Thermally activated tunneling in organic reactions

Abstract

In this review, we discuss quantum tunneling in the temperature range normally used to carry out reactions, ca.-78 °C and higher. Both heavy-and hydrogen-atom tunneling are included. Experimental data and computational results are discussed. Recent studies suggest that tunneling is widespread at typical reaction temperatures.

Conformer-specific [1,2]H-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC–C̈–OH)

“…We also attempted to stimulate [1,2] H -tunnelling in 7t through vibrationally activated tunnelling (VAT)18,19 (the [1,2] H -tunnelling half-life from an excited vibrational mode is computed to be around 2 h (Table S13†)) by near infrared (NIR) laser irradiation of the measured O–H stretching vibration overtone of 7t at 6840.2 cm –1 (1461.95 nm ≙ 19.6 kcal mol –1 ) (Fig. 4 and S21†).…”

Heavy‐Atom Tunneling in Organic Reactions

“…[26] TheA rrhenius plots of the rate constants for 7a!7a' ' and 8a!8a' ' reveal pronounced curvature that also indicates al arge tunneling contribution ( Figure 6). Thel evel portion of the solid blue curve in Figure 6b indicates "deep tunneling", that is,tunneling from the lowest vibrational state of 8a,w hereas the curved portions of the Arrhenius plots in Figure 6r epresent temperatures where thermally activated tunneling [17] is occurring,t hat is,f rom excited vibrational states of 7aand 8a.Inboth systems,bond-shifting is predicted to occur much faster than the corresponding (lower-barrier) conformation change below acertain temperature,providing aunique example of tunneling control in which the preferred process is dominated by motions of heavy atoms.A dditionally,t unneling of 16 carbons suggests that synchronous tunneling by multiple heavy atoms might be more prevalent than expected. Coupled with the work by Kozuch, it is not unreasonable to expect tunneling to dominate most planar p-BS reactions at temperatures up to 200 K(or higher in some cases);that is,automerization without tunneling would be the surprise.O ther candidates for degenerate p-BS reactions in annulenes in which tunneling might be present are [4n+ +2]annulenes,such as [14]-and [18]annulene.However, since these are aromatic systems,t he p-BS reactions would be expected to have extremely low barriers,m aking them rapid even without tunneling.…”

Carbon Tunneling in the Automerization of Cyclo[18]carbon

“…Indeed, our gas‐phase small curvature tunneling (SCT) computations point to a fast‐tunneling process for the automerization reaction of C 18 even close to the absolute zero ( k SCT =2.1×10 8 s −1 at the original experimental temperature, 5 K, see Table ), whereas over‐barrier canonical variational transition‐state theory (CVT) rates indicate that the reaction should be impossible almost up to 100 K. Therefore, only when including the QMT corrections the reaction is in a fast equilibrium between the two degenerate minima at cryogenic conditions ( t 1/2 =3 ns). It can be seen from Table that at any point below room temperature, where the SCT is still an order of magnitude faster than the CVT rate, the reaction is completely dominated by tunneling (albeit over 100 K it is mostly thermally activated tunneling from excited vibrational levels).…”