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Strength order and nature of the π-hole bond of cyanuric chloride and 1,3,5-triazine with halide
Hui Wang,Chen Li,Weizhou Wang,Wei Jun Jin
Physical Chemistry Chemical Physics Pub Date : 07/07/2015 00:00:00 , DOI:10.1039/C5CP03076E
Abstract

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.

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