Can a Cl–H···F Hydrogen Bond Replace a Cl···F Halogen Bond

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Can a Cl−H···F Hydrogen Bond Replace a Cl···F Halogen Bond? H2XP:ClY:ZH versus H2XP:ClY:HZ for Y, Z = F, Cl Published as part of The Journal of Physical Chemistry virtual special issue “Leo Radom Festschrift”. Janet E. Del Bene,*,‡ Ibon Alkorta,*,§ and Jose ́ Elguero§ ‡

Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States Instituto de Química Médica (CSIC), Juan de la Cierva, 3, Madrid E-28006, Spain

§

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S Supporting Information *

ABSTRACT: Ab initio MP2/aug’-cc-pVTZ (where MP2 is Møller−Plesset perturbation theory) calculations have been carried out on four series of complexes, H2XP:ClF:HCl, H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP:ClCl:HCl, to answer the question raised in the title of this paper. When X is F or Cl, binary complexes containing a P(V) molecule hydrogen bonded to an acid are found on all potential surfaces except H2ClP:ClF:HF, where an ion−pair complex exists. Ion−pair complexes also result from the optimization of H2XP:ClF:HF for X = NC, CN, and H. Changing the central molecule from ClF to ClCl has a dramatic effect on the nature of the optimized complexes when the substituents are NC, CN, and H. On the potential surfaces H2XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H3P:ClCl:FH leads to an ion pair. For H2(NC)P:ClCl:HCl and H2(CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of H3P:ClCl:HCl leads to a reaction in which H2ClP and a second HCl molecule are formed, and the resulting cyclic ternary complex is stabilized by two hydrogen bonds and a pnicogen bond. Thus, the type of complex resulting from the optimization of the starting ternary complex H2XP:ClY:HZ depends on the nature of the central molecule ClF or ClCl, the terminal molecule HCl or HF, and the substituent X. It is not possible to simply turn around the terminal HZ molecule in complexes H2XP:ClF:ZH for Z = F and Cl to give H2XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H2XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H2XP:ClCl:ZH are not.

1. INTRODUCTION

valence III, P(III), and molecules with the phosphorus with valence V, P(V), and the transition states, which connect these minima, were found on all potential surfaces. We then added the acid HCl to the binary complexes H2XP:ClF and turned the HCl molecule around relative to H2XP:ClF:ClH, so as to replace a halogen bond by a hydrogen bond in H2XP:ClF:HCl. The optimized complexes found on the potential surfaces were most interesting and unanticipated, to say the least. To gain further insight into the question posed in the title of this paper, we have varied the nature of the central and terminal molecules in the ternary complexes and determined the structures and

1

At a conference on Halogen Bonding, we presented a paper on ternary complexes H2XP:ClF:ClF and H2XP:ClF:ClH for a series of substituents X.2 During the discussion of this paper, we were asked if we had examined related complexes in which the terminal HCl molecule had been turned around, thereby giving H2XP:ClF:HCl and replacing a halogen bond3−5 by a hydrogen bond.6−8 At that time, we had not examined such complexes, but later discussions implied that a Cl−H···F hydrogen bond would probably replace the F···Cl halogen bond. We decided to explicitly address this question by first examining the potential surfaces of the binary complexes H2XP:ClF and H2XP:ClCl, for X = F, Cl, NC, CN, and H.9 Stable binary complexes with the phosphorus atoms with © XXXX American Chemical Society

Received: January 18, 2019 Revised: April 18, 2019

A

DOI: 10.1021/acs.jpca.9b00553 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A Scheme 1. Reaction Path for the Formation of Ternary P(III) and Binary Hydrogen-Bonded P(V) Complexes

pair to the Cl atom to form a P···Cl halogen bond. The F atom of the ClF molecule donates an electron pair to HCl to form a Cl−H···F hydrogen bond. In the transition state, the Cl−F bond elongates and begins to break, leaving a covalent P−Cl bond in the cation (H2XPCl)+ and an anion −(F···H···Cl). This anion migrates and donates a pair of electrons from Cl to P to form an axial P−Cl covalent bond. This same Cl is hydrogen bonded to the HF molecule in the P(V) complex H2XPClCl:HF. 3.1. H2XP:ClF:HCl Complexes. We initiated the present study in order to answer the question: Can a hydrogen bond replace a halogen bond in the ternary complexes H2XP:ClF:ClH simply by turning the terminal ClH molecule around? Even though a reasonable starting structure for the ternary P(III) complexes H2XP:ClF:HCl was employed, the optimization procedure led spontaneously, that is, without a barrier, to the hydrogen-bonded P(V) binary complexes H2XPClCl:HF, except for the complex with X = H. In order to follow this reaction, we have employed the following numbering system: H2XP1:Cl5F6:H7Cl8. The resulting binary complexes are composed of a P(V) molecule H2XPClCl with Cl5 bonded to P1 in an equatorial position and Cl8 bonded to P1 in an axial position. Cl8 also forms the F6−H7···Cl8 hydrogen bond. Figure 1a illustrates the structure of the

binding energies of three other series of complexes, namely, H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP:ClCl:HCl. It is the purpose of this paper to present the results of this study, to identify what factors influence the nature of the complexes found on these potential surfaces as a result of optimization, to determine whether or not turning the terminal HZ molecule around leads to ternary complexes H2XP:ClY:HZ that are minima on these surfaces, and to examine the structures and binding energies of the resulting P(III) and P(V) complexes.

2. METHODS Optimization of the complexes H2XP:ClY:HZ for X = F, Cl, NC, CN, and H and Y, Z = Cl, F was carried out at secondorder Møller−Plesset perturbation theory (MP2) with the aug’-cc-pVTZ basis set.10−13 This basis set was derived from the Dunning aug-cc-pVTZ basis set14,15 by removing diffuse functions from H atoms. The starting geometry for each optimization was the optimized binary complex H2XP:ClY to which HZ was added to form a ternary complex with a Z−H··· Y hydrogen bond. The binding energies (−ΔE) of the optimized complexes were computed as the negative reaction energy for the formation of the complex from the isolated monomers. Frequency calculations were carried out to confirm that the optimized complexes have no imaginary frequencies, indicating that they are stable structures on the potential surfaces. Optimization and frequency calculations were carried out using the Gaussian-16 program.16 The Natural Bond Orbital (NBO) method17 has been used to obtain the stabilizing charge-transfer interactions in complexes using the NBO-6 program.18 Since MP2 orbitals are nonexistent, charge-transfer interactions have been computed using the B3LYP functional with the aug’-ccpVTZ basis set at the MP2/aug’-cc-pVTZ complex geometries. This allows for the inclusion of at least some electron correlation effects. 3. RESULTS AND DISCUSSION In this study, the structures of the ternary complexes H2 XP:ClY:HZ have been optimized starting with the optimized H2XP:ClY binary complex and placing HZ in a position to form a Z−H···Y hydrogen bond. In some cases, the optimization spontaneously evolved to a hydrogen-bonded P(V) complex. In others, optimization led to an ion−pair or a stable P(III) ternary complex. In the latter cases, we did not search further for the hydrogen-bonded P(V) complex. Our emphasis is on the nature of the complex resulting from the initial optimization procedure. A plausible reaction path illustrating the formation of a P(III) ternary complex H2XP:ClY:HZ and then a hydrogenbonded P(V) binary complex is given in Scheme 1 for H2XP:ClF:HCl. In the ternary complex, P donates an electron

Figure 1. (a) P(V) complex H2(NC)PClCl:HF and (b) the P(III) ion−pair (H3PCl)+:−(Cl:HF).

H2(CN)PClCl:HF complex. Table S1 contains the structures, total energies, and molecular graphs of the equilibrium complexes, all of which have C1 symmetry. Table 1 presents selected bond distances and angles for the H2XPClCl:HF binary complexes. It is important to emphasize that the P(V) complexes form without a barrier, since no ternary P(III) complexes were found on these surfaces. When X = F, Cl, NC, and CN, the P(V) molecules have a distorted trigonal bipyramidal geometry around P and are hydrogen bonded through Cl8 by an F6−H7···Cl8 hydrogen bond, as illustrated for H2(NC)PClCl:HF in Figure 1a. The data of Table 1 show that the equatorial P−Cl5 distances are B


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The Journal of Physical Chemistry A

Table 1. P−Cl, F−Cl, and F−H Distances (R, Å), Binding Energies (−ΔE1 and −ΔE2), and Charge-Transfer Energies (Cl8lp → σ*H7−F6, kJ·mol−1) for Optimized Complexes Derived from H2XP:ClF:HCl X F Cl NC CN H

R(P1−Cl5)a

R(P1−Cl8)a

2.000 1.998 1.998 2.006 2.013

2.220 2.233 2.209 2.244 2.368

(2.010) (2.006) (2.007) (2.020) (2.043)

(2.160) (2.159) (2.148) (2.171)

R(F6−Cl8)

R(F6−H7)b

−ΔE1c

−ΔE2d

Cl8lp → σ*H7−F6

3.030 3.024 3.039 3.021 2.935

0.935 0.935 0.933 0.935 0.944

395.2 347.1 340.4 298.0 301.0

24.3 24.2 22.5 24.9 35.8e/491.5f

42.3 41.7 32.8 40.2 71.8/17.7g

a

P−Cleq and P−Clax distances in the isolated P(V) molecules are given in parentheses. bThe F−H distance in the F−H monomer is 0.922 Å. Binding energy of the complex relative to the isolated monomers H2XP, ClF, and HCl. dBinding energy of the complexes relative to the P(V) and HF molecules. This is the hydrogen bond energy. eThe F6−H7···Cl8 hydrogen bond energy. fBinding energy relative to the ions (H3PCl)+ and − (Cl:HF). gThe Cl8lp → σ*P1−H2 charge-transfer energy. c

Table 2. P−Cl, Cl−F, F−F, and F−H Distances (Å), Binding Energies (−ΔE1 and − ΔE2, kJ·mol−1), Charge-Transfer Energies (CT, kJ·mol−1), and the Nature of Charge Transfer for Complexes Derived from H2XP:ClF:HF −ΔE2

F8lp → σ*H7−F6

F

2.005

1.682

2.719

0.930

433.0

21.7d

Cl NC CN H

1.966 1.978 2.032 1.998

2.104 2.056 1.986 2.085

2.387 2.409 2.454 2.397

0.986 0.976 0.963 0.981

131.0 104.5 85.6 120.8

389.0f 408.3f 418.9f 385.3f

17.9 F6lp → σ*Cl5−P1e 126.2, 15.9 150.4, 20.0 211.3, 28.6 145.1, 17.8

X

R(P1−Cl5)

R(Cl5−F6) b

R(F6−F8)

R(F8−H7) c

−ΔE1a

F6lp → σ*H7−F8 215.9 186.2 142.9 204.1

a

Relative to the isolated monomers. bCl5 and F6 are not bonded. This is the P1−F8 distance. cThis is the F6−H7 distance. dRelative to the P(V) and FH molecules. This is the hydrogen bond energy. eTwo lone pairs of F6 donate electrons to σ*Cl5−P1. fRelative to (H2XPCl)+ and −(F···H− F).

(H3PCl)+:−(Cl:HF) ion−pair, the hydrogen bond energy is 36 kJ·mol−1. Thus, both structural and energetic data indicate that the F−H···Cl hydrogen bond in this complex is significantly stronger than the hydrogen bonds in the P(V) binary complexes. The binding energy of this complex relative to the ions (H3PCl)+ and −(Cl:HF) is 491 kJ·mol−1. The charge-transfer energies Cl8lp → σ*H7−F6 across the hydrogen bonds in the P(V) complexes are also reported in Table 1. These energies vary from 33 to 42 kJ·mol−1. There is a dramatic increase in the charge-transfer energy to 72 kJ·mol−1 in the ion−pair complex that has a short Cl8−F6 distance of 2.935 Å. In this ion−pair, there is also a second charge-transfer interaction Cl8lp → σ*P1−H2 of 18 kJ·mol−1. Thus, the charge-transfer interactions are consistent with the nature of the bonds in these complexes. 3.2. H2XP:ClF:HF Complexes. Do P(V) binary complexes form spontaneously on other related surfaces? If not, what factors influence the existence of P(III) complexes? To answer these questions, we have examined three related series of complexes: H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP:ClCl:HCl. We begin by keeping the central ClF molecule and replacing HCl by HF in H2XP:ClF:HF complexes. Table S2 provides the geometries, total energies, and molecular graphs of these complexes, and Table 2 provides values of selected distances and binding energies. There are two types of complexes that result from the optimization of H2XP:ClF:HF. When X = F, a hydrogen-bonded P(V) complex H2XPClF:HF with an F6−H7···F8 hydrogen bond forms without a barrier from the starting structure, while on the remaining surfaces, P(III) ion−pair complexes (H2XPCl5)+:−(F6···H7−F8) are found. These complexes are illustrated in Figure 2. This does not imply that the P(V) complexes do not exist on these surfaces but that the P(III) ion−pairs are stable, with a significant enough barrier between

significantly shorter than the P−Cl8 axial distances, a wellknown relationship for such P(V) molecules. An electron diffraction study of the PCl5 molecule reported P−Clax distances of 2.19 Å and P−Cleq distances of 2.04 Å.19 Moreover, the P−Cl5 distances in the complexes are also shorter than the corresponding P−Cl5 distances in the isolated P(V) molecules, while the P−Cl8 distances are longer in the complexes. The lengthening of the P−Cl8 distance is due in part to the presence of the distorted F6−H7···Cl8 hydrogen bond. The hydrogen bond F6−Cl8 distances lie between 3.02 and 3.04 Å. The F6−H7 distances are nearly constant with values of 0.933 or 0.935 Å. The F6−H7···Cl8 hydrogen bonds are nonlinear, with values of the H7−F6−Cl8 angle between 19° and 24°. The fifth complex in this series is a binary ion−pair (H3PCl5)+:−(Cl8:H7−F6), which is illustrated in Figure 1b. In this complex, Cl8 of the anion is an electron-pair donor to the cation at P1 and to F6−H7 across the F6−H7···Cl8 hydrogen bond. The basis for classifying this complex as an ion−pair is the P−Cl8 distance, which is longer than this distance in any of the P(V) complexes, the short F6−Cl8 distance of 2.93 Å compared to distances of 3.03 Å, and the long F6−H7 distance of 0.944 Å compared to 0.933 and 0.935 Å for the P(V) complexes. These data suggest that the F6−H7···Cl8 hydrogen bond is strongest in this complex. The binding energies (−ΔE1) of the H2XPClCl:HF P(V) and (H3PCl)+:−(Cl:HF) P(III) complexes relative to the isolated monomers H2XP, ClF, and HCl are given in Table 1. These binding energies are very large, varying between 298 and 395 kJ·mol−1, and decrease in the order F > Cl ≈ NC > H ≈ CN. In the P(V) complexes, they reflect the formation of 2 new covalent P−Cl bonds. These same complexes are further stabilized by distorted F6−H7···Cl8 hydrogen bonds. The hydrogen bond energies for the P(V) complexes, given in Table 1 as − ΔE2, lie between 23 and 25 kJ·mol−1. For the C


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unsaturated substituents NC and CN are more effective in stabilizing the cation. Table 2 presents the stabilizing charge-transfer energies of these complexes. In the P(V) complex H2FPClF:HF, charge transfer occurs across the hydrogen bond from an F8 lone pair to a σ antibonding H7−F6 orbital, as expected. In contrast, the ion−pairs present three charge-transfer interactions. From Table 2, it can be seen that there are two charge-transfer interactions involving F6 and Cl5−P1. These two arise because there are two lone pairs on F6 of the anion. The larger F6lp → σ*Cl5−P1 charge-transfer energies vary from 126 to 211 kJ· mol−1 and decrease in the order CN > NC ≈ H > Cl. The second F6lp → σ*Cl5−P1 charge-transfer interaction is much weaker, with energies ranging from 16 to 29 kJ·mol−1. This is the first time that we have observed two charge-transfer interactions across a single hydrogen bond involving two lone pairs of electrons on the electron-donor atom. The third charge-transfer interaction occurs within the anion as a lone pair on F6 donates a pair of electrons across the hydrogen bond to H7−F8. These charge-transfer energies are large, ranging from 143 to 216 kJ·mol−1, and decrease in the order Cl > H > NC > CN. At this point, it should be recognized that optimization of H2XP:ClF:HCl and H2XP:ClF:HF complexes does not produce ternary complexes with Cl−H···F or F−H···F hydrogen bonds. Thus, it is not possible to simply turn around the terminal HCl or HF molecule in H2XP:ClF:ClH and H2XP:ClF:FH complexes and replace a halogen bond by a hydrogen bond. 3.3. H 2 XP:ClCl:HF Complexes. In the complexes H2XP:ClCl:HF, the central ClF molecule in the complexes of Section 3.1 has been replaced by ClCl and the terminal HCl by HF. ClCl is a weaker electron-pair donor and acceptor than ClF, and HF is a weaker acid than HCl. To what extent does this double substitution determine the nature of the optimized complexes? The structures, total energies, and molecular graphs of these complexes are reported in Table S3. Three different types of complexes, all with Cs symmetry, have been found on the potential surfaces. The first contains a P(V) molecule hydrogen bonded to ClH and forms when the substituents are the strong electron-withdrawing groups F and Cl. The structure of the P(V) complex H2FPClF:HCl is illustrated in Figure 3a, and structural parameters are given in Table 3. These two complexes have P1−F8 distances of 1.67 Å and P1−Cl5 distances of 2.01 Å. The Cl6−F8 distances across the Cl6−H7···F8 hydrogen bonds in the H2FPClF:HCl and H2ClPClF:HCl complexes are significantly longer than the corresponding distances across F6−H7···Cl8 hydrogen bonds in H2FPClCl:HF and H2ClPClCl:HF.

Figure 2. (a) Hydrogen-bonded P(V) complex H2FPClF:HF and (b) the ion−pair [H2(CN)PCl]+: −(F···HF).

the P(III) and P(V) complexes to prevent the direct formation of the latter. These barriers will not be discussed in this paper, but only the properties of the P(III) and P(V) complexes that result when the starting ternary complexes are optimized. Table 2 presents values of selected inter- and intramolecular bond distances for complexes derived from H2XP:ClF:HF. The only hydrogen-bonded P(V) complex in this set is H2FPClF:HF, which has C1 symmetry and is illustrated in Figure 2a. The P1−Cl5 and F6−H7 distances are similar to those found in the P(V) complex H2FPClCl:HF but with a nearly linear F6−H7···F8 hydrogen bond. The binding energy of 433 kJ·mol−1 is 38 kJ·mol−1 greater than that of H2FPClCl:HF. The F6−H7···F8 hydrogen bond energy is 22 kJ·mol−1. The remaining complexes in this set are P(III) ion−pairs (H2XPCl)+:−(F···H−F), which have Cs symmetry and are illustrated by [H2(CN)PCl)]+:−(F···H−F) in Figure 2b. The P1−Cl5 covalent bond distances vary between 1.97 and 2.03 Å. The Cl5−F6 distances are between 1.99 and 2.10 Å and decrease in the order Cl > H > NC > CN. In the anions, the hydrogen bonds are nearly linear, with values of the H7−F8− F6 angle between 2° and 7°. The F6−F8 distance across the hydrogen bond decreases in the order CN > NC > H > Cl. The F8−H7 distance increases as the F6−F8 distance decreases, indicating that the strength of the hydrogen bond is increasing. The binding energies of the ion pairs relative to the isolated monomers vary from 86 to 131 kJ·mol−1 and decrease in the order Cl > H > NC > CN, which is the order of decreasing Cl5−F6 distance. Relative to the isolated ion pairs, the binding energies vary from 385 to 419 kJ·mol−1 and decrease in a different order CN > NC > Cl ≈ H. Thus,

Figure 3. Complexes derived from the optimization of H2XP:ClCl:HF: (a) H2FPFCl:HCl; (b) H2(CN)P:ClCl:HF; (c) (H3PCl)+:−(Cl···HF). D


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Table 3. P−F, P−Cl, Cl−F, Cl−Cl, Cl−H, and F−H Distances (Å), Binding Energies (−ΔE1 and − ΔE2), and Charge-Transfer Energies (CT, kJ·mol−1) for Complexes Derived from H2XP:ClCl:HF X

R(P1−Cl5)

R(Cl6−F8)

R(P1−F8)

R(Cl6−H7)

−ΔE1a

−ΔE2

F8lp → σ*H7−Cl8

F Cl

2.009 2.007

3.274 3.283

15.4b 14.5b

2.966 3.039

3.247 3.258

1.281 1.281 R(F8−H7) 0.927 0.927

292.4 242.1

NC CN

1.669 1.668 R(Cl5−Cl6) 2.031 2.023

H

2.076

2.975

2.440

0.948

41.0

18.8 17.7 Plp → σ*Cl5−Cl6 36.0 27.9 Cl6lp → σ*Cl5−P 226.8

a

22.5 21.3 366.0c

Cl6lp → σ*H7−F8 22.0 21.0 Cl6lp → σ*H7−F8 78.3

b

Relative to the isolated monomers. Relative to the isolated P(V) and Cl−H molecules. This is the hydrogen bond energy. cRelative to (H3PCl)+ and −(Cl···H−F).

Figure 4. Complexes derived from the optimization of H2XP:ClCl:HCl: (a) H2FPClCl:HCl; (b) H2(CN)P:ClCl:HCl; (c) H2ClP:ClH:ClH.

Table 4. P−Cl, Cl−Cl, and Cl−H Distances (R, Å) and Binding Energies (−ΔE1 and − ΔE2, kJ·mol−1) for Complexes Derived from H2XP:ClCl:HCl

a

X

R(P1−Cl5)

R(P1−Cl8)

R(Cl6−Cl8)

R(Cl6−H7)

−ΔE1a

−ΔE2

F Cl

2.003 2.000

2.203 2.211

20.1b 20.5b

3.068 3.121

3.283 3.418

H

2.060

3.719

1.287 1.287 R(Cl8−H7) 1.281 1.281 R(Cl6−Cl8) 3.673

256.6 209.0

NC CN

3.526 3.525 R(Cl5−Cl8) 3.675 3.702 R(P1−Cl6) 3.647

30.5 27.5 R(Cl−H) 1.282c/1.292d

213.2

29.3e

b

Relative to the isolated monomers H2XP, ClCl, and HCl. Relative to the isolated P(V) and Cl−H molecules. This is the hydrogen bond energy. R(Cl6−H7). dR(Cl8−H3). eRelative to the isolated monomers H2ClP and two HCl molecules.

c

complexes are about 22 kJ·mol−1. The charge-transfer energies P1lp → σ*Cl5−Cl6 across the halogen bonds are 36 and 28 kJ· mol−1 for the complexes with X = NC and CN, respectively. The charge-transfer energies Cl6lp → σ*H7−F8 across the hydrogen bonds are about 22 kJ·mol−1. The existence of hydrogen bonded complexes H 2 (NC)P:ClCl:HF and H2(CN)P:ClCl:HF might be taken to imply that it is possible to turn the HF molecule around in the complexes H2(NC)P:ClCl:FH and H2(CN)P:ClCl:FH and thereby replace a halogen bond by a hydrogen bond. However, we have searched the H2(NC)P:ClCl:HF and H2(CN)P:ClCl:HF surfaces and found that the corresponding halogen-bonded complexes are not equilibrium structures on these potential surfaces. The final complex in this set results from the optimization of H3P:ClCl:FH and is the ion−pair complex (H3PCl)+:−(Cl··· H−F) illustrated in Figure 3c. In this complex, Cl6 is a double electron-pair donor, first to the σ antibonding P−Cl5 orbital and then across the hydrogen bond to the σ*H7−F8 orbital. The binding energy of this complex relative to the isolated

Table 3 also reports the total binding energies and the hydrogen bond energies of the P(V) complexes H2FPFCl:HCl and H2ClPFCl:HCl. The total binding energies are 100 kJ· mol −1 less than the corresponding P(V) complexes H2FPClCl:HF and H2ClPClCl:HF. The Cl6−H7···F8 hydrogen bonds are nearly linear, with hydrogen bond energies of about 15 kJ·mol−1. These are significantly weaker than the hydrogen bonds in complexes with F6−H7···Cl8 hydrogen bonds. The charge-transfer interactions in these two complexes are 18.8 and 17.7 kJ·mol−1 and arise as F8 donates a pair of electrons across the hydrogen bond to a σ antibonding H7− Cl6 orbital. The complexes H2(NC)P:ClCl:HF and H2(CN)P:ClCl:HF are open ternary complexes stabilized by a P1···Cl5 halogen bond and an F8−H7···Cl6 hydrogen bond as illustrated in Figure 3b for H2(CN)P:ClCl:HF. The longer P1−Cl5 distances are now inter- rather than intramolecular distances. The Cl6−F8 distance and the F7−H8 distances are typical of traditional hydrogen bonds. The binding energies of these E


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The Journal of Physical Chemistry A Table 5. Charge-Transfer Energies (CT, kJ·mol−1) for Complexes Derived from H2XP:ClCl:HCl X

Cl8lp → σ*H7−Cl6

F Cl

31.5 28.2 P1lp → σ*Cl5−Cl6 26.0 21.5 P1lp → σ*H3−Cl8 40.2

NC CN H

Cl5lp → σ*H7−Cl8 13.4 14.5 Cl8lp → σ*H7−Cl6 20.6

monomers is 41 kJ mol−1 and relative to the isolated cation and ion is 366 kJ·mol−1. The charge-transfer interactions are consistent with the ion−pair nature of this complex as Cl6 acts as a double electron-pair donor. The charge-transfer interactions are Cl6lp → σ*Cl5−P1 and Cl6lp → σ*H7−Cl8 with charge-transfer energies of 227 and 78 kJ·mol−1, respectively. 3.4. H 2 XP:ClCl:HCl Complexes. The final set of complexes are those resulting from the optimization of H2XP:ClCl:HCl. These also have ClCl as the central molecule but the stronger acid HCl as the terminal molecule. The structures, total energies, and molecular graphs of the optimized complexes are reported in Table S4. Structures of the complexes with X = F, CN, and H are illustrated in Figure 4. Complexes with X = F, Cl, and H have C1 symmetry; those with X = NC and CN have Cs symmetry. The structures and binding energies of these complexes are reported in Table 4. The complexes with X = F and Cl are binary complexes containing P(V) molecules hydrogen bonded to Cl6−H7 through a Cl6−H7···Cl8 hydrogen bond. The P1−Cl5 distances are 2.00 Å. Once again, the equatorial P1−Cl5 distance is shorter than the axial P1−Cl8 distance by about 0.20 Å. The Cl6−Cl8 distances across the hydrogen bonds are 3.53 Å, while the Cl6−H7 distances are 1.29 Å. These distances are indicative of traditional hydrogen bonds. The hydrogen bonds are nonlinear with H7−Cl6−Cl8 angles of about 18°. The binding energies of the complexes H2FPClCl:HCl and H2ClPClCl:HCl relative to the isolated monomers are 257 and 209 kJ·mol−1, respectively. These energies are smaller than the binding energies of the complexes H2FPFCl:HCl and H2ClPFCl:HCl, which are 292 and 242 kJ·mol−1, respectively. They are significantly less than those of the complexes H2FPClCl:HF and H2ClPClCl:HF, which are 395 and 347 kJ·mol −1 , respectively. However, it is the complex H2FPClF:HF that has the greatest binding energy of 444 kJ· mol−1. The Cl6−H7···Cl8 hydrogen bond energies of H2FPClCl:HCl and H2ClPClCl:HCl are 20 kJ·mol−1. The charge-transfer energies of these complexes are reported in Table 5. Charge transfer occurs from a lone pair of Cl8 to the σ*H7−Cl6 orbital with energies of 32 and 28 kJ·mol−1 for X = F and Cl, respectively. The complexes obtained from the optimization of H2(NC)P:ClCl:HCl and H2(CN)P:ClCl:HCl are ternary P(III) complexes with cyclic structures, as illustrated in Figure 4b for H2(CN)P:ClCl:HCl. The P1−Cl5, P1−Cl8, and Cl5−Cl8 distances are 3.068, 3.283, and 3.675 Å, respectively, in the complex with X = NC and 3.121, 3.418, and 3.702 Å, respectively, in the complex with CN. The Cl8−H7 distance is 1.281 Å. The binding energies of these complexes relative to the isolated monomers are 31 and 28 kJ·mol−1 for X = NC and CN, respectively. Once again, although we have found stable

Cl8lp → σ*P1−A2 15.5 9.7 Cl6lp → σ*P1−H2 4.4

hydrogen-bonded complexes on these surfaces, the corresponding halogen-bonded complexes H2(NC)P:ClCl:ClH and H2(CN)P:ClCl:ClH are not stable structures on the same surfaces. The charge-transfer energies of these complexes are reported in Table 5. There are three different charge-transfer interactions. The Plp → σ*Cl5−Cl6 charge-transfer energy is 26 kJ mol−1 when X is NC and 22 kJ·mol−1 when X is CN. The direction of charge transfer indicates that the P1···Cl5 bond is a halogen bond. Charge transfer occurs across the Cl8−H7··· Cl5 hydrogen bond from a lone pair on Cl5 to the σ*H7−Cl8 orbital with energies of around 14 kJ·mol−1 for X = NC and CN. The final charge transfer is across the pnicogen bond20−22 from a lone pair on Cl8 to the σ*P1−A orbital, with A the X atom directly bonded to P, and has energies of 16 and 10 kJ· mol−1, respectively. Charge transfer occurs in a favorable headto-tail clockwise direction around the three-member ring defined by the atoms P1, Cl5, and Cl8 in Figure 4b. These rings are stabilized by three different intermolecular bonds: a halogen bond, a hydrogen bond, and a pnicogen bond. The final complex in this series is H2ClP:HCl:HCl, which is also illustrated in Figure 4c. During the optimization of this complex, both the Cl5−Cl6 and P1−H3 bonds break and two new covalent P1−Cl5 and Cl8−H3 bonds form. In the resulting cyclic ternary complex, Cl8−H3 is hydrogen bonded to P1 and H7 moves across the hydrogen bond from Cl8 to form a Cl6−H7 covalent bond, which is hydrogen bonded to Cl8. The intermolecular P1−Cl6 and P1−Cl8 distances of 3.65 and 3.72 Å, respectively, are consistent with intermolecular P− Cl pnicogen bond and Pl−Cl hydrogen bond distances. The third intermolecular bond in this complex is the Cl6−H7···Cl8 hydrogen bond, which has a Cl−Cl distance of 3.67 Å. The binding energy of this complex relative to the original starting molecules PH3, ClCl, and HCl is 213 kJ·mol−1, an unrealistic value for a neutral hydrogen-bonded ternary complex. However, the binding energy relative to the monomers which are present in the optimized complex, namely, PH2Cl and two HCl molecules, is much more realistic at 29 kJ·mol−1. Once again, the direction of the charge transfers that occur in this complex is consistent with the nature of the intermolecular bonds. In Table 5, the first charge transfer across a hydrogen bond involves the lone pair on P1, which is donated to the σ*H3−Cl8 orbital, with a charge-transfer energy of 40 kJ·mol−1. The second charge transfer also occurs across a hydrogen bond, from a lone pair of electrons on Cl8 to the antibonding σ H7−Cl6 orbital, with an energy of 21 kJ· mol−1. The third charge transfer occurs across the pnicogen bond from a lone pair on Cl6 to the antibonding σ P1−H2 orbital, with a charge-transfer energy of 4 kJ·mol−1. Each nonhydrogen atom that is the electron-donor for one interaction is the electron-pair acceptor for another. These charge-transfer interactions also occur around the ring defined F



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by P1, Cl6, and Cl8, this time in a head-to-tail counterclockwise direction.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 330-609-5593 (J.E.D.B.). *E-mail: [email protected]. Phone: +34 915622900 (I.A.).

4. CONCLUSIONS Complexes H2XP:ClF:HCl, H2XP:ClF:HF, H2XP:ClCl:HF, and H2XP:ClCl:HCl for X = F, Cl, NC, CN, and H have been optimized at MP2/aug’-cc-pVTZ. This study was carried out in order to answer the question: “Can a hydrogen bond replace a halogen bond in H 2 XP:ClF:ClH, H 2 XP:ClF:FH, H2XP:ClCl:FH, and H2XP:ClCl:FH complexes by simply turning the terminal HCl or HF molecule around?” The following statements are based on the structures of the complexes resulting from the optimization procedure. 1. With ClF as the central molecule, only binary hydrogenbonded P(V) and ion−pair complexes are found on the potential surfaces. 2. With ClCl as the central molecule, different types of complexes are found on the potential surfaces, depending on the nature of the terminal acid and the nature of the substituent X. a. For X = F or Cl, hydrogen-bonded P(V) binary complexes result from the optimization procedure, independent of the nature of the terminal acid. b. For X = CN or NC, open ternary complexes stabilized by a hydrogen bond and a halogen bond result when the terminal acid is HF. When the terminal acid is HCl, cyclic ternary complexes stabilized by a halogen bond, a hydrogen bond, and a pnicogen bond are found. c. For X = H, the optimization procedure yields an ion−pair complex when the acid is HF. When the acid is HCl, a reaction occurs in which one of the H atoms of PH3 is replaced by a Cl atom giving PH2Cl and HCl. The resulting ternary complex H2ClP:HCl:HCl is cyclic and stabilized by two hydrogen bonds and a pnicogen bond. 3. Optimization of H2XP:ClY:HZ for X = F and Cl leads to hydrogen-bonded P(V) binary complexes PH2XClZ:HY with only one exception. In the optimized P(V) complex, the equatorial Cl atom comes from ClY, the axial Z atom comes from the terminal acid HZ, and the Z atom is hydrogen bonded to YH. 4. It is not possible to replace a terminal halogen bond by a hydrogen bond by simply turning around the HCl or HF molecule in complexes H 2 XP:ClF:ClH and H2XP:ClF:FH, since H2XP:ClF:HCl and H2XP:ClF:HF are not stable ternary complexes on the potential surfaces. Complexes H2XP:ClCl:HCl and H2XP:ClCl:HF with X = NC and CN are stable ternary complexes with hydrogen bonds involving the terminal acid. However, the corresponding pnicogen-bonded complexes are not stable structures on the H2XP:ClCl:ClH and H2XP:ClCl:FH potential surfaces.

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Article

ORCID

Janet E. Del Bene: 0000-0002-9037-2822 Ibon Alkorta: 0000-0001-6876-6211 José Elguero: 0000-0002-9213-6858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the Ministerio de Ciencia, Innovación y universidades (PGC2018094644-B-C22) and Comunidad Autónoma de Madrid (P2018/EMT-4329 AIRTEC-CM). Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b00553. Structures, total energies, and molecular graphs of the complexes resulting from the optimization of complexes H2XP:ClF:ClH, H2XP:ClF:FH, H2XP:ClCl:HF, and H2XP:ClCl:HCl (PDF) G


Article

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