Type I-IV Halogen⋯Halogen Interactions: A Comparative Theoretical Study in Halobenzene⋯Halobenzene Homodimers

Abstract

In the current study, unexplored type IV halogen⋯halogen interaction was thoroughly elucidated, for the first time, and compared to the well-established types I−III interactions by means of the second-order Møller−Plesset (MP2) method. For this aim, the halobenzene⋯halobenzene homodimers (where halogen = Cl, Br, and I) were designed into four different types, parodying the considered interactions. From the energetic perspective, the preference of scouted homodimers was ascribed to type II interactions (i.e., highest binding energy), whereas the lowest binding energies were discerned in type III interactions. Generally, binding energies of the studied interactions were observed to decline with the decrease in the σ-hole size in the order, C6H5I⋯IC6H5 > C6H5Br⋯BrC6H5 > C6H5Cl⋯ClC6H5 homodimers and the reverse was noticed in the case of type IV interactions. Such peculiar observations were relevant to the ample contributions of negative-belt⋯negative-belt interactions within the C6H5Cl⋯ClC6H5 homodimer. Further, type IV torsional trans → cis interconversion of C6H5X⋯XC6H5 homodimers was investigated to quantify the π⋯π contributions into the total binding energies. Evidently, the energetic features illustrated the amelioration of the considered homodimers (i.e., more negative binding energy) along the prolonged scope of torsional trans → cis interconversion. In turn, these findings outlined the efficiency of the cis configuration over the trans analog. Generally, symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) demonstrated the predominance of all the scouted homodimers by the dispersion forces. The obtained results would be beneficial for the omnipresent studies relevant to the applications of halogen bonds in the fields of materials science and crystal engineering.

Keywords: QTAIM; SAPT-EDA; halogen bond; trans → cis interconversion; σ-hole.

Figure 1

Schematic representation for (i) the point-of-charge (PoC) calculations for halobenzene monomers (C₆H₅X; where X = Cl, Br, and I); and (ii) the potential energy surface (PES) scan for the designed halogen⋯halogen interactions within the C₆H₅X⋯XC₆H₅ homodimers.

Figure 2

Molecular electrostatic potential (MEP) maps of the halobenzene (i.e., C₆H₅X, where X = Cl, Br, and I) are plotted using 0.002 electron density contours. The electrostatic potential varies from −0.01 (red) to +0.01 (blue) au. The maximum positive electrostatic potentials (V_s,max) at σ-hole are given in kcal/mol.

Figure 3

Molecular stabilization/destabilization energies (E_{stabilization}/E_{destabilization}) of the C₆H₅X⋯PoC systems (where X = Cl, Br, and I) in the presence of ±0.50 au PoCs at X⋯PoC distance ranging from 2.5 to 5.0 Å along the x-axis and z-axis with ∠C-X⋯PoC angle of 180° and 90°, respectively.

Figure 4

Colored binding energy (E_binding) maps of type I halogen⋯halogen interactions within the C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) at X⋯X distance range of 2.5–5.0 Å and angle range of 90° < θ₁ ≈ θ₂ < 180°.

Figure 5

Binding energy curves for type II, III, and IV halogen⋯halogen interactions within the C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) estimated at MP2/aug–cc–pVDZ(PP) level of theory in kcal/mol at X–X distance range of 2.5–5.0 Å.

Figure 6

Quantum theory of atoms in molecules (QTAIM) diagrams for C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) in the fashion of type I–IV halogen⋯halogen interactions at the most favorable parameters. The gray, white, green, red, and purple colored balls represent the carbon, hydrogen, chlorine, bromine, and iodine atoms, respectively.

Figure 7

2D and 3D noncovalent interaction (NCI) isosurfaces for C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) in the pattern of type I–IV halogen⋯halogen interactions at the most favorable parameters. The isosurfaces were plotted with a reduced density gradient value of 0.50 au and colored from blue to red according to sign(λ₂)ρ ranging from −0.035 (blue) to 0.020 (red) au. The gray, white, green, red, and purple colored balls represent the carbon, hydrogen, chlorine, bromine, and iodine atoms, respectively.

Figure 8

Bar chart showed the physical components of total SAPT2+(3) energy, including electrostatic (E_elst), induction (E_ind), dispersion (E_disp), and exchange (E_exch) terms, for C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) in the fashion of type I–IV halogen⋯halogen interactions at the most favorable parameters.

Figure 9

Binding energy curves for torsional trans → cis interconversion of C₆H₅X⋯XC₆H₅ homodimers (where X = Cl, Br, and I) in the pattern of type IV halogen⋯halogen interactions at the most favorable parameters.