Calculations Predict That Carbon Tunneling Allows the Degenerate Cope Rearrangement of Semibullvalene to Occur Rapidly at Cryogenic Temperatures

Abstract

Calculations on the role of tunneling in the degenerate Cope rearrangements of semibullvalene (1) and barbaralane (3) predict that, at temperatures below 40 K, tunneling from the lowest vibrational level should make the temperature-independent rate constants k ) 1.43 × 10 -3 s -1 and k ) 7.28 × 10 -9 s -1 , respectively. An experiment, using semibullvalene-2(4)-d 1 , is proposed to test the prediction of rapid tunneling by 1 at cryogenic temperatures.

Reactions that involve tunneling by carbon and the role that calculations have played in their study

“…An experimental test of their prediction that semibullvalene should tunnel rapidly at cryogenic temperatures was proposed by Borden and coworkers . In semibullvalene‐d 1 ( 38b ), the isomer on the left in Figure was calculated to be of lower energy than the isomer on the right.…”

“…The calculated change in the CC distances in breaking and forming a CC σ bond in the degenerate rearrangement of 38a is 0.75 Å; and the energy barrier, both calculated and measured, is found to be low. Consequently, this reaction, which is shown in Figure , appeared to be a good candidate for being one in which tunneling by carbon would be rapid at cryogenic temperatures.…”

“…Consequently, this reaction, which is shown in Figure , appeared to be a good candidate for being one in which tunneling by carbon would be rapid at cryogenic temperatures. Indeed, SCT calculations predicted that, at temperatures below 40 K, the half‐time for the automerization of 38a would be less than 10 min …”

Atom Tunneling in Chemistry

“…[136,[268][269][270][271][272][273][274][275] Reactions of strained organic molecules often involve unusually strong heavy-atom tunneling:ahigh activation energy from the breaking of aC À Cbond combined with little movement of the involved atoms lead to narrow barriers,which enhance the probability for tunneling. Forexample,heavy-atom tunneling is prominent in the Cope rearrangement of semibullvalene [276] and the ring-opening reaction of cyclopropylcarbinyl radicals. [277][278][279][280][281] In the former case, 12 C/ 13 CKIEs of more than 5at40Kare predicted.…”

“…[277][278][279][280][281] In the former case, 12 C/ 13 CKIEs of more than 5at40Kare predicted. [276] In the latter case,t he reaction rate is nearly constant below 20 K. [279] Angewandte Chemie Reviews 3.6. Astrochemistry Astrochemistry describes the formation, distribution, and destruction of chemical substances in space.N oteworthy features,w hen considering reactions and reaction rates in the interstellar medium, are the low particle density and the strong radiation fields as well as the low temperature.…”