Classical Electrostatic Interaction Is the Origin for Blue-Shifting Halogen Bonds.

Studies have shown that, on the one hand, charge transfer (CT) plays a key role in halogen bonds (D···X-A) (D = donor; X = halogen atom; A = acceptor), suggesting considerable covalent character of halogen bonding. But, on the other hand, it has been proposed that halogen bonding is dominated by the electrostatic attraction between the electropositive σ-hole at the halogen atom X and the electronegative donor D. It has also been well-recognized that the CT from the donor D to the antibonding σ*(X-A) would weaken and lengthen the X-A bond. Yet, intriguingly, there is a blue-shifting phenomenon in halogen bonding, where the X-A bond contracts with an enhanced stretching vibrational frequency. Here we explored the nature of blue-shifting halogen bonds with the iconic case of H3N···ClNO2, which exhibits the blue-shifting phenomenon along with a strong CT interaction and its analogous H3N···XNY2 (X = Cl, Br, and I; Y = O, S). By decomposing the binding energy to a number of energy components and exploring their energy profiles along with the halogen-bonding distances with the block-localized wave function (BLW) method, we showed that the classical electrostatic interaction is the governing factor for the blue-shifting of the X-N bonds. This is further supported by the similar magnitudes of blue-shifting obtained when NH3 is replaced with atomic point charges in the complexes. Alternatively, by applying external electric field (E-field) along the X-N bond direction, the blue-to-red shifting transition can be identified. This is because both polarization and CT interactions tend to stretch the X-N bond, and both are enhanced simultaneously under the external E-field. Finally, roles of individual energy components are reconfirmed using the force analysis based on the BLW energy decomposition approach.