An Assessment of Radial Potential Functions for the Halogen Bond: Pseudo-Diatomic Models for Axially Symmetric Complexes B⋅⋅⋅ClF (B=N2 , CO, PH3 , HCN, and NH3 )

The radial potential energy is calculated ab initio at the explicitly correlated level of theory CCSD(T)(F12c)/cc-pVTZ-F12 for the five axially symmetric, halogen-bonded complexes B⋅⋅⋅ClF (B=N₂ , CO, PH₃ , HCN, and NH₃ ) as a function of the intermolecular distance r. The PE curves are fitted by the Hulburt-Hirschfelder analytical function under the assumption of a pseudo-diatomic model. The spectroscopic constants ω σ and ω σ x σ , and α σ of the intermolecular stretching mode υ σ are calculated by two closely related approaches. The first involves derivatives of a polynomial fitted to the ab initio calculated points near to r_e and evaluated at r=r_e . The second uses the constants of the fitted H-H function. Both procedures are tested on ³⁵ ClF by fitting (a) its RKR-type function and (b) the CCSD(T)(F12c)/cc-pVTZ-F12 version. The complexes OC⋅⋅⋅ClF and H₃ P⋅⋅⋅ClF behave differently from the other three. A point of inflection/secondary minimum with a shortened r(C⋅⋅⋅Cl) and an increased r(Cl-F) detected for B=CO, suggests a second isomer with a significant contribution from the valence-bond structure OC⁺ Cl⋅⋅⋅F^- . The shape of the ab initio calculated function for H₃ P⋅⋅⋅ClF is different from those involving B=N₂ , HCN, or NH₃ , a difference attributed to H₃ PCl⁺ ⋅⋅⋅F^- character. The ab initio generated curve for H₃ P⋅⋅⋅ClF is, nevertheless, satisfactorily fitted by the three-parameter H-H function.

An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study of the Halogen Bond with Explicit Analysis of Electron Correlation

Energy profiles of seven halogen-bonded complexes were analysed with the topological energy partitioning called Interacting Quantum Atoms (IQA) at MP4(SDQ)/6-31+G(2d,2p) level of theory. Explicit interatomic electron correlation energies are included in the analysis. Four complexes combine X₂ (X = Cl or F) with HCN or NH₃, while the remaining three combine ClF with HCN, NH₃ or N₂. Each complex was systematically deformed by translating the constituent molecules along its central axis linking X and N, and reoptimising its remaining geometry. The Relative Energy Gradient (REG) method (Theor. Chem. Acc. 2017, 136, 86) then computes which IQA energies most correlate with the total energy during the process of complex formation and further compression beyond the respective equilibrium geometries. It turns out that the covalent energy (i.e., exchange) of the halogen bond, X…N, itself drives the complex formation. When the complexes are compressed from their equilibrium to shorter X…N distance then the intra-atomic energy of N is in charge. When the REG analysis is restricted to electron correlation then the interatomic correlation energy between X and N again drives the complex formation, and the complex compression is best described by the destabilisation of the through-space correlation energy between N and the "outer" halogen.

Strengths of non-covalent interactions in hydrogen-bonded complexes B⋯HX and halogen-bonded complexes B⋯XY (X, Y = F, Cl): an ab initio investigation

The intermolecular quadratic stretching force constants kcalc.σ of a series of hydrogen-bonded and halogen bonded complexes B⋯HX and B⋯XY, where B is N₂, CO, HCCH, C₂H₄, H₂S, PH₃, HCN, or NH₃.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

A reduced radial potential energy function for the halogen bond and the hydrogen bond in complexes B···XY and B···HX, where X and Y are halogen atoms

It is shown by considering 76 halogen- and hydrogen-bonded complexes BXY and BHX (where B is a Lewis base N2, CO, C2H2, C2H4, H2S, HCN, H2O, PH3 or NH3 and X, Y are F, Cl, Br or I) that the intermolecular stretching force constants kσ (determined from experimental centrifugal distortion constants via a simple model) and the intermolecular dissociation energies Dσ (calculated at the CCSD(T)(F12*)/cc-pVDZ-F12 level of theory) are related by Dσ = Cσkσ, where Cσ = 1.50(3) × 10(3) m(2) mol(-1). This suggests that one-dimensional functions implying direct proportionality of Dσ and kσ, (e.g. a Morse or Rydberg function) might serve as reduced radial potential energy functions for such complexes.