In a recent study, Scheiner designed a double-germanium-based fluoride receptor that binds the halogen by means of strong tetrel bonds (Chem. Eur. J. 2016, 22, 18850). In this system the F binds to the germanium atoms in an asymmetric fashion, thereby producing a double-well potential in which the fluoride can jump from one germanium to the other as in a ping-pong game. Herein we prove through the use of computational tools that at cryogenic temperatures this rearrangement occurs by heavy-atom quantum mechanical tunneling. The inductive strength of the substituents and the polarity of the solvent can modify the barrier and the tunneling rate. But the strongest effect is observed upon modification of the geometry of the molecule by specific substitutions that affect the barrier width, the most critical factor in a tunneling mechanism. We postulate two experimental tests, one by microwave spectroscopy and one by cryogenic NMR spectroscopy, that can prove the predicted fluoride tunneling.

Anthracene can be used as a scaffold for intramolecular SN2 degenerate reactions of the “bell clapper” type, where a central boron atom or its isoelectronic carbocation bonds alternatively towards one or the other lateral Lewis bases at the first and eight anthracene positions. This ping-pong bond-switching reaction possesses a symmetrical double-well potential with low activation barrier and relatively narrow barrier width. Herein we show by computational means the active role played by heavy atom quantum tunneling in this degenerate rearrangement reaction at cryogenic temperatures. At these conditions the thermal “over the barrier” reaction is forbidden, whereas the tunneling effect enhances the rate of reaction up to an experimentally measurable half-life. Kinetic isotope effects and cryogenic NMR spectroscopy can, in principle, experimentally demonstrate the tunneling mechanism.