Theoretical description of halogen bonding – an insight based on the natural orbitals for chemical valence combined with the extended-transition-state method (ETS-NOCV)

In the present study we have characterized the halogen bonding in selected molecules H(3)N–ICF(3) (1-NH (3)), (PH(3))(2)C–ICF(3) (1-CPH (3)), C(3)H(7)Br–(IN(2)H(2)C(3))(2)C(6)H(4) (2-Br), H(2)–(IN(2)H(2)C(3))(2)C(6)H(4) (2-H (2) ) and Cl–(IC(6)F(5))(2)C(7)H(10)N(2)O(5) (3-Cl), containing from one halogen bond (1-NH (3), 1-CPH (3)) up to four connections in 3-Cl (the two Cl–HN and two Cl–I), based on recently proposed ETS-NOCV analysis. It was found based on the NOCV-deformation density components that the halogen bonding C–X(…)B (X-halogen atom, B-Lewis base), contains a large degree of covalent contribution (the charge transfer to X(…)B inter-atomic region) supported further by the electron donation from base atom B to the empty σ*(C–X) orbital. Such charge transfers can be of similar importance compared to the electrostatic stabilization. Further, the covalent part of halogen bonding is due to the presence of σ-hole at outer part of halogen atom (X). ETS-NOCV approach allowed to visualize formation of the σ-hole at iodine atom of CF(3)I molecule. It has also been demonstrated that strongly electrophilic halogen bond donor, [C(6)H(4)(C(3)H(2)N(2)I)(2)][OTf](2), can activate chemically inert isopropyl bromide (2-Br) moiety via formation of Br–I bonding and bind the hydrogen molecule (2-H (2)). Finally, ETS-NOCV analysis performed for 3-Cl leads to the conclusion that, in terms of the orbital-interaction component, the strength of halogen (Cl–I) bond is roughly three times more important than the hydrogen bonding (Cl–HN). [Figure: see text]