Quantum Tunneling Instability in Pericyclic Reactions

Abstract

Several cycloreversion reactions of the retro-Diels− Alder type were computationally assessed to understand their quantum tunneling (QT) reactivity. N 2 , CO, and other leaving groups were considered based on their strong exothermicity, as it reduces their thermodynamic and kinetic stabilities. Our results indicate that for many of these reactions, it is essential to take into account their QT decomposition rate, which can massively weaken their molecular stability and shorten their half-lives even at deep cryogenic temperatures. In practical terms, this indicates that many supposedly stable molecules will actually be unsynthesizable or unisolable, and therefore trying to prepare or detect them would be a futile attempt. In addition, we discuss the importance of tunneling to correctly understand the enthalpy of activation and the collective atomic effect on the tunneling kinetic isotope effects to test if third-row atoms can tunnel in a chemical reaction. This project raises the question of the importance of in silico chemistry to guide in vitro chemistry, especially in cases where the latter cannot solve its own uncertainties.

Quantum Tunneling Instability in Pericyclic Reactions: The Cheletropic, Coarctate, and Ene Cases

“…In a recent paper, we explored with computational tools the instability of certain molecules against their unimolecular degradation through a retro-Diels–Alder type of reaction . Such instability was thermodynamically driven by the large exothermicity of the reaction mostly obtained by using N 2 and CO as leaving groups.…”

“…Such instability was thermodynamically driven by the large exothermicity of the reaction mostly obtained by using N 2 and CO as leaving groups. The critical studied aspect of these reactions was the possibility of suffering a strong kinetic acceleration thanks to their quantum tunneling instability ( QTI ). − This means that, in principle, some supposedly stable molecules according to the semiclassical ( SC ) viewpoint will in practice be unsynthesizable, unobservable, and/or unisolable as a result of tunneling.…”

“…For the QTI to occur, the reactions must encompass a relatively low threshold energy and, more critically, a considerably narrow barrier, with small atomic trajectories. − This is essential, especially when dealing with “heavy” atom bond breaking (that is, anything heavier than He). ,− The high exothermicity of the reactions also favors the low and narrow barriers . The “retro” versions of some pericyclic reactions, such as the described retro-aza-Diels–Alder, might comply with these requirements, especially the narrow barrier because their bond-breaking processes involve only short movements [an introduction to quantum tunneling (QT) is included in the Supporting Information].…”

“…All of the computations were carried out at the exact same level as in our previous work, which provides reliable gas-phase results at a tractable computational time. In basic terms, the electronic structures were performed with Gaussian16 at the CCSD­(T)/CBS//M06-2X/6-311G­(d) level [all energies presented here are closed-shell singlets and include zero-point energy (ZPE)].…”

“…Co­(C 2 H 4 ) , probably thanks to the higher stability of the product, shows a much more moderate stability, with τ 1/2 of only 3 h at −90 °C, matching the experimental results of decomposition over this temperature. Note that the QT correction is significant in these conditions (κ equal to 3.7 and 5.2) but relatively small; basically, the stability of the compounds will be of the same order with or without tunneling at high temperatures, but we have to consider it if we want to achieve high accuracy, even for these reactions involving heavy atom tunneling. ,, …”