Halogen-bonded Complexes Research Articles

Relativistic Effects on NMR Parameters of Halogen-Bonded Complexes.

Relativistic effects are found to be important for the estimation of NMR parameters in halogen-bonded complexes, mainly when they involve the heavier elements, iodine and astatine. A detailed study of 60 binary complexes formed between dihalogen molecules (XY with X, Y = F, Cl, Br, I and At) and four Lewis bases (NH3, H2O, PH3 and SH2) was carried out at the MP2/aug-cc-pVTZ/aug-cc-pVTZ-PP computational level to show the extent of these effects. The NMR parameters (shielding and nuclear quadrupolar coupling constants) were computed using the relativistic Hamiltonian ZORA and compared to the values obtained with a non-relativistic Hamiltonian. The results show a mixture of the importance of the relativistic corrections as both the size of the halogen atom and the proximity of this atom to the basic site of the Lewis base increase.

Halogen-Bond Assisted Photoinduced Electron Transfer.

The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptor–donor–acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett. 2017, 8, 3927–3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S1 state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowski–Collins–Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond.

Isolation of a Halogen-Bonded Complex Formed between Methane and Chlorine Monofluoride and Characterisation by Rotational Spectroscopy and Ab Initio Calculations.

A halogen-bonded complex formed between methane and chlorine monofluoride has been isolated in the gas phase before the reaction between the components and has been characterised through its rotational spectrum, which is of the symmetric-top type but only exhibits K = 0 type transitions at the low effective temperature of the pulsed-jet experiment. Spectroscopic constants for two low-lying states that result from internal rotation of the CH4 subunit were detected for each of the two isotopic varieties H4C···35ClF and H4C···37ClF and were analysed to show that ClF lies on the symmetry axis with Cl located closer than F to the C atom, at the distance r0(C···Cl) Å and with an intermolecular stretching force constant kσ 4 N m−1. Ab initio calculations at the explicitly correlated level CCSD(T)(F12c)/cc-pVTZ-F12 show that in the equilibrium geometry, the ClF molecule lies along a C3 axis of CH4 and Cl is involved in a halogen bond. The Cl atom points at the nucleophilic region identified on the C3 axis, opposite the unique C–H bond and somewhere near the C atom and the tetrahedron face centre, with re(C···Cl) = 3.191 Å. Atoms-in-molecules (AIM) theory shows a bond critical point between Cl and C, confirming the presence of a halogen bond. The energy that is required to dissociate the complex from the equilibrium conformation into its CH4 and ClF components is only De 5 kJ mol−1. A likely path for the internal rotation of the CH4 subunit is identified by calculations at the same level of theory, which also provide the variation of the energy of the system as a function of the motion along that path. The barrier to the motion along the path is only 20 cm−1.

Motional Dynamics of Halogen-Bonded Complexes Probed by Low-Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization.

Halogen bonding is a subject of considerable interest owing to wide-ranging chemical, materials and biological applications. The motional dynamics of halogen-bonded complexes play a pivotal role in comprehending the nature of the halogen-bonding interaction. However, not many attempts appear to have been made to shed light on the dynamical characteristics of halogen-bonded species. For the first time, we demonstrate here that the combination of low-field NMR relaxometry and Overhauser dynamic nuclear polarization (ODNP) makes it possible to obtain a cogent picture of the motional dynamics of halogen-bonded species. We discuss here the advantages of this combined approach. Low-field relaxometry allows us to infer the hydrodynamic radius and rotational correlation time, whereas ODNP probes the molecular translational correlation times (involving the substrate as well as the organic radical) with high sensitivity at low field.

Comparison between Hydrogen and Halogen Bonds in Complexes of 6‐OX‐Fulvene with Pnicogen and Chalcogen Electron Donors

Quantum chemical calculations are applied to complexes of 6-OX-fulvene (X=H, Cl, Br, I) with ZH3 /H2 Y (Z=N, P, As, Sb; Y=O, S, Se, Te) to study the competition between the hydrogen bond and the halogen bond. The H-bond weakens as the base atom grows in size and the associated negative electrostatic potential on the Lewis base atom diminishes. The pattern for the halogen bonds is more complicated. In most cases, the halogen bond is stronger for the heavier halogen atom, and pnicogen electron donors are more strongly bound than chalcogen. Halogen bonds to chalcogen atoms strengthen in the order O<S<Se<Te, whereas the pattern is murkier for the pnicogen donors. In terms of competition, most halogen bonds to pnicogen donors are stronger than their H-bond analogues, but there is no clear pattern with respect to chalcogen donors. O prefers a H-bond, while halogen bonds are favored by Te. For S and Se, I-bonds are strongest, followed Br, H, and Cl-bonds in that order.

Benchmarking of Halogen Bond Strength in Solution with Nickel Fluorides: Bromine versus Iodine and Perfluoroaryl versus Perfluoroalkyl Donors.

The energetics of halogen bond formation in solution have been investigated for a series of nickel fluoride halogen bond acceptors; trans‐[NiF(2‐C5NF4)(PEt3)2] (A1), trans‐[NiF{2‐C5NF3(4‐H)}(PEt3)2] (A2), trans‐[NiF{2‐C5NF3(4‐NMe2)}(PEt3)2] (A3) and trans‐[NiF{2‐C5NF2H(4‐CF3)}(PCy3)2] (A4) with neutral organic halogen bond donors, iodopentafluorobenzene (D1), 1‐iodononafluorobutane (D2) and bromopentafluorobenzene (D3), in order to establish the significance of changes from perfluoroaryl to perfluoroalkyl donors and from iodine to bromine donors. 19F NMR titration experiments have been employed to obtain the association constants, enthalpy, and entropy for the halogen bond formed between these donor‐acceptor partners in protiotoluene. For A2–A4, association constants of the halogen bonds formed with iodoperfluoroalkane (D2) are consistently larger than those obtained for analogous complexes with the iodoperfluoroarene (D1). For complexes formed with A2–A4, the strength of the halogen bond is significantly lowered upon modification of the halogen donor atom from I (in D1) to Br (in D3) (for D1: 5≤K 285≤12 m −1, for D3: 1.0≤K 193≤1.6 m −1). The presence of the electron donating NMe2 substituent on the pyridyl ring of acceptor A3 led to an increase in −ΔH, and the association constants of the halogen bond complexes formed with D1–D3, compared to those formed by A1, A2 and A4 with the same donors.

Halogen bonding with the halogenabenzene bird structure, halobenzene, and halocyclopentadiene.

The ability of the "bird-like" halogenabenzene molecule, referred to as X-bird (XCl to At), to form halogen-bonded complexes with the nucleophiles H2 O and NH3 was investigated using double-hybrid density functional theory and the aug-cc-pVTZ/aug-cc-pVTZ-PP basis set. The structures and interaction energies were compared with 5-halocyclopenta-1,3-diene (halocyclopentadiene; an isomer of halogenabenzene) and halobenzene, also complexed with H2 O and NH3 . The unusual structure of the X-bird, with the halogen bonded to two carbon atoms, results in two distinct σ-holes, roughly at the extension of the C-X bonds. Based on the behavior of the interaction energy (which increases for heavier halogens) and van der Waals (vdW) ratio (which decreases for heavier halogens), it is concluded that the X-bird forms proper halogen bonds with H2 O and NH3 . The interaction energies are larger than those of the halogen-bonded complexes involving halobenzene and halocyclopentadiene, presumably due to the presence of a secondary interaction. © 2019 Wiley Periodicals, Inc.

Liquid crystal and photo-induced properties of polymers carrying pyridylazobenzene groups and iodopentafluorobenzene rings self-assembled through halogen bond

In this work, we report on the synthesis, liquid crystal behavior and photo-induced optical properties of a polymer and a copolymer, both carrying lateral pyridylazobenzene groups and iodopentafluorobenzene (IPFB) rings self-assembled through halogen bond. The formation of the halogen-bonded complexes was confirmed by Fourier transform infrared (FTIR) spectroscopy; the mesomorphic behavior was determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD); and the optical properties were photo-induced with a 405 nm laser beam. The pyridylazobenzene-functionalized polymer showed a smectic-type order (smectic A and crystE phases) whereas the copolymer and the halogen-bonded complexes with IPFB rings formed aggregates whose molecular order in the z-coordinates is suppressed (no layers). The photo-isomerization (in solution) of the polymer and copolymer led to a trans-to-cis isomer conversion (%cis) of 82, and 95, respectively. In film, the %cis values resulted much inferior but still high enough to induce optical properties, as for instance, in films of the copolymer (%cis = 60) and its halogen-bonded complex (%cis = 46) that responded quite well in periodical light-induced writing/erasure experiments. Only the halogen-bonded copolymer complex allowed us to register regular surface relief gratings (SRGs) suggesting that it would be a good candidate for optical data storage applications.

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.

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.

Halogen bonding and host-guest chemistry between N-alkylammonium resorcinarene halides, diiodoperfluorobutane and neutral guests.

Single crystal X-ray structures of halogen-bonded assemblies formed between host N-hexylammonium resorcinarene bromide (1) or N-cyclohexylammonium resorcinarene chloride (2), and 1,4-diiodooctafluorobutane and accompanying small solvent guests (methanol, acetonitrile and water) are presented. The guests’ inclusion affects the geometry of the cavity of the receptors 1 and 2, while the divalent halogen bond donor 1,4-diiodooctafluorobutane determines the overall nature of the halogen bond assembly. The crystal lattice of 1 contains two structurally different dimeric assemblies A and B, formally resulting in the mixture of a capsular dimer and a dimeric pseudo-capsule. 1H and 19F NMR analyses supports the existence of these halogen-bonded complexes and enhanced guest inclusion in solution.

Oriented External Electric Fields: Tweezers and Catalysts for Reactivity in Halogen-Bond Complexes.

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 either in the gas phase (where the X-Y bond dissociates 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 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 catalyze the process, by tens of kcal/mol, thus rendering the reaction allowed. 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 the structure and reactivity of XB species. Valence bond modeling is used to elucidate the means whereby OEEFs exert their control.

Hydrogen Bond versus Halogen Bond in HXOn (X = F, Cl, Br, and I) Complexes with Lewis Bases

We have theoretically studied the formation of hydrogen-bonded (HB) and halogen-bonded (XB) complexes of halogen oxoacids (HXOn) with Lewis bases (NH3 and Cl−) at the CCSD(T)/CBS//RIMP2/aug-cc-pVTZ level of theory. Minima structures have been found for all HB and XB systems. Proton transfer is generally observed in complexes with three or four oxygen atoms, namely, HXO4:NH3, HClO3:Cl−, HBrO3:Cl−, and HXO4:Cl−. All XB complexes fall into the category of halogen-shared complexes, except for HClO4:NH3 and HClO4:Cl−, which are traditional ones. The interaction energies generally increase with the number of O atoms. Comparison of the energetics of the complexes indicates that the only XB complexes that are more favored than those of HB are HIO:NH3, HIO:Cl−, HIO2:Cl−, and HIO3:Cl−. The atoms-in-molecules (AIM) theory is used to analyze the complexes and results in good correlations between electron density and its Laplacian values with intermolecular equilibrium distances. The natural bon orbital (NBO) is used to analyze the complexes in terms of charge-transfer energy contributions, which usually increase as the number of O atoms increases. The nature of the interactions has been analyzed using the symmetry-adapted perturbation theory (SAPT) method. The results indicate that the most important energy contribution comes from electrostatics, followed by induction.

Theoretical and conceptual DFT study of pnicogen- and halogen-bonded complexes of PH2X---BrCl.

The pnicogen and halogen bonding interactions in the PH2X---BrCl(X = H, F, OH, OCH3 and CH3) complexes have been studied at the MP2/aug-cc-pVTZ level. Analysis of interaction energies shows that the pnicogen-bonded structures are less stable than the corresponding halogen-bonded structures. The pnicogen and halogen bonds were also studied by conceptual DFT reactivity indices. Noncovalent interaction (NCI) and SAPT analysis reveals that the dispersion interactions dominate the pnicogen-bonded complexes of PH2X---BrCl in nature, while the halogen-bonded complexes are dominantly electrostatic energy. Graphical abstract It is found that the local softness s+ or s-on the basic center P of PH2X is related to the interaction energies (ΔECP) of halogen- or pnicogen-bonded complexes.

Systematic behaviour of electron redistribution on formation of halogen-bonded complexes BXY, as determined via XY halogen nuclear quadrupole coupling constants.

Equilibrium nuclear quadrupole coupling constants associated with the di-halogen molecule XY in each of 60 complexes BXY (where B is one of the Lewis bases N2, CO, HCN, H2O, H2S, HCCH, C2H4, PH3, NH3 or (CH3)3N and XY is one of the di-halogens Cl2, BrCl, Br2, ICl, IBr or I2) have been calculated ab initio. The Townes-Dailey model for interpreting the changes in the coupling constants when XY enters the complex was used to describe the electron redistribution in the di-halogen molecule in terms of the fraction δi of an electron transferred from the Lewis base B to atom X and the fraction δp of an electron transferred simultaneously from atom X to atom Y. Systematic relationships between the δi values for the six series are established. It is shown that, in reasonable approximation, δi decays exponentially as the first ionisation energy IB of the Lewis base B increases, that is δi = A exp(-bIB). It is concluded from the results for the series BBrCl, BBr2, BICl, BIBr and BI2 that the coefficients A and b in regression fits to the corresponding logarithmic version ln(δi) = ln(A) -b(IB) of the equation are not strongly dependent on either the halogen atom X directly involved in the halogen bond in BXY or, for a given X, on the nature of Y. The behaviour of PH3 as a Lewis base appears to be anomalous. Values of δi and δp calculated by the quantum theory of atoms-in-molecules and natural bond orbital methodologies are very close to those from application of the Townes-Dailey approach described.

Reactions of Arylcarbenes with Lewis Acids.

The reactions of the three triplet ground state arylcarbenes diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 with the Lewis acids H2 O, ICF3 , and BF3 were studied under the conditions of matrix isolation. H2 O was selected as typical hydrogen bond donor, ICF3 as halogen bond donor, and BF3 as strong Lewis acid. H2 O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet-triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding. With ICF3 , both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF3 , all three carbenes form zwitterionic Lewis acid/base complexes.

Halogen Bond Asymmetry in Solution.

Halogen bonding is the noncovalent interaction of halogen atoms in which they act as electron acceptors. Whereas three-center hydrogen bond complexes, [D···H···D]+ where D is an electron donor, exist in solution as rapidly equilibrating asymmetric species, the analogous halogen bonds, [D···X···D]+, have been observed so far only to adopt static and symmetric geometries. Herein, we investigate whether halogen bond asymmetry, i.e., a [D–X···D]+ bond geometry, in which one of the D–X bonds is shorter and stronger, could be induced by modulation of electronic or steric factors. We have also attempted to convert a static three-center halogen bond complex into a mixture of rapidly exchanging asymmetric isomers, [D···X–D]+ ⇄ [D–X···D]+, corresponding to the preferred form of the analogous hydrogen bonded complexes. Using 15N NMR, IPE NMR, and DFT, we prove that a static, asymmetric geometry, [D–X···D]+, is obtained upon desymmetrization of the electron density of a complex. We demonstrate computationally that conversion into a dynamic mixture of asymmetric geometries, [D···X–D]+ ⇄ [D–X···D]+, is achievable upon increasing the donor–donor distance. However, due to the high energetic gain upon formation of the three-center-four-electron halogen bond, the assessed complex strongly prefers to form a dimer with two static and symmetric three-center halogen bonds over a dynamic and asymmetric halogen bonded form. Our observations indicate a vastly different preference in the secondary bonding of H+ and X+. Understanding the consequences of electronic and steric influences on the strength and geometry of the three-center halogen bond provides useful knowledge on chemical bonding and for the development of improved halonium transfer agents.

An Ab Initio Investigation of the Geometries and Binding Strengths of Tetrel-, Pnictogen-, and Chalcogen-Bonded Complexes of CO₂, N₂O, and CS₂ with Simple Lewis Bases: Some Generalizations.

Geometries, equilibrium dissociation energies (De), and intermolecular stretching, quadratic force constants (kσ) are presented for the complexes B⋯CO2, B⋯N2O, and B⋯CS2, where B is one of the following Lewis bases: CO, HCCH, H2S, HCN, H2O, PH3, and NH3. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of De employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B⋯CO2 complexes involves the interaction of the electrophilic region around the C atom of CO2 (as revealed by the molecular electrostatic surface potential (MESP) of CO2) with non-bonding or π-bonding electron pairs of B. The conclusions for the B⋯N2O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B⋯CS2 series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C∞ axis of CS2 (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B⋯CO2, B⋯N2O, and B⋯CS2, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B⋯HX and B⋯XY. It is also shown that the dissociation energy De is directly proportional to the force constant kσ, with a constant of proportionality identical within experimental error to that found previously for many B⋯HX and B⋯XY complexes.

Reactions of Cyclopentadienylidenes with CF3I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom.

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF3I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF3I at 3 K. Bimolecular reactions between the carbenes and CF3I were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF3 group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.

Extremely Strong Halogen Bond. The Case of a Double-Charge-Assisted Halogen Bridge.

The stable model of a double (±)charge-assisted halogen bridge has been built on the basis of searches of the Crystal Structure Database. The model, investigated by DFT theory, consists of quinuclidine-like cation derivatives and a set of simple anions. These charged fragments form halogen-bonded complexes of which the energy of complexation in some cases reaches 100 kcal/mol. Even for such strong interactions, the QTAIM characteristics are similar to those of the more classic, relatively weak halogen bonds. An important effect of complexation is the charge transfer measured by means of QTAIM and NBO. It can also be supposed, on the basis of detailed structural and QTAIM analysis, that the delocalization of the charge in a quinuclidine moiety occurs through space and not necessarily along formal bonds. The analysis of only partially charged and fully neutral counterparts of a double (±)charge-assisted halogen bridge shows significantly weaker bonding, being less than 10 kcal/mol.

A theoretical investigation on Cu/Ag/Au bonding in XH2P⋯MY(X = H, CH3, F, CN, NO2; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes.

Intermolecular interaction of XH2P···MY (X = H, CH3, F, CN, NO2; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes was investigated by means of an ab initio method. The molecular interaction energies are in the order Ag < Cu < Au and increased with the decrease of RP···M. Interaction energies are strengthened when electron-donating substituents X connected to XH2P, while electron-withdrawing substituents produce the opposite effect. The strongest P···M bond was found in CH3H2P···AuF with -70.95 kcal/mol, while the weakest one was found in NO2H2P···AgI with -20.45 kcal/mol. The three-center/four-electron (3c/4e) resonance-type of P:-M-:Y hyperbond was recognized by the natural resonance theory and the natural bond orbital analysis. The competition of P:M-Y ↔ P-M:Y resonance structures mainly arises from hyperconjugation interactions; the bond order of bP-M and bM-Y is in line with the conservation of the idealized relationship bP-M + bM-Y ≈ 1. In all MF-containing complexes, P-M:F resonance accounted for a larger proportion which leads to the covalent characters for partial ionicity of MF. The interaction energies of these Cu/Ag/Au complexes are basically above the characteristic values of the halogen-bond complexes and close to the observed strong hydrogen bonds in ionic hydrogen-bonded species.