-pVTZ,23a MP2/cc-pVTZ,23b and CCSD(T)/cc-pVTZ.23c We also use multireference calculations, using the VB24 and MRCI25 methods. As the VB method, we selected the breathing-orbital VB (BOVB) method,26 which incorporates the changes of dynamic correlation along the reaction coordinate. For both VB and MRCI we used the 6-311G(d,p) basis set,27 hence the BOVB/6-311G(d,p) and MRCI/ 6-311G(d,p) levels. OEEF Calculations: The OEEF effects were studied by Gaussian 09 package, using the Z-matrix coordinates to define the OEEF axis, its direction, and its magnitude. The electric field in Gaussian is allowed to be oriented along the X-, Y-, and Z- axes of the reactants as defined in Chart 3. In our cases, the Z-axis defines as the “reaction axis”, which involves the new bonds made and broken during the transformation. It is noteworthy that in Gaussian 09 package, the positive direction of electric field is defined from the negative to the positive charge (see Chart 3),12d which is different with the conventional definition in physics. The unit of electric field is au, 1 au = 51.4 V/Å.

4. RESULTS The many numerical results are summarized in the supporting information (SI) document, while the key ones are provided or discussed below. 4.1. Halogen bond complexes in the Gas Phase and Solvents Geometric Features, Bonding Energies and Dipole Moments: The target XB complexes in the four series, in Scheme 1, involve as Lewis bases, B:, ammonia, pyridine, and trimethylamine (Me3N). Upon geometry optimization, all the systems gave rise to XB complexes in local minima. These XB complexes possess linear structures whereby the N---X-Y angle is almost 180°. Table 1, lists the main bond lengths


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(in Å) of the XB complexes for the target species, the percentage of X-X and X-CF3 bond elongation, visà-vis the free molecules (%DRXY/R0XY), and the bonding energy, DEb (kcal/mol).

Table 1: Computed M06-2X Geometric Features (bond-length R in Å, BXY angles in degrees), Dipole Moments (µ, in Debye (D)) and Binding Energies (DEb in kcal/mol). Part a X RN-X RX-X Ð µ ΔEb 100 [ΔRXY/ R0XY] Part b X RN-X RX-X Ð µ ΔEb 100 [ΔRXY/ R0XY]

H3N---X2 Cl Br I 2.69 2.60 2.76 2.02 2.33 2.70 179.8 179.8 179.3 3.3 4.6 5.1 5.5 7.9 8.4 1.4 2.0 1.7 Pyridine---XCF3 F Cl Br I 3.42 2.95 2.92 2.95 1.31 1.75 1.93 2.15 179.5 177.2 178.6 179.9 2.4 3.6 4.3 5.4 0.1 2.6 4.3 6.2 -0.3 -0.4 -0.2 0.1 F 2.56 1.38 177.9 2.2 1.7 1.0

F 2.56 1.38 179.2 2.8 1.5 0.9

Pyridine---X2 Cl Br 2.65 2.50 2.02 2.34 179.8 180.0 4.3 6.3 5.3 8.0 1.4 2.4 Me3N---XF

F 2.38 1.40 179.6 1.7 2.7 2.4

I 2.66 2.71 178.9 6.9 8.8 2.0

Cl 2.16 1.72 180.0 6.1 16.2 5.7

It is seen that the acceptor molecules, X-X (X-Y), undergo minor bond elongation of 1-6% (or minute shortening for CF3-X), where the higher extent is for the strongest base Me3N, while the X-CF3 bond barely changes, ~0.1%. The bonding energies of the XB complexes are in the range of 0.1-16.2 kcal/mol, where the higher values are obtained for the acceptor XY molecules where X = I. As argued before,4a,b the bonding energies are either dominated (for the dihalides) or have substantial contribution from charge-transfer mixing, and hence some covalent character typifies in the B---X interactions (see charge distributions in the SI, pp. S4-S21). Some evidence for the CT character is provided by the NBO charges, in Figure 1. These values which range from 0.23 e– for the best donoracceptor pair, Me3N---ClF, down to 0.03e– for the complexes with pyridine as a base (the values for all ACS Paragon Plus Environment

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the XB are tabulated in the SI). The enhanced ionicity for the XB Me3N---ClF matches the findings of Legon et al. from microwave spectroscopy.2b,5a

Figure 1: NBO Group Charges (M06-2X functional) for the Best Donor-Acceptor Pairs in Various XB Types.

Gas-Phase Behavior of XBs Along the Displacement Reaction-Coordinate: We have investigated all the 14 complexes for their displacement reactivity in the gas-phase without electric field or solvents. Figure 2 shows the typical energy profile, using the Me3N---Cl-F XB-complex. It is seen that the energy keeps rising and converges at the products limit (RCl---F = 12Å). For the M06-2X functional, the energy at RCl---F = 12Å is 112.8 kcal/mol above the XB minimum. However, the group charges at this geometry are still partial (-0.57 for the departing F), thus indicating SIE (or delocalization error), which is typical to DFT. Using the LC-wPBE functional for the products limit led to the two ions, (Me3NCl)+ + F–. At the same time, the energy of the ionic product is 133.3 kcal/mol above the XB minimum. As shown in Figure 2c, performing OSS calculations using M06-2X gave rise to the diradical product, (Me3N: •Cl) + •F, which was lower in energy by 59.6 kcal/mol.


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Figure 2: The energy profiles (kcal/mol) for the reaction of Me3N: with Cl-F, via the XB Me3N---Cl-F, and all the way to the products limit, Me3N-Cl + F. Geometric details, NBO charges and energies (kcal/mol) relative to the XB species are shown for two extreme species. (a) M06-2X. (b) LC-wPBE with IOP(3/107 = 2000000000). (c) Using OSS in M06-2X.

The same applies to all the target systems (see SI, pp. S25-S27). For example, for H3N + Cl2, the products limit at RCl---Cl = 12-15Å, gave in BOVB two solutions, one a diradical, (H3N: •Cl) + Cl’•, the other an ion-pair, (H3N-Cl)+ + :Cl’–; the latter being 72.5 kcal/mol higher than the former (at 15Å).. Using M06-2X(OSS) calculations leads to a higher preference of the diradical solution over the ionic product limit. Using similarly, the XB complexes of pyridine (C5H5N) with X-CF3 (X = I, Br, Cl, F), the dissociation limit converged to diradicals, (C5H4N: •X) + •CF3, with ion-pairs being much higher. Generally, the energy rises by 40-79 kcal/mol for the various complexes, at the M06-2X level (see SI,


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Table S34). As such, the XB complex in the gas phase is trapped in its minimum, and is unable to lead to a displacement reaction. Solvent-Phase Behavior of XBs Along the Displacement Reaction Coordinate: In order to ascertain the behavior in a condensed phase, we used the reaction of Me3N: with Cl-F, in CH2Cl2 and CH3CN. The results are summarized in Figure 3. As shown in Figure 3a, the solvent drives the XB slightly along the displacement coordinate, by shortening the N---Cl and lengthening the Cl---F distances. However, Figure 3b shows that the energy profile still goes up 36.5 kcal/mol from the XB geometry towards the product limit at RCl---F = 15Å, without any TS. In the solvent phase, the product limit is fully ionic without a lower lying OSS solution. Thus, the solvents we used do not appear capable of separating the ions and generating exothermic reactions. The XB is still trapped in its minimum and requires a high energy to perform the displacement of F–.


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Figure 3: (a) Geometries (distances in Å) of the XB Me3N---Cl-F in the gas phase, and in CH2Cl2 and CH3CN as solvents. (b) The energy profile (kcal/mol) for the displacement reaction, Me3N---Cl-F ® (Me3N-Cl)+ + F–, in CH2Cl2 and CH3CN. Energies (E, in kcal/mol) of the extreme species at dissociation, relative to the XBs, are indicated.

We tested a few other reactions for H3N and Pyridine + X2 (X = Cl, I), in CH2Cl2 and in CH3CN, which led to similar general conclusions (See Figure S15-S18 in the SI). Thus, even though in one case with CH3CN as solvent, the dissociation limit is rather low lying (~10 kcal/mol), the reaction is endothermic; the solvent does mot separate the ions and the XB remains trapped in its minimum. The only reaction, which shows a potential for realizing the displacement reaction in a solvent is, Me3N---F-F’ ® (Me3N-F)+ + F’–, which involves base with the lowest ionization potential (IP), 7.82 eV, and a very good electron-acceptor molecule, F2, having also a weak F-F bond. Figure 4 shows the results for this reaction. Thus, in the gas phase (Fig. 4a), the XB has a normal geometry, with a short F-F’ bond and a long N-F bond, with an energy profile (Figure 4b) that goes up to 8.8 kcal/mol, leading to the diradical product limit, (Me3N: •F) + •F’ (60.3 kcal/mol lower than the ionic limit). On the other hand, as shown in Figure 4a, in solution the XB is a tight ion-pair species, with a short RN-F and a long RF-F’. Thus, for example, in CH2Cl2, the species exhibits a net one-electron transfer from Me3N towards the F-F’ bond, leading to a short N-F bond (1.38Å) and a longish F---F’ bond (2.49Å). As such, in CH2Cl2 there is an inner-sphere ET, and as shown in Figure 4c, the departure of the F’– anion involves a small escape barrier of 4.8 kcal/mol, which becomes merely 1.6 kcal/mol in CH3CN. This XB mimics the observations of Rosokha et al. with the good electron-donor base, TMPD.10


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Figure 4: (a) Geometries (distances in Å) of the XB Me3N---F-F’ in the gas phase and in solvents, CH2Cl2 (blue color) and CH3CN (pink color). Below and above the structures are the color-coded NBO charges of the XB complexes in the two solvents. (b) The energy profile (kcal/mol) for the gas phase reaction Me3N---F-F’ ® (Me3N-F)• + F’•; the relative energy of the extreme species E = 8.8 kcal/mol is indicated. (c) The energy profiles in CH2Cl2 and CH3CN, relative energies (kcal/mol) are indicated for the extreme species.

The finding in Figure 4 is solitaire in our set of 14 XBs. The great majority exhibits an XB which is trapped in the gas phase and in solvents with large escape barriers. The displacement reaction, whether in the gas-phase or in a solvent, is restricted to cases of very good donor-acceptor pairs, which form very reactive and labile XBs, like Me3N---F2.

4.2. Halogen Bond Complexes Under OEEF


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XB Species Under OEEF Along the Reaction Axis: The behavior of the XB under OEEF along the B---X-Y axis (the “reaction axis”)12,13,28,29 is completely different compared with the reactions in the absence of OEEF. This has been verified for all the XBs in Scheme 1. Figure 5 uses H3N---Cl-Cl’ as an example, and provides a typical pattern of the OEEF impact on the potential energy profile.

(a)

Fz z

=H =N = Cl

2.69

2.02

µ 3.3 D FZ = 0 au

2.86 2.21

2.01 2.16

1.3 D FZ = -0.01 au 8.5 D FZ = +0.01 au

Z

Figure 5: (a) Geometries (distances in Å) and dipole moments of the XB H3N---Cl-Cl’ in the gas phase, and in different OEEF values: FZ = 0, FZ = -0.01 and FZ = +0.01 au. (b) The energy barriers as a function of FZ. For the Fz = 0 au, energy barrier is given at RCl-Cl’ = 12 Å. Note that at a field ≥ 0.02 au, the displacement to (H3NCl)+ + Cl’– occurs spontaneously.

Figure 5a shows the bond lengths of the XB complex in the gas phase, without a field, and in positive and negative fields along the “reaction axis” Z. A negative FZ is seen to lengthen the N---Cl bond and shorten the Cl-Cl’ bond. In contrast, a positively-oriented field does exactly the opposite; it shortens the N---Cl bond and lengthens Cl-Cl’. At the same time, the dipole moments (µZ) in Figure 5a show that negative FZ reduces the dipole moment of XB because the negative field destabilizes the native charge distribution (H3N---X)d+Cld–, whereas a positive FZ increases the dipole moment (in D units), since it stabilizes the same charge distribution and further polarizes the XB to increase charge transfer towards


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the terminal chlorine atom. In summary, Figure 5a shows that the directionality the Z-field can control the displacement reaction at will. The field can drive the XB along the displacement coordinate when FZ > 0, or shift it in the opposite direction when FZ < 0, thus inhibiting the displacement process. Figure 5b shows the reaction barriers for the displacement reaction as a function of FZ in the range of 0-0.02 au (1 au = 51.4 V/Å). The barrier is huge at FZ = 0 au and FZ = 0.005 au; 103.4 and 65.1 kcal/mol. Nevertheless unlike the field free situation where there is no TS, at FZ = 0.005 au there is a TS that connects to the product ion pair. As the field increases the barrier is reduced almost exponentially, down to 4.9 kcal/mol for FZ = 0.0175 au. At a high value of 0.02 au, there occurs a spontaneous displacement reaction. This behavior is general, though the quantitative numbers change. All the data are relegated to the SI document (see SI, pp. S5-S6). Transition State Geometries and Barriers Under OEEF: Figure 6 shows the barriers and the location of the transition states (TS) for the same reaction of Figure 5. Firstly, on Figure 6a, it is seen that as the field becomes increasingly more positive, the location of the TS moves to an earlier position with less Cl-Cl’ cleavage. Figure 6b shows explicitly, the two bond lengths of the H3N----Cl----Cl’ species in the TS. It is seen that the main geometric variation is the Cl---Cl’ distance, while the N-Cl distance is almost invariant. This behavior is also common to the various reactions in the set (see SI, pp. S4-S21). Thus, as a rule, it appears that the main activation mechanism is the X-Y bond cleavage, while the N-X bond making is already advanced at the XB stage.


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Figure 6: (a) M06-2X/cc-pVTZ energy profiles of the H3N---Cl-Cl’ ® (H3N-Cl)+ + Cl’– reaction at different OEEF values (in au), and the Cl---Cl distances (in Å) in the corresponding transition states (TSs). (b) N-Cl and Cl-Cl distances (Å) in the various TSs. Structure-Reactivity Patterns for Various XB Complexes Under OEEF: In order to assess the trends in the XB set, we recalculated all the barriers under the same field value of FZ = 0.0125 au. Figure 7 collects the TS structures, the group NBO charges on the base molecule (QB) and the terminal group (QX’, QF, QCF3), and the barriers relative (DE‡) to the corresponding XB complexes.

X=F X = Cl X = Br X=I

R1 1.35 1.75 1.92 2.14

R2 3.50 4.16 4.51 4.82

QB +1.10 +0.69 +0.59 +0.46

QX’ -0.99 -0.97 -0.96 -0.94

ΔE‡ 12.5 21.8 24.9 21.1

X=F X = Cl

R1 1.38 1.77

R2 3.03 4.00

QB +1.16 +0.75

QF -0.99 -0.98

ΔE‡ 0.8 34.0


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R1 1.34 1.72 1.89 2.10

R2 2.96 3.78 4.11 4.46

QB +1.10 +0.70 +0.61 +0.49

QX’ -0.99 -0.98 -0.97 -0.96

ΔE‡ 0.6 4.8 9.7 9.0


R1 1.54 1.72 1.89 2.10

R2 2.24 3.80 4.13 4.43

QB +0.87 +0.70 +0.61 +0.50

QCF3 -0.64 -0.95 -0.94 -0.94

ΔE‡ 108.6 59.1 52.5 46.1

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Figure 7: TS Geometries (R1 and R2 distances in Å), Group Charges (Q), and Barriers (DE‡ in kcal/mol) at FZ = 0.0125 au, for (a) the H3N---X-X’ series, (b) Me3N---X-F’, (c) Pyridine----X-X’ series, and (d) Pyridine---X-CF3 series.

It is seen that in all cases, the group charge on the terminal group is negative < -0.5 (-0.64 à -0.99). Similarly, the base acquires a considerable positive charge ( ~ 0.49à1.16), and so does the X on the site of attack. As such, the TS at FZ = 0.0125 au is rather close to being an ion pair, (B—X)+---Y–, e.g. H3N-X+---X–. Therefore, as the leaving group Y– (X– or CF3–) gets farther away from cation (B—X)+ (H3N-X+ or Pyridine-X+), the dipole-moment of the (B—X)+---Y– species increases ( > 9 D, see e.g., SI, Tables S2-S29) and interacts more and more strongly with the field,12d,30 such that the potential energy of the ion-pair gradually decreases towards the displacement product (B—X)+ + Y–, and a TS is achieved on the energy profile. The energy barriers in Figure 7 span a range of 0.6-108.6 kcal/mol, and enable us to arrange the relative reactivity as follows: Pyridine + F2 ~ Me3N + F2 > Pyridine + X2 >> H3N + X2 >> Me3N + ClF >> Pyridine + X-CF3; F-CF3 has a huge barrier. Thus, for a given acceptor molecule, X2, the displacement reaction is faster (has a lower barrier) for the base Me3N, which possesses the lowest ionization potential, and when the formed N-X+ bond is stronger, such for pyridine vs. NH3. Among the dihalides, X2, the most reactive is F2, which possesses the weakest X-X bond [see Table S1 in the SI] and is the best electron ACS Paragon Plus Environment

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acceptor (electron affinity = 71.5 kcal/mol31). The X-CF3 molecules have strong X-C bonds [see Table S1 in the SI], and are not as good acceptors as the dihalides. As such, these molecules are nonreactive and would require higher fields to undergo displacement of the CF3– group. It appears therefore that, for a given electric field strength, the trends in the barrier are shaped by the relative bond strength; N-X vs. XX and N-X vs. X-CF3, as well as by the electron-accepting power (electron affinity) of the X-X or CF3-X molecules. Testing the DFT Barriers in OEEF: In order to assess the possible accuracy of the barriers in Figure 7, we selected the reaction H3N + Cl2, and applied: (a) double-hybrid functional, (b) MP2, (c) MRCI/6311G(d,p), and (d) BOVB/6-311G(d,p), calculations. For MRCI, the barriers were corrected for sizeconsistency, while for BOVB we used the localized L-BOVB, and the delocalized D-BOVB versions.26,32 The barriers are collected in Table 2 (more details are in the SI document, pp. S29-S40).

Table 2: Energy Barriers (kcal/mol) for the Displacement Reaction, H3N----Cl-Cl’ ® H3NCl+ + Cl’– under and OEEF, FZ = 0.0125 au.

a

1 2 3 4

Method M06-2X/cc-pVTZ B2PLYP/cc-pVTZ MP2/cc-pVTZ CCSD(T)/cc-pVTZ

5

MRCI/6-311G(d,p)

6 7

L-BOVB/6-311G(d,p)c D-BOVB/6-311G(d,p)e

Energy Barriers 21.8 24.8 18.5 21.6 13.1a 13.5b 11.7;11.8d 12.4f

Davison correction; b Pople correction; c,eAll geometries used here are corresponding to Table S39.

d

L-BOVB/cc-pVTZ; f nonactive orbitals are delocalized (D).


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It is seen that the DFT methods lead to barriers of 21.8 and 24.8 kcal/mol. MP2 yields a smaller barrier, 18.5 kcal/mol, while CCSD(T) leads to a barrier extremely close to M06-2X. However, when we proceed to the multireference methods, we get lower barriers, MRCI with size consistency corrections to 13.1-13.5 kcal/mol, while L-BOVB leads to 11.7-11.8 kcal/mol, and DBOVB to 12.4 kcal/mol. These multi-reference methods predict rather close barriers, which are lower than those obtained by DFT and the other single-reference methods, by approximately 8-10 kcal/mol. Since the OEEF polarizes the reacting species extensively, methods which allow each configuration/VB-structure to have its own optimized orbitals in the presence of all other configurations/VB-structures, and in a self-consistent manner, may have an advantage over methods that start with a delocalized electronic structure calculation and treat the various configurations at a mean field manner. We think that the barrier should indeed be 810 kcal/mol lower than the M06-2X value. If this difference is commonly applied, then even the reaction of Pyridine with CF3-I may transpire, albeit slowly, under OEEF catalysis. OEEF Control of the XB Orientation: One of the questions that arises for using OEEF is how to orient the molecule vis-à-vis the field?12d,30 As argued,30,33a a sufficiently strong OEEF can orient the molecular dipole along the axis of the OEEF, which serves thereby as tweezers that prepare the XB for the displacement reaction along the same axis. To test this option, we selected the same reaction H3N + Cl2, and applied the same field FZ = 0.0125 au, which exhibit catalysis. Indeed, this field orients the complex H3N----Cl-Cl’ along the Z axis, with a barrier of 25.3 kcal/mol for a 90° rotation, at the M062X/cc-pVTZ level (see Table S35 in the SI). In the same direction, this field also catalyzes the reaction with a barrier of 21.8 kcal/mol (M06-2X/cc-pVTZ, see Table 2). As such, the same field will fix the molecule in space and catalyze its displacement reaction. It is interesting to note that enzymes achieve the tweezer effect33b by use of protein residues which hold the substrate (that has to undergo chemical


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modification), and juxtapose it vis-à-vis the active species of the enzyme. A good example is cytochrome P450, e.g., P450cam, which orients the 5-C-Hexo bond of camphor in the trajectory of the Fe=O moiety of the active species, and thereby bringing about selective activation of this C-H bond.33b The OEEF in this study achieves the same by means of electrostatic restriction.

5. DISCUSSION Our computational results reveal the following findings about the potential of stimulating XB complexes (B:---X-Y) to undergo displacement reactions on the halide X: (a) In the gas phase, the XB complexes are trapped in their deep minima, and face very high barriers for displacement reactions, ranging from 69-188 kcal/mol. In most cases, the product limit in the gas phase is a diradical, (B:•X) + •Y rather than the classical “displacement product”, BX+ + :Y–. Furthermore, the gas phase processes for the target systems exhibit no transition state (TS) species along the reaction coordinate, and the energy keeps rising all the way to the products limit. Nevertheless, in regions of the energy curve before the product limit, there is some contribution from CT mixing, as revealed by the VB calculations. (b) Solvents and even polar ones, like acetonitrile, do not change the displacement pattern; while the solvent lowers the energy needed to reach the ionic product-limit, there is still no TS species, and the energy rises to an endothermic dissociation limit. The exceptional cases are the XBs which are composed of very good donor-acceptor pairs line Me3N---F2 and Me3N---ClF. (c) External oriented electric fields (OEEFs) along the “reaction axis” B---X---Y (defined as the Z axis in Chart 3) can however control the displacement reaction, even in the gas phase. Thus, a negatively oriented OEEF FZ < 0 will weaken the XB and inhibit the displacement process. In contrast, just flipping ACS Paragon Plus Environment

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the field to FZ > 0 it will acts as tweezers, which orient the XB along the reaction axis, and at the same time the field enables the displacement reaction.33 Thus, the OEEF orients the molecules, generates a TS and lowers the energy barrier by tens of kcal/mol. (d) Every XB has a threshold OEEF, which leads to a spontaneous displacement reaction. To comprehend these trends, we shall present here a VB analysis of the energy profile for the displacement reaction from a typical XB, H3N:----Cl-Cl’, which we investigated in details.

5.1. A Valence Bond Analysis of the Displacement Energy Profile Under OEEF The VB Structure Set: Scheme 2 shows the six VB structures, which are needed for the description of the target displacement reaction, H3N----Cl-Cl’ ® (H3N-Cl)+ + Cl’–. The first two structures describe the covalent bonds in the XB and the displacement product (P), respectively. As we usually do,32 the covalent bond is depicted here using an arched line connecting the two single electrons in the respective bonds, ClCl’ in FC,XB and (N-Cl)+ in FC,P. There are three ionic type structures, labelled as FI with a qualifier of the location of the positive charge. Thus, FI,Cl+ and FI,Cl’+ provide the ionic components of the respective Lewis bonds; FI,Cl+ contributes to the two bonds, while FI,Cl’+ contributes only to the Cl-Cl’ bond, and finally FI,N2+ only to (N-Cl)+. The last VB structure, FLB is a long-bond (LB) structure, which couples the single electrons on H3N•+ and the terminal Cl’•.

Scheme 2: VB Structures for the description of the displacement reaction, H3N----Cl-Cl’ ® (H3NCl)+ + Cl’–. Near the structures we show the respective dipole moments (in Debye units) in the absence of the field.


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To comprehend the effect of OEEF on these VB structures, let us consider first the principal ones, FC,XB and FC,P. FC,XB is the main descriptor of the Cl-Cl’ bond in the absence of the field, while FC,P is the main descriptor of the (N-Cl)+ bond in the displacement product. In the absence of an electric field, the BOVB weights of these structures in the respective bonds are 0.60 and 0.64, respectively. The third most important structure is FI,Cl+ which contributes to the two bonds, with weights of ~0.21 and ~0.30 for Cl-Cl’ and (N-Cl)+, respectively. The fourth but much less important is the long-bond structure, FLB, which contributes to the description of the XB, with a weight of 0.03 in the field-free situation. Note that at the XB geometry, FC,P is related to FC,XB by one-electron transfer from the ammonia into the terminal chlorine atom of the Cl-Cl’ bond (which becomes Cl• :Cl’–). Another such structure is the long bond structure, FLB, which is related to FC,XB by one-electron transfer from the ammonia to the Cl atom of the Cl-Cl’ linkage (which becomes Cl: – •Cl’). Thus, taken together, FC,P and FLB constitute the charge transfer (CT) state, YCT (H3N•+---(Cl: – •Cl’ Û Cl• :Cl’–)), which is initially an excited state of the XB ground-state species, YXB. Scheme 2 adds near these four structures, their intrinsic dipole moments (µ); FC,XB has a rather small dipole (2.5 D), whereas FC,P possesses a large dipole moment (22.2 D). The FI,Cl+ and FLB structures possess intermediate values (10.3 and 14.4 D, respectively). As such, when the OEEF is turned on, in the


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positive direction (FZ), the weights of these structures will be affected to extents, which depend on their respective dipole moments.12d,30 Spontaneous Displacement Reactions Elicited by OEEFs: Let us try first to comprehend the occurrence of spontaneous displacement reactions that are elicited by the field at a critically intense value (e.g. FZ = 0.02 au for the target reaction H3N + Cl-Cl’). Figure 8 shows the variations of the weights (W) for the four main VB structures of the XB complex, as a function of FZ, ranging from zero to 0.02 au (the critical field). It is seen that in the absence of the field (FZ = 0 au), FC,XB possesses the highest weight (W = 0.60). But as the field increases to 0.0125 au this weight drops almost twofold, to W = 0.36, and further to W = 0.08 at FZ = 0.02 au. At the same time, at FZ = 0.0125 au, FC,P increase to W = 0.21 and FLB to W = 0.13. Thus at this field, the CT state achieves an almost identical weight to the fundamental VB structure of the XB, FC,XB, and accordingly, the dipole moment of the XB complex increases from 3.9 D (FZ = 0 au) to 10.2 D (FZ = 0.0125 au). At FZ = 0.02, the weight of FC,P increase to W = 0.50, and it becomes the major VB structure; the dipole moment further increase to 19.8 D. FI,Cl+ which contributes to the Lewis bonds, Cl-Cl’ and (N-Cl)+, maintains a roughly constant weight, W = 0.21-0.31. These changes in the weights of the VB structures reflect the dipole moments of the respective structures. Thus, the field interacts, selectively, with the VB structures depending on their individual dipole moments. And even without major geometric changes a sufficiently enough field along the reaction axis will induce interchange of the FC,XB and FC,P VB structures.12,32


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Figure 8: The changes of weights (W) of the four main VB structures, for the XB, H3N----Cl-Cl’, complex as a function of FZ.

The results of Figure 8 highlight the following key findings: the critical field drives the XB state towards the product state, and leads to a spontaneous barrier-free displacement of H3N----Cl-Cl’ to the ion-pair (H3N-Cl)+ Cl’–. Additionally, while at FZ = 0 au, the amount of CT state-mixing into the ground state of XB is rather small, as the field increases e.g., to FZ = 0.0125 au, the combined weights of the CT structures FC,P and FLB reaches W = 0.34, which is only slightly smaller than the corresponding weight of the FC,XB structure (0.36). The FZ direction serves also as a control measure. Thus, as we showed in the results section, flipping of the field’s direction to FZ < 0 inhibits the process and keeps the XB intact. The flipped field, FZ < 0, destabilizes the CT and ionic structures, and will generate a loosely bound XB complex, which is nonreactive. VB analysis of energy profile and the wave function along the reaction coordinate: Let us address now the observation that FZ generates “normal” energy profiles, even when the field is a moderate one, 0.005 au (0.26 V/Å). Figure 9 shows the energy profile, and the evolution of the weights (W) of the VB


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structures, for the target reaction, H3N----Cl-Cl’ ® (H3N-Cl)+ + Cl’–, in fields ranging from zero to 0.0125 au.

µXB = 3.9 D µTS µP = 5.4 D

µXB = 5.6 D µTS = 37.6 D µP = 81.1 D

µXB = 10.9 D µTS = 28.3 D µP = 81.6 D

Figure 9: Energy profiles (kcal/mol), without and with OEEF, along the N---Cl---Cl’ axis (Z), with relative energies of the XB, TS, and P. The reaction coordinate is RCl---Cl’ (Å). All energy values are relative to the halogen-bond (XB) species. All geometries were taken from DFT calculations. Relative energies in brackets refer to BOVB/6-311G(d,p) values, while within parentheses to M06-2X values. (a) FZ = 0 au. (b) FZ = 0.005 au. (c) FZ = 0.0125 au. The P species for each profile, are characterized by the charge on the terminal Cl’, QCl’, whereas the XB and TS species are characterized by the WCT values, which are weights of the charge-transfer VB structures (the sum of the respective weights of FC,P and FLB; see Scheme 2). The boxes underneath the energy profiles provide the dipole moments (µ) of the XB, TS, and P species (in Debye).

In the absence of a field (Figure 9a), the DFT energy profile does not exhibit a TS and the energy rises to the OSS diradical product limit, (H3N: •Cl) + •Cl’, which is a FC,XB having a broken Cl•---•Cl’ bond (RCl---Cl’ ~15Å). The ion-pair limit (H3N-Cl)+ + Cl’–, lies higher in energy (by 72.5 kcal/mol at the BOVB level) and the two electronic wave functions do not mix at the dissociation limit. The XB complex which is trapped in a deep energy well, exhibits a weak CT mixing with a weight (WCT) of 0.06. ACS Paragon Plus Environment

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Interestingly at an intermediate Cl---Cl’ distance (~4-4.5Å), there is a significant CT mixing (WCT = 0.40), which gradually vanishes as the Cl---Cl’ distance further increases. Thus, in the absence of the field, the CT state behaves as a parabola with a deep minimum that approaches the diradical state, at an intermediate distance, as the energy of the latter curve rises. The proximity of the diradical and CT curves may give rise to a barrier-like feature (see SI, Figure S21 for the BOVB curve). However, this is not observed in the M06-2X energy profile, in Figure 9a. Figure 9b shows the impact of a rather small OEEF of 0.005 au. Despite of the fact that the WCT evolution is similar to Figure 9a, here there is a TS with a high barrier (M06-2X value: 65.1 kcal/mol; BOVB value: 52.3 kcal/mol), which leads to an endothermic ion pair product, (H3N-Cl)+ + Cl’–. The CT mixing in the XB complex has a weight WCT = 0.12. However, at the TS WCT = 0.59, and the product configuration FC,P dominates the wave function; as expected from the TS geometry having a short N-Cl (1.79 Å) and a long Cl-Cl’ (6.24 Å). At the reaction coordinate limit, the product (P) is the pair of ions, (H3N-Cl)+ + Cl’– . As the field’s intensity increases to FZ = 0.0125 au, in Figure 9c, the CT mixing into the XB complex increases to WCT = 0.34, while the TS maintains the same wave function as in the weaker field. What change dramatically, at this field’s intensity, is the barrier that drops to ~11-12 kcal/mol (BOVB-DBOVB) 21.8 kcal/mol (M06-2X), and the ion-pair product which is -111.8 (-98.3) kcal/mol exothermic relative to the XB complex. What is the reason for these dramatic changes which occur in shifting from Figure 9b to 9c? In both cases, the CT ion-pair state lies below the radical state. However, the difference reflects the impact of the field on the structure and energy of the ion-pair species.12d Thus, on the one hand, separating the (H3NCl)+ and Cl’– ions lowers their intrinsic electrostatic interaction, thus bringing about an energy rise. But,


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on the other hand, the electric field stabilizes the ion-pair in direct proportion to its dipole moment, thus lowering the energy.12d,30 Since a looser ion pair has also a larger dipole moment (81.6 D in Figure 9c), the OEEF exerts a larger stabilization, which increases as the ions get more and more distant. Furthermore, when the field is strong enough as in Figure 9c, it overrides the intrinsic tendency of the opposite ions to remain close, and stabilizes increasingly more the looser ion-pair geometries. Thereby the strong field bends the energy profile past the TS stage, and converts the energy profile from being endothermic as in Figure 9b to exothermic as in Figure 9c. The OEEF effect is very different than the effect of solvents shown above in Figure 3. Why does the OEEF orient the XB along the reaction axis? As we mentioned in the results section, the OEEF (FZ = 0.0125 au) also restricts the H3N----Cl-Cl’ XB along the reaction axis and exerts a large barrier for rotation off axis (25.3 kcal/mol). As shown in Table S35, the dipole moment of the XB is 10.2 D (note that in BOVB it is 10.9 D) due to mixing of the CT character. As such, the stabilization of the XB due to interaction of its dipole with the co-linear field is strong. In contrast, as the XB rotates by 90°, its dipole moment is now perpendicular to the field, and the interaction energy is zero.12d This significant lowering of the stabilization energy creates a barrier for rotation off-reaction axis, and the field acts as tweezers for fixing the XB in space.33

5.2. A General Mechanism for OEEF Catalysis of Displacement Reactions of Halogen Bonds The behavior of the displacement reaction for NH3 + Cl2 (Figure 9) is an archetype that represents the set of B:-----(X—Y) systems, with the exception of quantitative aspects. The six VB structures which are depicted in Scheme 2 above can be converted to two-state curves, which intersect along the reaction coordinate, mix, avoid the crossing and give rise to the transition state and energy barrier for the


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displacement reaction of the XB.11 The so generate two-states diagram is called a VB state-correlation diagram (VBSCD),11c,32 and is depicted below in Figure 10 which involves the numerical data for the target reaction, while using the generic symbols, B:-----(X—Y) ® (B—X)+ + :Y–, to emphasize the generality of the VB diagram.

B ---(X Y) Z Fz > 0 µz

Figure 10: A VB State-Correlation Diagram (VBSCD) for the reaction B:-----(X—Y) ® (B—X)+ + :Y–, Under a positive OEEF along the reaction axis (FZ > 0). The left inset in the Figure depicts the direction of FZ > 0, and the direction of the dipole (µZ) which will be stabilized by the positive FZ. (a) The state curves in the absence of an OEEF, FZ = 0 au. (b) and (c) State curves for FZ = 0.005 and FZ = 0.0125 au, respectively. The mixing of the state curves generates the full states, and the energy profile is shown in ACS Paragon Plus Environment

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the bold blue curves. The energy values are imported to the general diagram, from the VB study of the reaction H3N:-----(Cl—Cl’) ® (H3N:—Cl)+ + :Cl’–.

Figure 10a depicts the two state curves at FZ = 0 au, without mixing them, hence the states are symbolized as Y0. One state curve, Y0XB, involves a base with its electron pair, B:, and a molecule with a Lewis bond X—Y. This state is a mixture of the covalent structure (B: X•—•Y) between X and Y, and the two ionic structures, B: X: – Y+ and B: X+ :Y–, which are required to generate a Lewis 2e-bond in the molecule X—Y. Along the reaction coordinate this state correlates to the singlet diradical-state species, 1

[(B: •X) + •Y], which has three electrons in the BX linkage and a single electron on Y. The two single

electrons on X and Y are paired to a singlet spin-state, and hence this is simply the covalent structure of the Y0XB state, taken along the reaction coordinate, to the products side. Above the Y0XB state, there is a CT state, in which one electron from B: is transferred to the molecule X—Y (to the s* orbital), to generate Y0CT,XB, which involves the radical ions, B•+ and (XY)• –. Again the single electrons are paired into a singlet spin state. The Y0CT,XB state correlates along the reaction coordinate to YP, which is the ion-pair product, (B-X)+ + :Y–. For the bases used in the present study, the ionization potential IPB: is large (e.g. 239 kcal/mol for B = NH3),34-36 and the X-Y molecules have moderate electron affinities (e.g., EAXY = 54-70 kcal/mol for the dihalides31). Hence, in the absence of the field the energy gap between the two states at the side of the XB complex is high (IPB: - EAXY). While, the energy of Y0CT,XB is lowered along the reaction coordinate due to B-X+ bond making and electrostatic interaction between the charges, it rises quickly in energy as the ions B-X+ and :Y – separate. Therefore, the CT state remains above the Y0XB curve, as shown in Figure 10a.


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Note that in the intermediate region of the reaction coordinate region, the minimum of the Y0CT,XB reaches rather closely to the Y0XB curve. This region (painted in yellow) will result in mixing of the two states, and hence the ground-state wave function will acquire some ionicity, as we found in the calculations (see Figure 9), and may even exhibit a barrier feature. Nevertheless, since at the product end the two-state curves do not cross, the product limit is the diradical state, labelled as (B: •X) •Y (see also Figure S21 the actual curves for H3N:-----(Cl—Cl’) ® (H3N:—Cl)+ + :Cl’). Thus, Figure 10a describes quite well our computational findings. In the absence of an electric field, the displacement reaction is not allowed. Figures 10a and 10b describe what transpires when a positive field, along the reaction axis, is turned on. The field is responsible for two major effects: (a) Firstly, it reduces the IPB: and increases the EAXY, and as such it shrinks the large gap from the ground state curve; and importantly (b) since the field prefers higher dipole moments, the CT state curve gets stabilized increasingly more with the increase in the distance between the ions, (B-X)+ and :Y–. Consequently, the CT state loses its minimum,12d and gets markedly stabilized, so much so that it crosses below the diradical state and enables thereby the displacement reaction, leading to (B-X)+ and :Y–. A moderate field like in Figure 10b will exhibit late crossing along the RX-Y coordinate and a high barrier (6.24 Å and 50.0 kcal/mol for H3N + Cl2, See Figure 9b), while a stronger field, 0.0125 au as in Figure 10c, will have a dramatic effect on the stabilization of the CT and will lead to an earlier crossing (e.g., 4.2 Å for H3N + Cl2), with a very low barrier (~11-12 kcal/mol), and an exothermic reaction. This qualitative description in Figure 10 is quite general, and it provides a framework for conceptualizing the above computational observations (e.g., Figures 2, 3, 5, 6, 8, and 9, as well as Figures S1-S14). Clearly, the electric field will generally enable the anionic displacement reaction for halogen bonds, and their congeners, e.g., Chalcogen bonds,5f etc. ACS Paragon Plus Environment

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4. CONCLUSIONS The above study focuses on an uncommon chemical process nascent from halogen bonds (XB); the anionic displacement reaction, B:---(X—Y) ® (B—X)+ + :Y–, which transpires by nucleophilic attack of the base, B: on the halogen X. In most of the cases examined, these reactions exhibited very high barriers in either the gas phase (where the X-Y bond dissociates actually to radicals) or in solvents such as CH2Cl2 and CH3CN, which lead to endothermic processes. Thus, generally the XB species are trapped in deep minima and their reactions are not allowed under normal chemical conditions, even in some polar solvents. However, when an oriented-external electric field (OEEF) is directed along the reaction axis B---X---Y (with a positive pole in the direction of Y), the process enjoys intense catalysis, by tens of kcal/mol, and the reaction becomes allowed (Figures 9c and 10c). This intense catalysis brought about by the OEEF 37 can be contrasted with the sluggish ability of the intrinsic electric fields of solvents to enable these reactions. The qualitative valence bond (VB) model12, 32 shows that the OEEF affects primarily the chargetransfer state (CTS), which generates the displacement product (Figure 10). Thus, the electric field overcomes the intrinsic electrostatic interactions between the ions, (B—X)+ and :Y–, and converts the CTS from a high lying “parabolic state” to one which gets highly stabilized as the distance between the ions increases (hence leading to an enormous increase of the molecular dipole moment).11,12d,30 With this transformed shape, the CTS crosses the reactants state and gives rise to a highly exothermic displacement reaction, with barriers which can be as low as 12 kcal/mol and even lower (Figure 7 and Table 2). We further show that for each XB, there is a critical OEEF, which leads to spontaneous, and barrier-free ion pair, (B—X)+ + :Y–. This sudden polarization is not limited to halogen bonds, and will apply also to chalcogen bonds, and a variety of donor-acceptor complexes.


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Obviously, the simple displacement reaction may not seem at first glance to be overly important in the landscape of chemical reactions. Nevertheless, the intrinsic importance of the study is the demonstration that OEEF can function as the means to control reactivity of otherwise impossible reactions: (a) the field orients the molecule along its axis and as such it acts as electric tweezers that hold the XB and prepare it for the displacement reaction, and (b) the OEEF is capable of catalyzing an otherwise ‘forbidden’ reaction. The control of orientation and catalysis is an attractive feature, which enzymes achieve by utilizing protein residues which hold the reacting molecule(s) juxtaposed to the enzymatic active species.33b The sudden polarization typical of the reaction studied here may eventually find its uses.

ASSOCIATED CONTENT Supporting Information Tables S1-S48, Figures S1-S21 and Scheme S1-S2 of DFT, VB and MRCI computational results, Cartesian coordinates of the optimized structures, full citation of Gaussian 09.

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected] ORCID Chao Wang: 0000-0001-5664-2350 David Danovich: 0000-0002-8730-5119 Hui Chen: 0000-0003-0483-8786 Sason Shaik: 0000-0001-7643-9421 Notes: The authors declare no competing financial interest. ACS Paragon Plus Environment

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ACKNOWLEDGMENTS This work was supported by the Israel Science Foundation (Grant ISF 520/18) and the China Scholarship Council (CSC). The paper is dedicated to Walter Thiel on the occasion of his 70th birthday.

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(37) Use of OEEF to catalyzing chemical reactions is in principle scalable. See: Lin, Z.; Zeng, X.; Yu, S. Enhancement of Ethanol-Acetic Acid Esterification Under Room Temperature and Non-Catalytic Condition via Pulsed Electric Field Application. Food Bioprocess Technol. 2012, 5, 2637-2645.

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Oriented External Electric Fields - Tweezers and Catalysts for

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