Strength order and nature of the π-hole bond of cyanuric chloride and 1,3,5-triazine with halide

The 13C NMR chemical shift moving upfield indicates the main model of π-hole⋯X− bond between cyanuric chloride/1,3,5-triazine (3ClN/3N), which possess both the π-hole and σ-hole, and X−. 13C NMR and UV absorption titration in acetonitrile confirmed that the bonding abilities of 3ClN/3N with X− follow the order I− > Br− > Cl−, which is apparently the order of the charge transfer ability of halide to 3ClN/3N. Chemical calculations showed that the bonding abilities in solution were essentially consistent with those obtained by titration experiments. However, the results in the gas phase were the reverse, i.e., π-hole⋯Cl− > π-hole⋯Br− > π-hole⋯I− in bonding energy, which obeys the order of electrostatic interaction. In fact, the π-hole bond and σ-hole bond compete with solvation and possible anion-hydrogen bond between a solvent molecule and a halide in solution. An explanation is that the apparent charge transfer order of π-/σ-hole⋯I− > π-/σ-hole⋯Br− > π-/σ-hole⋯Cl− occurs for weak π-hole bonds and σ-hole bonds, whereas the order of electrostatic attraction of π-/σ-hole⋯Cl− > π-/σ-hole⋯Br− > π-/σ-hole⋯I− is valid for strong bonds. It can be concluded by combining energy decomposition analysis and natural bond orbital analysis that the π-hole⋯X− bond and σ-hole⋯X− bond are electrostatically attractive in nature regardless of whether the order is I− > Br− > Cl− or the reverse.