Heavy‐Atom Tunneling in Semibullvalenes: How Driving Force, Substituents, and Environment Influence the Tunneling Rates

Abstract

The Cope rearrangement of selectively deuterated isotopomers of 1,5‐dimethylsemibullvalene 2 a and 3,7‐dicyano‐1,5‐dimethylsemibullvalene 2 b were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy‐atom tunneling. The driving force for the rearrangements is the small difference in the zero‐point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in 2 a and 2 b, two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of 2 b in cryogenic matrices were found to be smaller than the ones of 2 a under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of 2 b compared to that of 2 a results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for 2 a in a qualitatively different way than for 2 b.

Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide

“…Also, the matrix material might interact differently with various derivatives of the same compound class. A very recent study in the realm of heavy‐atom tunneling [72] addresses the interplay of these effects [73] . Disentangling the influences on tunneling half‐lives to draw a clearer picture how to effectively control QMT remains an ongoing endeavor.…”

Switch chemistry at cryogenic conditions: quantum tunnelling under electric fields

“…This system has a similar double-well potential as PL, but it has the advantage of having been thoroughly studied not only theoretically but also experimentally in cryogenic matrices. 53,[58][59][60][61] Previous SCT computations at the B3LYP/6-31G(d) level 60 gave k = 1.4×10 -3 s -1 below 40 K (H ‡ = 19 kJmol -1 ), while the experiments in cryogenic matrices produced rate constants of the order of 10 -4 s -1 , 58,59 depending on the matrix. This attests to the fortuitous error cancellation with B3LYP that provides accurate QT results only when compared to interacting environments (which tend to accelerate the reaction).…”

“…1A).  Semibullvalene (SBV) Cope rearrangement, 53,[58][59][60][61] another degenerate reaction (Fig. 1B).…”

Heavy‐Atom Tunneling Through Crossing Potential Energy Surfaces: Cyclization of a Triplet 2‐Formylarylnitrene to a Singlet 2,1‐Benzisoxazole

“…[14][15][16] So far only about ad ozen examples of direct experimental observations of heavy-atom QMT reactions in cryogenic conditions have been reported. [17][18][19][20][21][22][23][24][25][26][27][28][29][30] Even fewer cases with clear experimental evidence of heavy-atom QMT contribution are known for reactions in solution. [2] Thei ncreasing number of recent reports on experimental and theoretical heavy-atom QMT reactions, however, seems to suggest that this phenomenon occurs more frequently than had been assumed and can have significant implications for many chemical reactions.…”

“…Because tunneling probability decreases exponentially with the square root of the moving mass, tunneling of atoms heavier than hydrogen (such as carbon, 12 times heavier) is predicted to be very unlikely . So far only about a dozen examples of direct experimental observations of heavy‐atom QMT reactions in cryogenic conditions have been reported . Even fewer cases with clear experimental evidence of heavy‐atom QMT contribution are known for reactions in solution .…”