Toward a uniform description of hydrogen bonds and halogen bonds: correlations of interaction energies with various geometric, electronic and topological parameters

Abstract

Halogen bonds, which are specific non-covalent interactions similar to hydrogen bonds, play crucial roles in fields as diverse as supramolecular assemblies, crystal engineering, and biological systems. A total of 108 halogen-bonded and hydrogen-bonded complexes formed by different electron acceptors and NH₃, namely, R–A⋯NH₃ (A = H, Cl, Br or I), have been investigated at the MP2(full)/aug-cc-pVDZ(-PP) level of theory. The relationships between the interaction strengths and various geometric and electronic structures, as well as topological properties, were established, with a particular focus on the uniformity of these two types of interaction. The dependence of the BSSE-corrected interaction energy (ΔE^cor) on the interatomic distance (r_A⋯N) appeared to be nonlinear for both halogen-bonded and hydrogen-bonded systems; the relationship between ΔE^cor and the difference between r_A⋯N and the sum of the van der Waals radii (Δr_A⋯N) can be fitted to a combined quadratic regression equation. Furthermore, we demonstrated that the linear correlations between ΔE^cor and ρ_b(BCP) (the electron density at bond critical points in the A⋯N bond) and its Laplacian ∇²ρ_b(BCP) can be used to provide a combined description of hydrogen bonds and halogen bonds, with correlation coefficients of 0.964 and 0.956, respectively. The dependence of the interaction strength on the electrostatic potential corresponding to an electron density of 0.002 a.u. along the R–A bond vector (ESP_0.002), the amount of charge transferred (Q_CT) and the second-order perturbation stabilization energies of n(NH₃) → σ*(R–A) (E⁽²⁾) were also examined. Strong halogen-bonded complexes were found to exhibit different linear correlations from weak halogen-bonded and hydrogen-bonded systems. Nevertheless, for the latter two types of system, a uniform regression equation can be constructed. These relationships not only improve our understanding of the nature of halogen bonding but also provide a feasible approach for predicting or determining the relative strengths of hydrogen bonds and halogen bonds, in particular when both types of non-covalent interaction coexist and compete with each other.