Ping‐Pong Tunneling Reactions: Can Fluoride Jump at Absolute Zero?

Abstract

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.

Effect of confinement on ammonia inversion

“…The signature of tunneling is evident in the behavior of the rate coefficients for certain hydrogen transfer reactions at low temperatures (at energies below the reaction barrier) [31,32]. Tunneling seems to be also important in certain organic reactions where heavy atoms like carbon, oxygen, sulfur and nitrogen are involved [35][36][37][38][39]. The effect of tunneling is manifested in the predicted and measured kinetic isotope effects [40,41].…”

Heavy-atom tunnelling in Cu(ii)N6 complexes: theoretical predictions and experimental manifestation

“…This measure is similar to the effective trajectory of a QMT process, 17,60,61 and therefore their low values (combined with the low activation energies, see Table 1 ), suggest a high QMT probability. Solid-state experimental R JT seems to be shorter than our gas phase values (for example, 0.33 vs. 0.42 Å for Cu(tach) 2 2+ ) due to the crystal pressure (see above), 35 which points to a significantly faster QMT rate for the former.…”

Heavy‐Atom Tunneling in Organic Reactions

“…The “ping‐pong” reactions (Scheme ) in which an atom or group of atoms “bounces” between two groups are notable in that they provide predictions of both fluorine and boron tunneling at cryogenic temperatures. DFT calculations by Kozuch on variations of 26 (with R=H) showed that changing Y from SO 2 (the original system proposed by Scheiner) to SeO 2 increases k SCT at 6 K from 0.17 s −1 to 2.9×10 9 s −1 . Although far from the reactive site, increasing the size of Y produces a geometric change that translates to a lower and narrower barrier for hopping of the fluoride ion from one germanium to the other.…”