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Dual Noncovalent Bonding to Planar Tetracoordinated Oxygen
Wiktor Zierkiewicz1, Mariusz Michalczyk1, Steve Scheiner2
1Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław 50-370, Poland.
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The OTr4 molecule, where Tr refers to a triel atom Al, Ga, In, or Tl, places the O at the center of a square. The region directly above the O can host the halogen X atom of FX or MeX to form a T-shaped complex. Quantum chemical calculations measure various aspects of the binding which is found to consist of two separate components. Charge can be transferred from the pz orbital of O to the antibonding σ*(XF/XC) orbital in a classical halogen bond. At the same time, charge can shift in the opposite direction, from X lone pairs to empty Tr lone pair orbitals, so as to form Tr···X triel bonds to all four Tr centers. Both forms of bonding are present in all dyads, but one or the other takes precedence depending on the identity of the FX/MeX molecule and the Tr atom. FX forms much stronger bonds to OTr4 than does MeX, with interaction energies approaching 40 kcal/mol.
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